Urinary System Flashcards
In addition to urea, the kidneys secrete creatinine and uric acid. Creatinine is a waste product that results from the breakdown of creatine phosphate, a high-energy phosphate reserve molecule in muscles. Uric acid is formed from the metabolic processing of nucleotides (such as adenine and thymine). Uric acid is rather insoluble. If too much uric acid is present in blood, crystals form and precipitate out. Crystals of uric acid sometimes collect in the joints, producing a painful ailment called gout.
What else do kidneys secrete?
What is creatinine?
What is uric acid?
What happens if one has too much uric acid?
What’s a floating kidney ?
What is a “floating kidney”?
A floating kidney, a condition also known as nephroptosis, occurs when the kidney becomes detached from its position and moves freely beneath the peritoneum. A floating kidney may develop in people who are very thin or in someone who has recently received a sharp blow to the back. When the kidney becomes dislodged, it may form a kink in the ureter, causing urine to back up into the kidney. This can result in damage to the structures inside the kidney. Surgery can correct a floating kidney by reattaching it to the abdominal wall.
The organ that converts ammonia from amino acid breakdown to the less toxic compound urea is the
liver
removal of metabolic waste from body
This is excretion
Hormones regulate the reabsorption of sodium and water.
Aldosterone is a hormone secreted by the adrenal glands, which sit atop the kidneys. This hormone promotes ion exchange at the distal convoluted tubule. Potassium ions (K+) are excreted, and sodium ions (Na+) are reabsorbed into the blood. The release of aldosterone is set into motion by the kidneys. The juxtaglomerular apparatus is a region of contact between the afferent arteriole and the distal convoluted tubule (Fig. 11.8). When blood volume (and, therefore, blood pressure) falls too low for filtration to occur, the juxtaglomerular apparatus can respond to the decrease by secreting renin. Renin is an enzyme that ultimately leads to secretion of aldosterone by the adrenal glands. Research scientists speculate that excessive renin secretion—and thus, reabsorption of excess salt and water—might contribute to high blood pressure.
Distal convoluted tubule- reabsorption & hormones, renin, juxtaglomerular apparatus
a waste product of amino acid metabolism; primary nitrogenous end product of metabolism in humans.
(broken down amino acids) ammonia (very toxic) so + liver quickly mixes it with carbon dioxide- in liver
What is urea?
What does the maintance of acid base balance entail?
The kidneys regulate the acid-base balance of the blood. For a person to remain healthy, the blood pH should be just about 7.4. The kidneys monitor and help control blood pH, mainly by excreting hydrogen ions (H+) and reabsorbing the bicarbonate ions (HCO3−) as needed to keep blood pH at 7.4. Urine usually has a pH of 6 or lower, because our diet often contains acidic foods.
What is the excretion of metabolic waste?
The metabolic waste of humans consists primarily of nitrogenous waste, such as urea, creatinine, ammonium, and uric acid. Urea, a waste product of amino acid metabolism, is the primary nitrogenous end product of metabolism in humans. In the liver, the breakdown of amino acids releases ammonia, a compound that is very toxic to cells. The liver rapidly combines the ammonia with carbon dioxide to produce urea, which is much less harmful. Normally, urea levels in the blood are between 10 and 20 milligrams per deciliter (mg/dl). Elevated urea levels in the blood may cause uremia, a condition that causes cardiac arrhythmia, vomiting, respiratory problems, and potentially death. Treatments for elevated urea levels are discussed in Section 11.5.
In addition to urea, the kidneys secrete creatinine and uric acid. Creatinine is a waste product that results from the breakdown of creatine phosphate, a high-energy phosphate reserve molecule in muscles. Uric acid is formed from the metabolic processing of nucleotides (such as adenine and thymine). Uric acid is rather insoluble. If too much uric acid is present in blood, crystals form and precipitate out. Crystals of uric acid sometimes collect in the joints, producing a painful ailment called gout.
Why is blood pressure higher in the glomerular?
Is it the
Each nephron has its own blood supply, including two capillary regions (Fig. 11.4). From the renal artery, an afferent arteriole transports blood to the glomerulus, a knot of capillaries inside the glomerular capsule. Blood leaving the glomerulus is carried away by the efferent arteriole. Blood pressure is higher in the glomerulus, because the efferent arteriole is narrower than the afferent arteriole. The efferent arteriole divides and forms the peritubular capillary network, which surrounds the rest of the nephron. Blood from the efferent arteriole travels through the peritubular capillary network. The blood then goes into a venule that carries blood into the renal vein.
Nephron flow
1) maintain salt, water, ph homeostasis of blood
Major functions of urinary system
The cuboidal epithelial cells lining this part of the nephron have numerous microvilli, about 1 micrometer (µm) in length, that are tightly packed and form a brush border (Fig. 11.5). A brush border greatly increases the surface area for the tubular reabsorption of filtrate components. Each cell also has many mitochondria, which can supply energy for active transport of molecules from the lumen to the peritubular capillary network.
proximal convoluted tubule and features
The bladder wall is expandable, because it contains a middle layer of circular fibers of smooth muscle and two layers of longitudinal smooth muscle. The epithelium of the mucosa becomes thinner, and folds in the mucosa called rugae disappear as the bladder enlarges. The bladder’s rugae are similar to those of the stomach.
What is the structure of the bladder wall?
Normally, urea levels in the blood are between 10 and 20 milligrams per deciliter (mg/dl). Elevated urea levels in the blood may cause uremia, a condition that causes cardiac arrhythmia, vomiting, respiratory problems, and potentially death. Treatments for elevated urea levels are discussed in Section 11.5.
What is uremia ?
Memorize this card
The formation of urine involves three stages: glomerular filtration, tubular reabsorption, and tubular secretion.
What are the three stages of the formation of urine?
11.3 Urine Formation
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Summarize the three processes involved in the formation of urine.
List the components of the glomerular filtrate.
Describe how tubular reabsorption processes nutrient and salt molecules.
Explain the substances removed from the blood by tubular secretion.
The formation of urine involves three stages: glomerular filtration, tubular reabsorption, and tubular secretion. Figure 11.6 provides an overview of these processes.
Figure 11.6 An overview of urine production. The three main processes in urine formation are described in boxes and color-coded to arrows that show the movement of molecules into or out of the nephron at specific locations. In the end, urine is composed of the substances within the collecting duct (see brown arrow).
Tutorial: Urine Formation
Glomerular Filtration
Glomerular filtration occurs when whole blood enters the glomerulus by way of the afferent arteriole. The afferent arteriole (Fig. 11.6, inset) has a larger diameter than the efferent arteriole, resulting in an increase in glomerular blood pressure. Because of this, water and small molecules move from the glomerulus to the inside of the glomerular capsule. This is a filtration process, because large molecules Page 224and formed elements are unable to pass through the capillary wall. In effect, then, blood in the glomerulus has two portions: the filterable components and the nonfilterable components.
Table Summary: Table is divided into two columns to list the names of filterable blood components and nonfilterable blood components. Empty cells in column 2 signify that there are only two types of nonfilterable blood components.
Filterable Blood ComponentsNonfilterable Blood Components
WaterFormed elements (blood cells and platelets)
Nitrogenous wastesPlasma proteins
Nutrients
Salts (ions)
The nonfilterable components leave the glomerulus by way of the efferent arteriole. The glomerular filtrate inside the glomerular capsule now contains the filterable blood components in approximately the same concentration as plasma.
As indicated in Table 11.1, nephrons in the kidneys filter 180 liters of water per day, along with a considerable amount of small molecules (such as glucose) and ions (such as sodium). If the composition of urine were the same as that of the glomerular filtrate, the body would continually lose water, salts, and nutrients. Therefore, we can conclude that the composition of the filtrate must be altered as this fluid passes through the remainder of the tubule.
Table 11.1Reabsorption from Nephrons
Table Summary: Data is provided for four substances, the names of which are listed in column 1. The data for each substance appear in their respective columns.
SubstanceAmount Filtered (per Day)Amount Excreted (per Day)Reabsorption (%)
Water (l) 180 1.8 99.0
Sodium (g) 630 3.2 99.5
Glucose (g) 180 0.0 100.0
Urea (g) 5430.0 44.0
l = liters; g = grams
SCIENCE IN YOUR LIFE
Is urine sterile?
This is a common misconception from pop culture. In fact, the urine from a healthy individual can contain thousands of bacteria per milliliter of urine, and scientists have identified over 30 types of bacteria normally found in urine. Where do these bacteria come from? Some are naturally present in the urinary system, while others are collected from the skin during urination.
The good news is that having bacteria in your urine is not necessarily bad. Bacteria form part of the microbiota of our bodies, and their presence may actually help fight some infections. However, having a high number of bacteria, or changes in the types of bacteria, can indicate a possible urinary tract infection (UTI).
Tubular Reabsorption
Tubular reabsorption occurs as molecules, especially ions, are passively and actively reabsorbed from the nephron into the blood of the peritubular capillary network. The osmolarity of the blood is maintained by the presence of plasma proteins and salt. Sodium ions (Na+) are actively transported by one of two types of transport proteins. First, the movement of sodium may be coupled to the movement of larger solutes, such as amino acids or glucose. This is called a symport, because both solutes are being moved in the same direction. The second mechanism involves an antiport protein, which moves Na+ ions into the cell while transporting H+ ions out of the cell. This also regulates the pH balance of the blood, because the movement of H+ ions outward reduces the acidity of the blood. As sodium ions are being moved, chloride ions (Cl−) follow passively. The reabsorption of salt (NaCl) increases the osmolarity of the blood compared with the filtrate. Therefore, water moves passively from the tubule into the blood. About 65% of Na+ is reabsorbed at the proximal convoluted tubule.
Cotransport
Nutrients such as glucose and amino acids return to the peritubular capillaries almost exclusively at the proximal convoluted tubule. This is a selective process, because only molecules recognized by carrier proteins are actively reabsorbed. Glucose is an example of a molecule that ordinarily is completely reabsorbed because there is a plentiful supply of carrier proteins for it. However, every substance has a maximum rate of transport. After all its carriers are in use, any excess in the filtrate will appear in the urine. In diabetes mellitus, because the liver and muscles fail to store glucose as glycogen, the blood glucose level is above normal and glucose appears in the urine. The presence of excess glucose in the filtrate raises its osmolarity. Therefore, less water is reabsorbed into the peritubular capillary network. The frequent urination and increased thirst experienced by people with untreated diabetes are due to less water being reabsorbed from the filtrate into the blood.
We have seen that the filtrate that enters the proximal convoluted tubule is divided into two portions: components reabsorbed from the tubule into blood, and components not reabsorbed that continue to pass through the nephron to be further processed into urine.
Table Summary:
Reabsorbed Filtrate ComponentsNonreabsorbed Filtrate Components
Most waterSome water
NutrientsMuch nitrogenous waste
Required salts (ions)Excess salts (ions)
The substances not reabsorbed become the tubular fluid, which enters the loop of the nephron.
Tubular Secretion
Tubular secretion is the second way by which substances are removed from blood and added to the tubular fluid. Hydrogen ions (H+), creatinine, and drugs such as penicillin are some of the substances moved by active transport from blood into the kidney tubule. In the end, urine contains substances that have undergone glomerular filtration but have not been reabsorbed, as well as substances that have undergone tubular secretion. Tubular secretion occurs along the length of the kidney tubule. The Health feature “Urinalysis” explains how the contents of the urine may be used as a diagnostic tool for assessing an individual’s health.
This is how urine forms
The formation of urine involves three stages: glomerular filtration, tubular reabsorption, and tubular secretion.
Glomerular Filtration
blood enters the glomerulus by way of the afferent arteriole
glomerular blood pressure. Because of this, water and small molecules move from the glomerulus to the inside of the glomerular capsule. This is a filtration process, because large molecules Page 224and formed elements are unable to pass through the capillary wall. In effect, then, blood in the glomerulus has two portions: the filterable components and the nonfilterable components.
Table Summary: Table is divided into two columns to list the names of filterable blood components and nonfilterable blood components. Empty cells in column 2 signify that there are only two types of nonfilterable blood components.
Filterable Blood ComponentsNonfilterable Blood Components
WaterFormed elements (blood cells and platelets)
Nitrogenous wastesPlasma proteins
Nutrients
Salts (ions)
The nonfilterable components leave the glomerulus by way of the efferent arteriole. The glomerular filtrate inside the glomerular capsule now contains the filterable blood components in approximately the same concentration as plasma.
As indicated in Table 11.1, nephrons in the kidneys filter 180 liters of water per day, along with a considerable amount of small molecules (such as glucose) and ions (such as sodium). If the composition of urine were the same as that of the glomerular filtrate, the body would continually lose water, salts, and nutrients. Therefore, we can conclude that the composition of the filtrate must be altered as this fluid passes through the remainder of the tubule.
Table 11.1Reabsorption from Nephrons
Table Summary: Data is provided for four substances, the names of which are listed in column 1. The data for each substance appear in their respective columns.
SubstanceAmount Filtered (per Day)Amount Excreted (per Day)Reabsorption (%)
Water (l) 180 1.8 99.0
Sodium (g) 630 3.2 99.5
Glucose (g) 180 0.0 100.0
Urea (g) 5430.0 44.0
l = liters; g = grams
SCIENCE IN YOUR LIFE
Is urine sterile?
This is a common misconception from pop culture. In fact, the urine from a healthy individual can contain thousands of bacteria per milliliter of urine, and scientists have identified over 30 types of bacteria normally found in urine. Where do these bacteria come from? Some are naturally present in the urinary system, while others are collected from the skin during urination.
The good news is that having bacteria in your urine is not necessarily bad. Bacteria form part of the microbiota of our bodies, and their presence may actually help fight some infections. However, having a high number of bacteria, or changes in the types of bacteria, can indicate a possible urinary tract infection (UTI).
Tubular Reabsorption
Tubular reabsorption occurs as molecules, especially ions, are passively and actively reabsorbed from the nephron into the blood of the peritubular capillary network. The osmolarity of the blood is maintained by the presence of plasma proteins and salt. Sodium ions (Na+) are actively transported by one of two types of transport proteins. First, the movement of sodium may be coupled to the movement of larger solutes, such as amino acids or glucose. This is called a symport, because both solutes are being moved in the same direction. The second mechanism involves an antiport protein, which moves Na+ ions into the cell while transporting H+ ions out of the cell. This also regulates the pH balance of the blood, because the movement of H+ ions outward reduces the acidity of the blood. As sodium ions are being moved, chloride ions (Cl−) follow passively. The reabsorption of salt (NaCl) increases the osmolarity of the blood compared with the filtrate. Therefore, water moves passively from the tubule into the blood. About 65% of Na+ is reabsorbed at the proximal convoluted tubule.
Cotransport
Nutrients such as glucose and amino acids return to the peritubular capillaries almost exclusively at the proximal convoluted tubule. This is a selective process, because only molecules recognized by carrier proteins are actively reabsorbed. Glucose is an example of a molecule that ordinarily is completely reabsorbed because there is a plentiful supply of carrier proteins for it. However, every substance has a maximum rate of transport. After all its carriers are in use, any excess in the filtrate will appear in the urine. In diabetes mellitus, because the liver and muscles fail to store glucose as glycogen, the blood glucose level is above normal and glucose appears in the urine. The presence of excess glucose in the filtrate raises its osmolarity. Therefore, less water is reabsorbed into the peritubular capillary network. The frequent urination and increased thirst experienced by people with untreated diabetes are due to less water being reabsorbed from the filtrate into the blood.
We have seen that the filtrate that enters the proximal convoluted tubule is divided into two portions: components reabsorbed from the tubule into blood, and components not reabsorbed that continue to pass through the nephron to be further processed into urine.
Table Summary:
Reabsorbed Filtrate ComponentsNonreabsorbed Filtrate Components
Most waterSome water
NutrientsMuch nitrogenous waste
Required salts (ions)Excess salts (ions)
The substances not reabsorbed become the tubular fluid, which enters the loop of the nephron.
Tubular Secretion
Tubular secretion is the second way by which substances are removed from blood and added to the tubular fluid. Hydrogen ions (H+), creatinine, and drugs such as penicillin are some of the substances moved by active transport from blood into the kidney tubule. In the end, urine contains substances that have undergone glomerular filtration but have not been reabsorbed, as well as substances that have undergone tubular secretion. Tubular secretion occurs along the length of the kidney tubule. The Health feature “Urinalysis” explains how the contents of the urine may be used as a diagnostic tool for assessing an individual’s health.
This is how urine forms
11.3 Urine Formation
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Summarize the three processes involved in the formation of urine.
List the components of the glomerular filtrate.
Describe how tubular reabsorption processes nutrient and salt molecules.
Explain the substances removed from the blood by tubular secretion.
The formation of urine involves three stages: glomerular filtration, tubular reabsorption, and tubular secretion. Figure 11.6 provides an overview of these processes.
Figure 11.6 An overview of urine production. The three main processes in urine formation are described in boxes and color-coded to arrows that show the movement of molecules into or out of the nephron at specific locations. In the end, urine is composed of the substances within the collecting duct (see brown arrow).
Tutorial: Urine Formation
Glomerular Filtration
Glomerular filtration occurs when whole blood enters the glomerulus by way of the afferent arteriole. The afferent arteriole (Fig. 11.6, inset) has a larger diameter than the efferent arteriole, resulting in an increase in glomerular blood pressure. Because of this, water and small molecules move from the glomerulus to the inside of the glomerular capsule. This is a filtration process, because large molecules Page 224and formed elements are unable to pass through the capillary wall. In effect, then, blood in the glomerulus has two portions: the filterable components and the nonfilterable components.
Table Summary: Table is divided into two columns to list the names of filterable blood components and nonfilterable blood components. Empty cells in column 2 signify that there are only two types of nonfilterable blood components.
Filterable Blood ComponentsNonfilterable Blood Components
WaterFormed elements (blood cells and platelets)
Nitrogenous wastesPlasma proteins
Nutrients
Salts (ions)
The nonfilterable components leave the glomerulus by way of the efferent arteriole. The glomerular filtrate inside the glomerular capsule now contains the filterable blood components in approximately the same concentration as plasma.
As indicated in Table 11.1, nephrons in the kidneys filter 180 liters of water per day, along with a considerable amount of small molecules (such as glucose) and ions (such as sodium). If the composition of urine were the same as that of the glomerular filtrate, the body would continually lose water, salts, and nutrients. Therefore, we can conclude that the composition of the filtrate must be altered as this fluid passes through the remainder of the tubule.
Table 11.1Reabsorption from Nephrons
Table Summary: Data is provided for four substances, the names of which are listed in column 1. The data for each substance appear in their respective columns.
SubstanceAmount Filtered (per Day)Amount Excreted (per Day)Reabsorption (%)
Water (l) 180 1.8 99.0
Sodium (g) 630 3.2 99.5
Glucose (g) 180 0.0 100.0
Urea (g) 5430.0 44.0
l = liters; g = grams
SCIENCE IN YOUR LIFE
Is urine sterile?
This is a common misconception from pop culture. In fact, the urine from a healthy individual can contain thousands of bacteria per milliliter of urine, and scientists have identified over 30 types of bacteria normally found in urine. Where do these bacteria come from? Some are naturally present in the urinary system, while others are collected from the skin during urination.
The good news is that having bacteria in your urine is not necessarily bad. Bacteria form part of the microbiota of our bodies, and their presence may actually help fight some infections. However, having a high number of bacteria, or changes in the types of bacteria, can indicate a possible urinary tract infection (UTI).
Tubular Reabsorption
Tubular reabsorption occurs as molecules, especially ions, are passively and actively reabsorbed from the nephron into the blood of the peritubular capillary network. The osmolarity of the blood is maintained by the presence of plasma proteins and salt. Sodium ions (Na+) are actively transported by one of two types of transport proteins. First, the movement of sodium may be coupled to the movement of larger solutes, such as amino acids or glucose. This is called a symport, because both solutes are being moved in the same direction. The second mechanism involves an antiport protein, which moves Na+ ions into the cell while transporting H+ ions out of the cell. This also regulates the pH balance of the blood, because the movement of H+ ions outward reduces the acidity of the blood. As sodium ions are being moved, chloride ions (Cl−) follow passively. The reabsorption of salt (NaCl) increases the osmolarity of the blood compared with the filtrate. Therefore, water moves passively from the tubule into the blood. About 65% of Na+ is reabsorbed at the proximal convoluted tubule.
Cotransport
Nutrients such as glucose and amino acids return to the peritubular capillaries almost exclusively at the proximal convoluted tubule. This is a selective process, because only molecules recognized by carrier proteins are actively reabsorbed. Glucose is an example of a molecule that ordinarily is completely reabsorbed because there is a plentiful supply of carrier proteins for it. However, every substance has a maximum rate of transport. After all its carriers are in use, any excess in the filtrate will appear in the urine. In diabetes mellitus, because the liver and muscles fail to store glucose as glycogen, the blood glucose level is above normal and glucose appears in the urine. The presence of excess glucose in the filtrate raises its osmolarity. Therefore, less water is reabsorbed into the peritubular capillary network. The frequent urination and increased thirst experienced by people with untreated diabetes are due to less water being reabsorbed from the filtrate into the blood.
We have seen that the filtrate that enters the proximal convoluted tubule is divided into two portions: components reabsorbed from the tubule into blood, and components not reabsorbed that continue to pass through the nephron to be further processed into urine.
Table Summary:
Reabsorbed Filtrate ComponentsNonreabsorbed Filtrate Components
Most waterSome water
NutrientsMuch nitrogenous waste
Required salts (ions)Excess salts (ions)
The substances not reabsorbed become the tubular fluid, which enters the loop of the nephron.
Tubular Secretion
Tubular secretion is the second way by which substances are removed from blood and added to the tubular fluid. Hydrogen ions (H+), creatinine, and drugs such as penicillin are some of the substances moved by active transport from blood into the kidney tubule. In the end, urine contains substances that have undergone glomerular filtration but have not been reabsorbed, as well as substances that have undergone tubular secretion. Tubular secretion occurs along the length of the kidney tubule. The Health feature “Urinalysis” explains how the contents of the urine may be used as a diagnostic tool for assessing an individual’s health.
This is how urine forms
11.3 Urine Formation
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Summarize the three processes involved in the formation of urine.
List the components of the glomerular filtrate.
Describe how tubular reabsorption processes nutrient and salt molecules.
Explain the substances removed from the blood by tubular secretion.
The formation of urine involves three stages: glomerular filtration, tubular reabsorption, and tubular secretion. Figure 11.6 provides an overview of these processes.
Figure 11.6 An overview of urine production. The three main processes in urine formation are described in boxes and color-coded to arrows that show the movement of molecules into or out of the nephron at specific locations. In the end, urine is composed of the substances within the collecting duct (see brown arrow).
Tutorial: Urine Formation
Glomerular Filtration
Glomerular filtration occurs when whole blood enters the glomerulus by way of the afferent arteriole. The afferent arteriole (Fig. 11.6, inset) has a larger diameter than the efferent arteriole, resulting in an increase in glomerular blood pressure. Because of this, water and small molecules move from the glomerulus to the inside of the glomerular capsule. This is a filtration process, because large molecules Page 224and formed elements are unable to pass through the capillary wall. In effect, then, blood in the glomerulus has two portions: the filterable components and the nonfilterable components.
Table Summary: Table is divided into two columns to list the names of filterable blood components and nonfilterable blood components. Empty cells in column 2 signify that there are only two types of nonfilterable blood components.
Filterable Blood ComponentsNonfilterable Blood Components
WaterFormed elements (blood cells and platelets)
Nitrogenous wastesPlasma proteins
Nutrients
Salts (ions)
The nonfilterable components leave the glomerulus by way of the efferent arteriole. The glomerular filtrate inside the glomerular capsule now contains the filterable blood components in approximately the same concentration as plasma.
As indicated in Table 11.1, nephrons in the kidneys filter 180 liters of water per day, along with a considerable amount of small molecules (such as glucose) and ions (such as sodium). If the composition of urine were the same as that of the glomerular filtrate, the body would continually lose water, salts, and nutrients. Therefore, we can conclude that the composition of the filtrate must be altered as this fluid passes through the remainder of the tubule.
Table 11.1Reabsorption from Nephrons
Table Summary: Data is provided for four substances, the names of which are listed in column 1. The data for each substance appear in their respective columns.
SubstanceAmount Filtered (per Day)Amount Excreted (per Day)Reabsorption (%)
Water (l) 180 1.8 99.0
Sodium (g) 630 3.2 99.5
Glucose (g) 180 0.0 100.0
Urea (g) 5430.0 44.0
l = liters; g = grams
SCIENCE IN YOUR LIFE
Is urine sterile?
This is a common misconception from pop culture. In fact, the urine from a healthy individual can contain thousands of bacteria per milliliter of urine, and scientists have identified over 30 types of bacteria normally found in urine. Where do these bacteria come from? Some are naturally present in the urinary system, while others are collected from the skin during urination.
The good news is that having bacteria in your urine is not necessarily bad. Bacteria form part of the microbiota of our bodies, and their presence may actually help fight some infections. However, having a high number of bacteria, or changes in the types of bacteria, can indicate a possible urinary tract infection (UTI).
Tubular Reabsorption
Tubular reabsorption occurs as molecules, especially ions, are passively and actively reabsorbed from the nephron into the blood of the peritubular capillary network. The osmolarity of the blood is maintained by the presence of plasma proteins and salt. Sodium ions (Na+) are actively transported by one of two types of transport proteins. First, the movement of sodium may be coupled to the movement of larger solutes, such as amino acids or glucose. This is called a symport, because both solutes are being moved in the same direction. The second mechanism involves an antiport protein, which moves Na+ ions into the cell while transporting H+ ions out of the cell. This also regulates the pH balance of the blood, because the movement of H+ ions outward reduces the acidity of the blood. As sodium ions are being moved, chloride ions (Cl−) follow passively. The reabsorption of salt (NaCl) increases the osmolarity of the blood compared with the filtrate. Therefore, water moves passively from the tubule into the blood. About 65% of Na+ is reabsorbed at the proximal convoluted tubule.
Cotransport
Nutrients such as glucose and amino acids return to the peritubular capillaries almost exclusively at the proximal convoluted tubule. This is a selective process, because only molecules recognized by carrier proteins are actively reabsorbed. Glucose is an example of a molecule that ordinarily is completely reabsorbed because there is a plentiful supply of carrier proteins for it. However, every substance has a maximum rate of transport. After all its carriers are in use, any excess in the filtrate will appear in the urine. In diabetes mellitus, because the liver and muscles fail to store glucose as glycogen, the blood glucose level is above normal and glucose appears in the urine. The presence of excess glucose in the filtrate raises its osmolarity. Therefore, less water is reabsorbed into the peritubular capillary network. The frequent urination and increased thirst experienced by people with untreated diabetes are due to less water being reabsorbed from the filtrate into the blood.
We have seen that the filtrate that enters the proximal convoluted tubule is divided into two portions: components reabsorbed from the tubule into blood, and components not reabsorbed that continue to pass through the nephron to be further processed into urine.
Table Summary:
Reabsorbed Filtrate ComponentsNonreabsorbed Filtrate Components
Most waterSome water
NutrientsMuch nitrogenous waste
Required salts (ions)Excess salts (ions)
The substances not reabsorbed become the tubular fluid, which enters the loop of the nephron.
Tubular Secretion
Tubular secretion is the second way by which substances are removed from blood and added to the tubular fluid. Hydrogen ions (H+), creatinine, and drugs such as penicillin are some of the substances moved by active transport from blood into the kidney tubule. In the end, urine contains substances that have undergone glomerular filtration but have not been reabsorbed, as well as substances that have undergone tubular secretion. Tubular secretion occurs along the length of the kidney tubule. The Health feature “Urinalysis” explains how the contents of the urine may be used as a diagnostic tool for assessing an individual’s health.
This is how urine forms
The formation of urine involves three stages: glomerular filtration, tubular reabsorption, and tubular secretion. Figure 11.6 provides an overview of these processes.
Figure 11.6 An overview of urine production. The three main processes in urine formation are described in boxes and color-coded to arrows that show the movement of molecules into or out of the nephron at specific locations. In the end, urine is composed of the substances within the collecting duct (see brown arrow).
Tutorial: Urine Formation
Glomerular Filtration
Glomerular filtration occurs when whole blood enters the glomerulus by way of the afferent arteriole. The afferent arteriole (Fig. 11.6, inset) has a larger diameter than the efferent arteriole, resulting in an increase in glomerular blood pressure. Because of this, water and small molecules move from the glomerulus to the inside of the glomerular capsule. This is a filtration process, because large molecules Page 224and formed elements are unable to pass through the capillary wall. In effect, then, blood in the glomerulus has two portions: the filterable components and the nonfilterable components.
Table Summary: Table is divided into two columns to list the names of filterable blood components and nonfilterable blood components. Empty cells in column 2 signify that there are only two types of nonfilterable blood components.
Filterable Blood ComponentsNonfilterable Blood Components
WaterFormed elements (blood cells and platelets)
Nitrogenous wastesPlasma proteins
Nutrients
Salts (ions)
The nonfilterable components leave the glomerulus by way of the efferent arteriole. The glomerular filtrate inside the glomerular capsule now contains the filterable blood components in approximately the same concentration as plasma.
As indicated in Table 11.1, nephrons in the kidneys filter 180 liters of water per day, along with a considerable amount of small molecules (such as glucose) and ions (such as sodium). If the composition of urine were the same as that of the glomerular filtrate, the body would continually lose water, salts, and nutrients. Therefore, we can conclude that the composition of the filtrate must be altered as this fluid passes through the remainder of the tubule.
Table 11.1Reabsorption from Nephrons
Table Summary: Data is provided for four substances, the names of which are listed in column 1. The data for each substance appear in their respective columns.
SubstanceAmount Filtered (per Day)Amount Excreted (per Day)Reabsorption (%)
Water (l) 180 1.8 99.0
Sodium (g) 630 3.2 99.5
Glucose (g) 180 0.0 100.0
Urea (g) 5430.0 44.0
l = liters; g = grams
SCIENCE IN YOUR LIFE
Is urine sterile?
This is a common misconception from pop culture. In fact, the urine from a healthy individual can contain thousands of bacteria per milliliter of urine, and scientists have identified over 30 types of bacteria normally found in urine. Where do these bacteria come from? Some are naturally present in the urinary system, while others are collected from the skin during urination.
The good news is that having bacteria in your urine is not necessarily bad. Bacteria form part of the microbiota of our bodies, and their presence may actually help fight some infections. However, having a high number of bacteria, or changes in the types of bacteria, can indicate a possible urinary tract infection (UTI).
Tubular Reabsorption
Tubular reabsorption occurs as molecules, especially ions, are passively and actively reabsorbed from the nephron into the blood of the peritubular capillary network. The osmolarity of the blood is maintained by the presence of plasma proteins and salt. Sodium ions (Na+) are actively transported by one of two types of transport proteins. First, the movement of sodium may be coupled to the movement of larger solutes, such as amino acids or glucose. This is called a symport, because both solutes are being moved in the same direction. The second mechanism involves an antiport protein, which moves Na+ ions into the cell while transporting H+ ions out of the cell. This also regulates the pH balance of the blood, because the movement of H+ ions outward reduces the acidity of the blood. As sodium ions are being moved, chloride ions (Cl−) follow passively. The reabsorption of salt (NaCl) increases the osmolarity of the blood compared with the filtrate. Therefore, water moves passively from the tubule into the blood. About 65% of Na+ is reabsorbed at the proximal convoluted tubule.
Cotransport
Nutrients such as glucose and amino acids return to the peritubular capillaries almost exclusively at the proximal convoluted tubule. This is a selective process, because only molecules recognized by carrier proteins are actively reabsorbed. Glucose is an example of a molecule that ordinarily is completely reabsorbed because there is a plentiful supply of carrier proteins for it. However, every substance has a maximum rate of transport. After all its carriers are in use, any excess in the filtrate will appear in the urine. In diabetes mellitus, because the liver and muscles fail to store glucose as glycogen, the blood glucose level is above normal and glucose appears in the urine. The presence of excess glucose in the filtrate raises its osmolarity. Therefore, less water is reabsorbed into the peritubular capillary network. The frequent urination and increased thirst experienced by people with untreated diabetes are due to less water being reabsorbed from the filtrate into the blood.
We have seen that the filtrate that enters the proximal convoluted tubule is divided into two portions: components reabsorbed from the tubule into blood, and components not reabsorbed that continue to pass through the nephron to be further processed into urine.
Table Summary:
Reabsorbed Filtrate ComponentsNonreabsorbed Filtrate Components
Most waterSome water
NutrientsMuch nitrogenous waste
Required salts (ions)Excess salts (ions)
The substances not reabsorbed become the tubular fluid, which enters the loop of the nephron.
Tubular Secretion
Tubular secretion is the second way by which substances are removed from blood and added to the tubular fluid. Hydrogen ions (H+), creatinine, and drugs such as penicillin are some of the substances moved by active transport from blood into the kidney tubule. In the end, urine contains substances that have undergone glomerular filtration but have not been reabsorbed, as well as substances that have undergone tubular secretion. Tubular secretion occurs along the length of the kidney tubule. The Health feature “Urinalysis” explains how the contents of the urine may be used as a diagnostic tool for assessing an individual’s health.
This is how urine forms
What is meant by excretion?
Collectively, these organs (Fig 11.1) carry out the process of excretion, or the removal of metabolic wastes from the body.
Excretion in humans is performed by the formation and discharge of urine from the body.
11.2 Kidney Structure
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Identify the structures of a human kidney.
Identify the structures of a nephron and state the function of each.
A lengthwise section of a kidney shows that many branches of the renal artery and renal vein reach inside a kidney (Fig. 11.3a, b). If the blood vessels are removed, it is easier to identify the three regions of a kidney:
The renal cortex is an outer, granulated layer that dips down in between a radially striated inner layer called the renal medulla.
The renal medulla consists of cone-shaped tissue masses called renal pyramids.
The renal pelvis is a central space, or cavity, continuous with the ureter (Fig. 11.3c, d).
Figure 11.3 The anatomy of a human kidney. a. A longitudinal section of the kidney showing the blood supply. The renal artery divides into smaller arteries, and these divide into arterioles. Venules join to form small veins, which join to form the renal vein. b. A procedure called an angiogram highlights blood vessels by injecting a contrast medium that is opaque to X-rays. c. The same section without the blood supply. d. A diagram of the renal cortex; the renal medulla; and the renal pelvis, which connects with the ureter. The renal medulla consists of the renal pyramids. e. An enlargement showing the placement of nephrons.
(photos) (b): ©James Cavallini/Science Source; (c): ©Kage Mikrofotografie/Medical Images
Microscopically, the kidney is composed of over 1 million nephrons, sometimes called renal, or kidney, tubules (Fig. 11.3e). Page 221The nephrons filter the blood and produce urine. Each nephron is positioned so that the urine flows into a collecting duct. Several nephrons enter the same collecting duct. The collecting ducts eventually enter the renal pelvis.
Anatomy of a Nephron
Each nephron has its own blood supply, including two capillary regions (Fig. 11.4). From the renal artery, an afferent arteriole transports blood to the glomerulus, a knot of capillaries inside the glomerular capsule. Blood leaving the glomerulus is carried away by the efferent arteriole. Blood pressure is higher in the glomerulus, because the efferent arteriole is narrower than the afferent arteriole. The efferent arteriole divides and forms the peritubular capillary network, which surrounds the rest of the nephron. Blood from the efferent arteriole travels through the peritubular capillary network. The blood then goes into a venule that carries blood into the renal vein.
Figure 11.4 The structure of a nephron. A nephron is made up of a glomerular capsule, the proximal convoluted tubule, the loop of the nephron, and the distal convoluted tubule. The collecting duct collects fluid from several nephrons. The photomicrographs show the microscopic anatomy of these structures. The arrows indicate the path of blood around the nephron.
(glomerulus): Steve Gschmeissner/Science Source; (proximal convoluted tubule): Science Photo Library/Getty Images
Parts of a Nephron
Each nephron is made up of several parts (Fig. 11.4). Some functions are shared by all parts of the nephron. However, the specific structure of each part is especially suited to a particular function.
First, the closed end of the nephron is pushed in on itself to form a cuplike structure called the glomerular capsule (formally Page 222called the Bowman’s capsule). The outer layer of the glomerular capsule is composed of squamous epithelial cells. The inner layer is made up of podocytes that have long, cytoplasmic extensions. The podocytes cling to the capillary walls of the glomerulus and leave pores that allow easy passage of small molecules from the glomerulus to the inside of the glomerular capsule. This process, called glomerular filtration, produces a filtrate of the blood.
Next, there is a proximal convoluted tubule. The cuboidal epithelial cells lining this part of the nephron have numerous microvilli, about 1 micrometer (µm) in length, that are tightly packed and form a brush border (Fig. 11.5). A brush border greatly increases the surface area for the tubular reabsorption of filtrate components. Each cell also has many mitochondria, which can supply energy for active transport of molecules from the lumen to the peritubular capillary network.
Figure 11.5 The specialized cells of the proximal convoluted tubule. a. This photomicrograph shows that the cells lining the proximal convoluted tubule have a brushlike border composed of microvilli, which greatly increase the surface area exposed to the lumen. The peritubular capillary network surrounds the cells. b. Diagrammatic representation of (a) shows that each cell has many mitochondria, which supply the energy needed for active transport, the process that moves molecules (green) from the lumen of the tubule to the capillary, as indicated by the arrows.
(a): ©Joseph F. Gennaro Jr./Science Source
Simple squamous epithelium appears as the tube narrows and makes a U-turn called the loop of the nephron (loop of Henle). Each loop consists of a descending limb and an ascending limb. The descending limb of the loop allows water to diffuse into tissue surrounding the nephron. The ascending limb actively transports salt from its lumen to interstitial tissue. As we will see, this activity facilitates the reabsorption of water by the nephron and collecting duct.
The cuboidal epithelial cells of the distal convoluted tubule have numerous mitochondria, but they lack microvilli. This means that the distal convoluted tubule is not specialized for reabsorption. Instead, its primary function is ion exchange. During ion exchange, cells reabsorb certain ions, returning them to the blood. Other ions are secreted from the blood into the tubule. The distal convoluted tubules of several nephrons enter one collecting duct. Many collecting ducts carry urine to the renal pelvis.
As shown in Figure 11.4, the glomerular capsule and the convoluted tubules always lie within the renal cortex. The loop of the nephron dips down into the renal medulla. A few nephrons have a very long loop of the nephron, which penetrates deep into the renal medulla. Collecting ducts are also located in the renal medulla, and together they give the renal pyramids their appearance.
What is the structure of a kidney?
This part of the kidney consists of cone-shaped tissue masses called renal pyramids.
The renal medulla
sodium ion reabsorption and potassium ion excretion by the distal convoluted tubules
Aldosterone
Parts of a Nephron
Each nephron is made up of several parts (Fig. 11.4). Some functions are shared by all parts of the nephron. However, the specific structure of each part is especially suited to a particular function.
First, the closed end of the nephron is pushed in on itself to form a cuplike structure called the glomerular capsule (formally Page 222called the Bowman’s capsule). The outer layer of the glomerular capsule is composed of squamous epithelial cells. The inner layer is made up of podocytes that have long, cytoplasmic extensions. The podocytes cling to the capillary walls of the glomerulus and leave pores that allow easy passage of small molecules from the glomerulus to the inside of the glomerular capsule. This process, called glomerular filtration, produces a filtrate of the blood.
Next, there is a proximal convoluted tubule. The cuboidal epithelial cells lining this part of the nephron have numerous microvilli, about 1 micrometer (µm) in length, that are tightly packed and form a brush border (Fig. 11.5). A brush border greatly increases the surface area for the tubular reabsorption of filtrate components. Each cell also has many mitochondria, which can supply energy for active transport of molecules from the lumen to the peritubular capillary network.
Parts of a nephron
What is the urethra?
The urethra is a small tube that extends from the urinary bladder to an external opening. Its function is to remove urine from the body. The urethra has a different length in females than in males. In females, the urethra is about 4 cm long. The short length of the female urethra makes bacterial invasion of the urinary tract easier. In males, the urethra averages 20 cm when the penis is flaccid (limp, nonerect). As the urethra leaves the male urinary bladder, it is encircled by the prostate gland. The prostate sometimes enlarges, restricting the flow of urine in the urethra. The Health feature “Urinary Difficulties Due to an Enlarged Prostate” in Section 11.5 discusses this problem in men.
In females, the reproductive and urinary systems are not connected. However, in males, the urethra carries urine during urination and sperm during ejaculation
aldosterone- promotes ion exchange in the distal convoluted tubule
promotes ion exchange in the distal convoluted tubule
ADH -required to open aquaporin channels in the distal convoluted tubule
required to open aquaporin channels in the distal convoluted tubule
ANH, promotes the secretion of sodium, the process of natriuresis
promotes the secretion of sodium, the process of natriuresis
Hormones
Figure 11.3 The anatomy of a human kidney. a. A longitudinal section of the kidney showing the blood supply. The renal artery divides into smaller arteries, and these divide into arterioles. Venules join to form small veins, which join to form the renal vein. b. A procedure called an angiogram highlights blood vessels by injecting a contrast medium that is opaque to X-rays. c. The same section without the blood supply. d. A diagram of the renal cortex; the renal medulla; and the renal pelvis, which connects with the ureter. The renal medulla consists of the renal pyramids. e. An enlargement showing the placement of nephrons.
(photos) (b): ©James Cavallini/Science Source; (c): ©Kage Mikrofotografie/Medical Images
Notes on the anatomy of a kidney
The metabolic waste of humans consists primarily of nitrogenous waste, such as urea, creatinine, ammonium, and uric acid. Urea, a waste product of amino acid metabolism, is the primary nitrogenous end product of metabolism in humans.
These are the 4 types of human primary nitrogenous waste.
Figure 11.5 The specialized cells of the proximal convoluted tubule. a. This photomicrograph shows that the cells lining the proximal convoluted tubule have a brushlike border composed of microvilli, which greatly increase the surface area exposed to the lumen. The peritubular capillary network surrounds the cells. b. Diagrammatic representation of (a) shows that each cell has many mitochondria, which supply the energy needed for active transport, the process that moves molecules (green) from the lumen of the tubule to the capillary, as indicated by the arrows.
(a): ©Joseph F. Gennaro Jr./Science Source
Simple squamous epithelium appears as the tube narrows and makes a U-turn called the loop of the nephron (loop of Henle). Each loop consists of a descending limb and an ascending limb. The descending limb of the loop allows water to diffuse into tissue surrounding the nephron. The ascending limb actively transports salt from its lumen to interstitial tissue. As we will see, this activity facilitates the reabsorption of water by the nephron and collecting duct.
The cuboidal epithelial cells of the distal convoluted tubule have numerous mitochondria, but they lack microvilli. This means that the distal convoluted tubule is not specialized for reabsorption. Instead, its primary function is ion exchange. During ion exchange, cells reabsorb certain ions, returning them to the blood. Other ions are secreted from the blood into the tubule. The distal convoluted tubules of several nephrons enter one collecting duct. Many collecting ducts carry urine to the renal pelvis.
As shown in Figure 11.4, the glomerular capsule and the convoluted tubules always lie within the renal cortex. The loop of the nephron dips down into the renal medulla. A few nephrons have a very long loop of the nephron, which penetrates deep into the renal medulla. Collecting ducts are also located in the renal medulla, and together they give the renal pyramids their appearance.
The substance and structure of the nephron
What are the organs of the urinary system?
The urinary system consists of the kidneys, ureters, urinary bladder, and urethra
What happens when you need to urinate?
Figure 11.2
Sensory impulses trigger a desire to urinate. As the bladder fills with urine, sensory impulses go to the spinal cord and then to the brain. The brain can override the urge to urinate. When urination occurs, motor nerve impulses cause the bladder to contract and the sphincters to relax.
1) renal artery 2) afferent arteriole 3) glomerulus 4) efferent arteriole 5) peritubular capillary network, which surrounds the rest of the nephron 6) venule 7) renal vein.
I am the order of the urinary system.
What are the urinary system organs/glands and locate them
Review this slide and take notes
What are the kidneys like, where are they located?
The kidneys are a pair of organs located one on each side of the vertebral column at the same level as the small of the lower back. They lie behind the peritoneum, the membrane that lines the abdominal cavity, where they receive some protection from the lower rib cage. Due to the shape of the liver, the right kidney is positioned slightly lower than the left. The kidneys are bean-shaped and reddish-brown in color. The fist-sized organs are covered by a tough capsule of fibrous connective tissue, called a renal capsule. Masses of adipose tissue adhere to each kidney. The concave side of a kidney has a depression where a renal artery enters and a renal vein and a ureter exit the kidney. The renal artery transports blood to be filtered to the kidneys, and the renal vein carries filtered blood away from the kidneys.
What are additional functions of the kidneys?
The kidneys also reabsorb filtered nutrients and participate in the synthesis of vitamin D. Vitamin D is a hormone that promotes calcium ion (Ca2+) absorption from the digestive tract.
Excretion of metabolic wastes 2. Regulate aspects of homeostasis a. Maintenance of water-sodium balance b. Electrolyte balance c. Maintenance of acid-base balance in blood and therefore pH of body d. Blood pressure 3. Hormone secretion: renin and erythropoietin (EPO) 4. Reabsorb filtered nutrients and convert vitamin D
Functions of the Urinary System
secreted by the atria of the heart when cardiac cells are stretched due to increased blood volume. ANH inhibits the secretion of renin by the juxtaglomerular apparatus and the secretion of aldosterone by the adrenal glands. Its effect, therefore, is to promote the excretion of sodium ions (Na+), called natriuresis. Normally, salt reabsorption creates an osmotic gradient that causes water to be reabsorbed. Thus, by causing salt excretion, ANH causes water excretion, too. If ANH is present, less water will be reabsorbed, even if ADH is also present.
Atrial natriuretic hormone (ANH)
11.5 Urinary System Disorders
LEARNING OUTCOMES
Upon completion of this section, you should be able to
List the major diseases of the urinary system and summarize their causes.
Describe how hemodialysis can help restore homeostasis of the blood in the event of kidney failure.
Many types of illnesses—especially diabetes, hypertension, and inherited conditions—cause progressive renal disease and renal failure. Infections are also contributory. If the infection is localized in the urethra, it is called urethritis. If the infection invades the urinary bladder, it is called cystitis. Finally, if the kidneys are affected, the infection is called pyelonephritis.
SCIENCE IN YOUR LIFE
Does cranberry juice really prevent or cure a urinary tract infection?
Research has supported the use of cranberry juice to prevent urinary tract infections. It appears to prevent bacteria that would cause infection from adhering to the surfaces of the urinary tract. However, cranberry juice has not been shown to be an effective treatment for an already existing urinary tract infection.
Urinary tract infections, an enlarged prostate gland (see the Health feature “Urinary Difficulties Due to an Enlarged Prostate”), pH imbalances, or an intake of too much calcium can lead to kidney stones. Kidney stones are hard granules made of calcium, phosphate, uric acid, and protein. Kidney stones form in the renal pelvis and usually pass unnoticed in the urine flow. If they grow to several centimeters and block the renal pelvis or ureter, a reverse pressure builds up and destroys nephrons. When a large kidney stone passes, strong contractions in a ureter can be excruciatingly painful.Page 231
BIOLOGY TODAY Health
Urinary Difficulties Due to an Enlarged Prostate
The prostate gland, part of the male reproductive system, surrounds the urethra at the point where the urethra leaves the urinary bladder (Fig. 11C). The prostate gland produces and adds a fluid to semen as semen passes through the urethra within the penis. At about age 50, the prostate gland often begins to enlarge, growing from the size of a walnut to that of a lime or even a lemon. This condition is called benign prostatic hyperplasia (BPH). As it enlarges, the prostate squeezes the urethra, causing urine to back up—first into the bladder, then into the ureters, and finally, perhaps, into the kidneys. While BPH is technically a disorder of the reproductive system, its symptoms are almost exclusively associated with urination and excretion, and therefore it is often discussed as a urinary system disorder.
Figure 11C Location of the prostate gland. Note the position of the prostate gland, which can enlarge to obstruct urine flow.
Treatment Emphasis Is on Early Detection
The treatment for BPH can involve (1) invasive procedures to reduce the size of the prostate or (2) medications that shrink the prostate and/or improve urine flow. For the former, prostate tissue can be destroyed by applying microwaves to a specific portion of the gland. In some cases, a physician may decide to surgically remove that prostate tissue. This may be accomplished by abdominal surgery, which requires an incision of the abdomen, or access to the prostate via the urethra. This operation, called transurethral resection of the prostate (TURP), requires careful consideration, because one study found that the death rate during the 5 years following TURP is much higher than that following abdominal surgery.
Some drug treatments recognize that prostate enlargement is due to a prostate enzyme (5-alpha-reductase) that acts on the male sex hormone testosterone, converting it into a substance that promotes prostate growth. Growth is fine during puberty, but continued growth in an adult is undesirable. These drugs contain substances that interfere with the action of the enzyme that promotes growth. One of the ingredients is an extract from a plant called the saw palmetto. It is particularly effective during the early stages of prostate enlargement. While it is sold in tablet form as an over-the-counter nutrient supplement, it should not be taken unless the need for it is confirmed by a physician. The prescription drugs finasteride (Propecia) and dutasteride (Avodart) are more powerful inhibitors of the same growth enzyme, but patients complain of erectile dysfunction and loss of libido while on the drugs.
Another common treatment for BPH involves the use of alpha-blockers, such as tamsulosin (Flomax). Alpha-blockers target specific receptors (called α-adrenergic receptors) on the surface of smooth muscle tissue. Tamsulosin inhibits the interaction of the nervous system with the smooth muscle, causing it to relax and promote urine flow. Similarly, drugs such as tadalafil (Cialis) inhibit an enzyme called phosphodiesterase type 5 (PDE5) in smooth muscle tissue, causing it to relax. Like tamsulosin, the use of tadalafil causes the relaxation of the prostate, enhancing the flow of urine.
Many men are concerned that BPH may be associated with prostate cancer, but the two conditions are not necessarily related. BPH occurs in the inner zone of the prostate, whereas cancer tends to develop in the outer area. If prostate cancer is suspected, blood tests and a biopsy, in which a tiny sample of prostate tissue is surgically removed, will confirm the diagnosis.
Enlarged Prostate and Cancer
Although prostate cancer is the second most common cancer in men, it is not a major killer. Typically, prostate cancer is so slow growing that the survival rate is about 98% if the condition is detected early.
Questions to Consider
What is the role of the prostate in the male reproductive system?
Given how alpha-blockers function, what other applications might they have in humans?
One of the first signs of nephron damage is albumin, white blood cells, or even red blood cells in the urine. As described in the Health feature “Urinalysis” (Section 11.3), a urinalysis can detect urine abnormalities rapidly. If damage is so extensive that more than two-thirds of the nephrons are inoperative, urea and other waste substances accumulate in the blood. This condition is Page 232called uremia. Although nitrogenous wastes can cause serious damage, the retention of water and salts is of even greater concern. The latter causes edema, fluid accumulation in the body tissues. Imbalance in the ionic composition of body fluids can lead to loss of consciousness and heart failure.
Hemodialysis
Patients with renal failure can undergo hemodialysis, using either an artificial kidney machine or continuous ambulatory peritoneal dialysis (CAPD). Dialysis is defined as the diffusion of dissolved molecules through a semipermeable natural or synthetic membrane that has pore sizes that allow only small molecules to pass through. In an artificial kidney machine (Fig. 11.11), the patient’s blood is passed through a membranous tube that is in contact with a dialysis solution, or dialysate. Substances more concentrated in the blood diffuse into the dialysate, and substances more concentrated in the dialysate diffuse into the blood. The dialysate is continuously replaced to maintain favorable concentration gradients. In this way, the artificial kidney can be used either to extract substances from blood, including waste products or toxic chemicals and drugs, or to add substances to blood—for example, bicarbonate ions (HCO3−) if the blood is acidic. In the course of a 3- to 6-hour hemodialysis, 50–250 g of urea can be removed from a patient, which greatly exceeds the amount excreted by normal kidneys. Therefore, a patient needs to undergo treatment only about twice a week.
Figure 11.11 Hemodialysis using an artificial kidney machine. As the patient’s blood is pumped through dialysis tubing, it is exposed to a dialysate (dialysis solution). Wastes exit from blood into the solution because of a preestablished concentration gradient. In this way, not only is blood cleansed but its water-salt and acid-base balances can also be adjusted.
©Gopixa/Shutterstock
CAPD is so named because the peritoneum is the dialysis membrane. A fresh amount of dialysate is introduced directly into the abdominal cavity from a bag that is temporarily attached to a permanently implanted plastic tube. The dialysate flows into the peritoneal cavity by gravity. Waste and salt molecules pass from the blood vessels in the abdominal wall into the dialysate before the fluid is collected 4 to 8 hours later. The solution is drained into a bag from the abdominal cavity by gravity, and then it is discarded. One advantage of CAPD over an artificial kidney machine is that the individual can go about his or her normal activities during CAPD.
Replacing a Kidney
Patients with renal failure sometimes undergo a kidney transplant operation, during which a functioning kidney from a donor is received. As with all organ transplants, there is the possibility of organ rejection. Receiving a kidney from a close relative has the highest chance of success. The current 1-year survival rate is 97% if the kidney is received from a relative and 90% if it is received from a nonrelative. In the future, transplantable kidneys may be created in a laboratory. Another option could be to use kidneys from specially bred pigs whose organs would not be antigenic to humans.
CHECK YOUR PROGRESS 11.5
List and detail a few common causes of renal disease.
Answer
Bacterial and viral infections of the kidney and urinary tract, elevated blood glucose levels from diabetes, hypertension leading to damaged renal capillaries, and the formation of kidney stones due to elevated calcium intake are causes of renal disease.
Provide examples of diseases associated with the urinary tract and kidneys.
Answer
Examples include infections of the urethra, bladder, and the kidneys; uremia; enlarged prostate; prostate cancer; and kidney stones.
Explain why hemodialysis would need to be done frequently in a patient with renal failure.
Answer
Hemodialysis uses an artificial kidney machine to filter the blood, removing wastes and reabsorbing needed nutrients and water, just like the functions of a kidney. Because wastes build up continuously, and water-salt and acid-base balance are essential for homeostasis, hemodialysis needs to be performed frequently.
CONNECTING THE CONCEPTS
For more information on organ transplants, refer to the following discussions:
Section 5.7 describes the options available for individuals experiencing heart failure.
Section 7.5 examines how the immune system potentially interferes with organ transplants.
Section 20.4 provides an overview of how bone marrow transplants can be used to treat cancer.
CONCLUSION
In this chapter, we explored the role of the kidneys in filtering the blood. In Karla’s case, her kidney stones were caused because of an overabundance of uric acid, which was probably caused by a combination of a protein-rich diet and dehydration. However, sometimes genetic factors also contribute to the formation of these types of kidney stones. Normally, kidney stones do not present a health problem. But larger stones, such as Karla’s, can sometimes block the urinary tract and cause problems.
The treatment for Karla’s condition involved first breaking the stones into smaller pieces so they could pass through the urinary tract. This was done using a procedure called extracorporeal shock wave lithotripsy (ESWL), which uses sound waves to break up the kidney stones. In addition, the doctor prescribed a drug called allopurinol to reduce the uric acid levels in her blood, along with moderation of protein-rich foods and more water consumption.
These are urinary system disorders
What is the renal cortex ?
an outer, granulated layer that dips down in between a radially striated inner layer called the renal medulla.
• What are the parts and functions of the urinary system? • What is the macroscopic and microscopic structure of the kidney? • What are the 3 processes in urine formation? • How is the kidney involved with regulating water-salt and acid-base balance of blood? • What are the common kidney disorders? • How is the kidney involved with maintaining homeostasis along with other body systems? • What is the structure and function of other urinary system organs? • What are the gender differences in urethra structure? • What is the process of micturition?
Key concepts for urinary system
11.4 Kidneys and Homeostasis
LEARNING OUTCOMES
Upon completion of this section, you should be able to
Summarize how the kidney maintains the water-salt balance of the body.
State the purpose of ADH and aldosterone in homeostasis.
Explain how the kidneys assist in the maintenance of the pH levels of the blood.
The kidneys play a major role in homeostasis, from maintaining the water-salt balance in the body to regulating the pH of the blood. In doing so, the kidneys interact with every other organ system of the human body (Fig. 11.7).
Figure 11.7 The urinary system and homeostasis. The urinary system works primarily with these systems to bring about homeostasis.
Kidneys Excrete Waste Molecules
In Section 9.4, we compared the liver to a sewage treatment plant because it removes poisonous substances from the blood and prepares them for excretion. Similarly, the liver produces urea, the primary nitrogenous end product of humans, which is excreted by the kidneys. If the liver is a sewage treatment plant, the tubules of the kidney are like the trucks that take the sludge, prepared waste, away from the town (the body).
Metabolic waste removal is absolutely necessary for maintaining homeostasis. The blood must constantly be cleansed of the nitrogenous wastes, end products of metabolism. The liver produces urea, and muscles make creatinine. These wastes, as well as uric acid from the cells, are carried by the cardiovascular system to the kidneys. The urine-producing kidneys are responsible for the excretion of nitrogenous wastes. They are assisted to a limited degree by the sweat glands in the skin, which excrete perspiration, a mixture of water, salt, and some urea. In times of kidney failure, urea is excreted by the sweat glands and forms a substance called uremic frost on the skin.
Water-Salt Balance
Most of the water in the filtrate is reabsorbed into the blood before urine leaves the body. The nephron is specialized for the reabsorption of water. The reabsorption of salt always precedes the reabsorption of water. In other words, water is returned to the blood by the process of osmosis. During the process of reabsorption, water passes through water channels, called aquaporins, within a plasma membrane protein.
Sodium ions (Na+) are important in plasma. Usually, more than 99% of the Na+ filtered at the glomerulus is returned to the blood. The kidneys also excrete or reabsorb other ions, such as potassium ions (K+), bicarbonate ions (HCO3−), and magnesium ions (Mg2+) as needed.
Reabsorption of Salt and Water from Cortical Portions of the Nephron
The proximal convoluted tubule, the distal convoluted tubule, and the cortical portion of the collecting ducts are present in the renal cortex. Most of the water (65%) that enters the glomerular capsule is reabsorbed from the nephron into the blood at the proximal convoluted tubule. Na+ is actively reabsorbed, and Cl− follows passively. Aquaporins are always open, and water is reabsorbed osmotically into the blood.
Hormones regulate the reabsorption of sodium and water in the distal convoluted tubule. Aldosterone is a hormone secreted by the adrenal glands, which sit atop the kidneys. This hormone promotes ion exchange at the distal convoluted tubule. Potassium ions (K+) are excreted, and sodium ions (Na+) are reabsorbed into the blood. The release of aldosterone is set into motion by the kidneys. The juxtaglomerular apparatus is a region of contact between the afferent arteriole and the distal convoluted tubule (Fig. 11.8). When blood volume (and, therefore, blood pressure) falls too low for filtration to occur, the juxtaglomerular apparatus can respond to the decrease by secreting renin. Renin is an enzyme that ultimately leads to secretion of aldosterone by the adrenal glands. Research scientists speculate that excessive renin secretion—and thus, reabsorption of excess salt and water—might contribute to high blood pressure.
Figure 11.8 The juxtaglomerular apparatus of the nephron. This drawing shows that the afferent arteriole and the distal convoluted tubule usually lie next to each other. The juxtaglomerular apparatus occurs where they touch. The juxtaglomerular apparatus secretes renin, a substance that leads to the release of aldosterone by the adrenal cortex. Reabsorption of sodium ions and then water now occurs in the distal convoluted tubule. Thereafter, blood volume and blood pressure increase.
Aquaporins are not always open in the distal convoluted tubule. Another hormone, called antidiuretic hormone (ADH), must be present. ADH is produced by the hypothalamus and secreted by the posterior pituitary according to the osmolarity of the blood. If our intake of water has been low, ADH is secreted by the posterior pituitary. Water moves from the distal convoluted tubule and the collecting duct into the blood.
Atrial natriuretic hormone (ANH) is a hormone secreted by the atria of the heart when cardiac cells are stretched due to increased blood volume. ANH inhibits the secretion of renin by the juxtaglomerular apparatus and the secretion of aldosterone by the adrenal glands. Its effect, therefore, is to promote the excretion of sodium ions (Na+), called natriuresis. Normally, salt reabsorption creates an osmotic gradient that causes water to be reabsorbed. Thus, by Page 227causing salt excretion, ANH causes water excretion, too. If ANH is present, less water will be reabsorbed, even if ADH is also present.
Reabsorption of Salt and Water from Medullary Portions of the Nephron
The ability of the human body to regulate the tonicity of urine is dependent on the work of the medullary portions of the nephron (loop of the nephron) and the collecting duct.
The Loop of the Nephron
A long loop of the nephron (also called the loop of Henle), which typically penetrates deep into the renal medulla, is made up of a descending limb and an ascending limb. Salt (NaCl) passively diffuses out of the lower portion of the ascending limb. Any remaining salt is actively transported from the thick upper portion of the limb into the tissue of the outer medulla (Fig. 11.9). In the end, the concentration of salt is greater in the direction of the inner medulla. Surprisingly, however, the inner medulla has an even higher concentration of solutes than expected. It is believed that urea leaks from the lower portion of the collecting duct, contributing to the high solute concentration of the inner medulla.
Figure 11.9 Movement of salt and water within a nephron. Salt (NaCl) diffuses and is actively transported out of the ascending limb of the loop of the nephron into the renal medulla. Also, urea is believed to leak from the collecting duct and to enter the tissues of the renal medulla. This creates a hypertonic environment, which draws water out of the descending limb and the collecting duct. This water is returned to the cardiovascular system. (The thick black outline of the ascending limb means it is impermeable to water.)
Water leaves the descending limb along its entire length via a countercurrent mechanism because of the osmotic gradient within Page 228the medulla. Although water is reabsorbed as soon as fluid enters the descending limb, the remaining fluid within the limb encounters an increasing osmotic concentration of solute. Therefore, water continues to be reabsorbed, even to the bottom of the descending limb. The ascending limb does not reabsorb water, primarily because it lacks aquaporins (as indicated by the dark line in Fig. 11.9). Its job is to help establish the solute concentration gradient. Water is returned to the cardiovascular system when it is reabsorbed.
The Collecting Duct
Fluid within the collecting duct encounters the same osmotic gradient established by the ascending limb of the nephron. Therefore, water diffuses from the entire length of the collecting duct into the blood if aquaporins are open, as they are if ADH is present.
To understand the action of ADH, consider its name, antidiuretic hormone. Diuresis means “increased amount of urine,” thus antidiuresis means “decreased amount of urine.” When ADH is present, more water is reabsorbed (blood volume and pressure rise) and a decreased amount of urine results. ADH ultimately fine-tunes the tonicity of urine according to the needs of the body. For example, ADH is secreted at night when we are not drinking water, and this explains why the first urine of the day is more concentrated.
The Role of ANH
If blood does not have the usual water-salt balance, blood volume and blood pressure are affected. Without adequate blood pressure, exchange across capillary walls cannot take place, nor is glomerular filtration possible in the kidneys.
What happens if you have insufficient sodium ions (Na+) in your blood and interstitial fluid? This can occur due to prolonged heavy sweating, as in athletes running a marathon. When blood Na+ concentration falls too low, blood pressure falls and the renin–aldosterone sequence begins. The kidneys then increase Na+ reabsorption, conserving as much as possible. Subsequently, the osmolarity of the blood and the blood pressure return to normal.
A marathon runner should not drink too much water too fast. Quickly ingesting a large amount of pure water can dilute the Page 229body’s remaining Na+ and disrupt water-salt balance. Although the sports drinks preferred by athletes contain both sodium and water, they may also be high in calories and are not recommended for routine exercise lasting less than 90 minutes.
By contrast, think about what happens if you eat a big tub of salty popcorn at the movies. When salt (NaCl) is absorbed from the digestive tract, the Na+ content of the blood increases above normal. This results in increased blood volume. The atria of the heart are stretched by this increased blood volume, and the stretch triggers the release of ANH by the heart. ANH inhibits sodium and water reabsorption by the proximal convoluted tubule and collecting duct. Blood volume then decreases, because more sodium and water are excreted in the urine.
Diuretics
Diuretics are chemicals that increase the flow of urine. Drinking alcohol causes diuresis, because it inhibits the secretion of ADH. The dehydration that follows is believed to contribute to the symptoms of a hangover. Caffeine is a diuretic because it increases the glomerular filtration rate and decreases the tubular reabsorption of sodium ions (Na+). Diuretic drugs developed to counteract high blood pressure also decrease the tubular reabsorption of Na+. A decrease in water reabsorption and a decrease in blood volume and pressure follow.
Acid-Base Balance of Body Fluids
The pH scale, as discussed in Section 2.2, can be used to indicate the basicity (alkalinity) or the acidity of body fluids. A basic solution has a lower hydrogen ion concentration ([H+]) than the neutral pH of 7.0. An acidic solution has a greater [H+] than neutral pH. The normal pH for body fluids is between 7.35 and 7.45. This is the pH at which our proteins, such as cellular enzymes, function properly. If the blood pH rises above 7.45, a person is said to have alkalosis, and if the blood pH decreases below 7.35, a person is said to have acidosis. Alkalosis and acidosis are abnormal conditions that may need medical attention.
The foods we eat add basic or acidic substances to the blood, and so does metabolism. For example, cellular respiration adds carbon dioxide that combines with water to form carbonic acid, and fermentation adds lactic acid. The pH of body fluids stays at just about 7.4 via several mechanisms, primarily acid-base buffer systems, the respiratory center, and the kidneys.
Acid-Base Buffer Systems
The pH of the blood stays near 7.4 because the blood is buffered. A buffer is a chemical or a combination of chemicals that can take up excess hydrogen ions (H+) or excess hydroxide ions (OH−). One of the most important buffers in the blood is a combination of carbonic acid (H2CO3) and bicarbonate ions (HCO3−). When hydrogen ions (H+) are added to blood, the following reaction occurs:
When hydroxide ions (OH−) are added to blood, this reaction occurs:
These reactions temporarily prevent any significant change in blood pH. A blood buffer, however, can be overwhelmed unless a more permanent adjustment is made. The next adjustment to keep the pH of the blood constant occurs at pulmonary capillaries.
Respiratory Center
As discussed in Section 10.5, the respiratory center in the medulla oblongata increases the breathing rate if the hydrogen ion concentration of the blood rises. Increasing the breathing rate rids the body of hydrogen ions, because the following reaction takes place in pulmonary capillaries:
In other words, when carbon dioxide is exhaled, this reaction shifts to the right and the amount of hydrogen ions is reduced.
It is important to have the correct proportion of carbonic acid and bicarbonate ions in the blood. Breathing readjusts this proportion, so that this acid-base buffer system can continue to absorb H+ and OH− as needed.
The Kidneys
As powerful as the acid-base buffer and the respiratory center mechanisms are, only the kidneys can rid the body of a wide range of acidic and basic substances and, otherwise, adjust the pH. The kidneys are slower acting than the other two mechanisms, but they have a more powerful effect on pH. For the sake of simplicity, we can think of the kidneys as reabsorbing bicarbonate ions and excreting hydrogen ions as needed to maintain the normal pH of the blood (Fig. 11.10).
Figure 11.10 Blood pH is maintained by the kidneys. In the kidneys, bicarbonate ions (HCO3−) are reabsorbed and hydrogen ions +(H+) are excreted as needed to maintain the pH of the blood. Excess hydrogen ions are buffered, for example, by ammonia (NH3), which becomes ammonium +(NH4+). Ammonia is produced in tubule cells by the deamination of amino acids.
If the blood is acidic, hydrogen ions are excreted and bicarbonate ions are reabsorbed. If the blood is basic, hydrogen ions are not excreted and bicarbonate ions are not reabsorbed. The urine is usually acidic, so it follows that an excess of hydrogen ions is usually excreted. Ammonia (NH3) provides another means of buffering and removing the hydrogen ions in urine:
Ammonia (whose presence is obvious in the diaper pail or kitty litter box) is produced in tubule cells by the deamination of amino acids. Phosphate provides another means of buffering hydrogen ions in urine.
The importance of the kidneys’ ultimate control over the pH of the blood cannot be overemphasized. As mentioned, the Page 230enzymes of cells cannot continue to function if the internal environment does not have near-normal pH.
The Kidneys Assist Other Systems
Aside from producing renin, the kidneys assist the endocrine system and the cardiovascular system by producing erythropoietin (EPO), a hormone secreted by the kidneys. When blood oxygen decreases, EPO increases red blood cell synthesis by stem cells in the bone marrow (see Section 6.2). When the concentration of red blood cells increases, blood oxygen increases also. During kidney failure, the kidneys may produce less EPO, resulting in fewer red blood cells and symptoms of fatigue. Drugs such as epoetin (Procrit) represent a form of EPO produced by genetic engineering and biotechnology (see Section 22.3). Like normal EPO, epoetin increases red blood cell synthesis and energy levels. This drug is often used to stimulate red bone marrow production in patients in renal failure or recovering from chemotherapy.
The kidneys assist the skeletal, nervous, and muscular systems by helping regulate the amount of calcium ions (Ca2+) in the blood. The kidneys convert vitamin D to its active form needed for Ca2+ absorption by the digestive tract, and they regulate the excretion of electrolytes, including Ca2+. The kidneys also regulate the sodium ion (Na+) and potassium ion (K+) content of the blood. These ions, needed for nerve conduction, are necessary to the contraction of the heart and other muscles in the body.
This is how kidneys maintain homeostasis
First, the closed end of the nephron is pushed in on itself to form a cuplike structure called the this aka; Bowman’s capsule.
The outer layer composed of squamous epithelial cells.
The inner layer is made up of podocytes that have long, cytoplasmic extensions. The podocytes cling to the capillary walls of the glomerulus and leave pores that allow easy passage of small molecules from the glomerulus to the inside of the glomerular capsule.
This process, called filtration, produces a filtrate of the blood.
I am the glomerulus and the name of the process I do to produce a filtrate of the blood.
True or False?
A lengthwise section of a kidney shows that many branches of the renal artery and renal vein reach inside a kidney
True
What kind of epithelium allows the bladder to stretch and contain a greater volume of urine?
What is the maximum capacity of the bladder?
What other features allow the bladder to retain urine?
A layer of transitional epithelium enables the bladder to stretch and contain an increased volume of urine. The urinary bladder has a maximum capacity of between 700 and 800 ml.
The bladder has other features that allow it to retain urine. After urine enters the bladder from a ureter, small folds of bladder mucosa act as a valve to prevent backward flow. Two sphincters in close proximity are found where the urethra exits the bladder. The internal sphincter occurs around the opening to the urethra. It is composed of smooth muscle and is involuntarily controlled. An external sphincter is composed of skeletal muscle that can be voluntarily controlled.
When the urinary bladder fills to about 250 ml with urine, stretch receptors are activated by the enlargement of the bladder. These receptors send sensory nerve signals to the spinal cord. Subsequently, motor nerve impulses from the spinal cord cause the urinary bladder to contract and the sphincters to relax, so that urination, also called micturition, is possible (Fig. 11.2).
What is the urinary bladder?
stores urine until it is expelled from the body.
three openings: two for the ureters and one for the urethra, which drains the bladder (Fig. 11.2).
What hormones does the urinary system excrete?
he kidneys assist the endocrine system in hormone secretion. The kidneys release renin, an enzyme that leads to aldosterone secretion. Aldosterone is a hormone produced by the adrenal glands, which lie atop the kidneys. As described in Section 11.4, aldosterone is involved in regulating the water-salt balance of the blood. The kidneys also release erythropoietin (EPO), a hormone that regulates the production of red blood cells.
is a central space, or cavity, continuous with the ureter (Fig. 11.3c, d).
The renal pelvis
– small, muscular tubes that carry urine from the kidneys to the bladder
Ureters (2)
by product of normal activities of cells and tissues;
formation and discharge of urine
How is urination different from defecation?
These metabolic waste materials are the by-products of the normal activities of the cells and tissues. In comparison to excretion, defecation is a process of the digestive system (see Section 9.1) that eliminates undigested food and bacteria in the form of feces. Excretion in humans is performed by the formation and discharge of urine from the body.
What does the urinary system do?
The urinary system is the organ system of the body that plays a major role in maintaining the salt, water, and pH homeostasis of the blood.
Microscopically, the kidney is composed of over 1 million nephrons, sometimes called renal, or kidney, tubules (Fig. 11.3e). Page 221The nephrons filter the blood and produce urine. Each nephron is positioned so that the urine flows into a collecting duct. Several nephrons enter the same collecting duct. The collecting ducts eventually enter the renal pelvis.
Anatomy of a Nephron
Each nephron has its own blood supply, including two capillary regions (Fig. 11.4). From the renal artery, an afferent arteriole transports blood to the glomerulus, a knot of capillaries inside the glomerular capsule. Blood leaving the glomerulus is carried away by the efferent arteriole. Blood pressure is higher in the glomerulus, because the efferent arteriole is narrower than the afferent arteriole. The efferent arteriole divides and forms the peritubular capillary network, which surrounds the rest of the nephron. Blood from the efferent arteriole travels through the peritubular capillary network. The blood then goes into a venule that carries blood into the renal vein.
Blood from the efferent arteriole travels through the peritubular capillary network. The blood then goes into a venule that carries blood into the renal vein.
Nephrons are
Figure 11.6 provides an overview of these processes.
Figure 11.6 An overview of urine production. The three main processes in urine formation are described in boxes and color-coded to arrows that show the movement of molecules into or out of the nephron at specific locations. In the end, urine is composed of the substances within the collecting duct (see brown arrow).
Be able to read a visual overview of urine production and trace the flow.
Reversed
Memorize this card
Reversed
What is the structure of a kidney?
- The renal cortex** is an outer, granulated layer that dips down in between a radially striated inner layer called the **renal medulla.
- The renal medulla consists of cone-shaped** tissue masses called **renal pyramids.
- The renal pelvis is a central space, or cavity, continuous with the ureter (Fig. 11.3c, d).
- Figure 11.3 The anatomy of a human kidney. a. A longitudinal section of the kidney showing the blood supply. The renal artery divides into smaller arteries, and these divide into arterioles. Venules join to form small veins, which join to form the renal vein. b. A procedure called an angiogram highlights blood vessels by injecting a contrast medium that is opaque to X-rays. c. The same section without the blood supply. d. A diagram of the renal cortex; the renal medulla; and the renal pelvis, which connects with the ureter. The renal medulla consists of the renal pyramids. e. An enlargement showing the placement of nephrons.
- (
- Microscopically, the kidney is composed of over 1 million nephrons, sometimes called renal, or kidney, tubules (Fig. 11.3e). Page 221The nephrons filter the blood and produce urine. Each nephron is positioned so that the urine flows into a collecting duct. Several nephrons enter the same collecting duct. The collecting ducts eventually enter the renal pelvis.
- Anatomy of a Nephron
- Each nephron has its own blood supply, including two capillary regions (Fig. 11.4). From the renal artery, an afferent arteriole transports blood to the glomerulus, a knot of capillaries inside the glomerular capsule. Blood leaving the glomerulus is carried away by the efferent arteriole. Blood pressure is higher in the glomerulus, because the efferent arteriole is narrower than the afferent arteriole. The efferent arteriole divides and forms the peritubular capillary network, which surrounds the rest of the nephron. Blood from the efferent arteriole travels through the peritubular capillary network. The blood then goes into a venule that carries blood into the renal vein.
- Figure 11.4 The structure of a nephron. A nephron is made up of a glomerular capsule, the proximal convoluted tubule, the loop of the nephron, and the distal convoluted tubule. The collecting duct collects fluid from several nephrons. The photomicrographs show the microscopic anatomy of these structures. The arrows indicate the path of blood around the nephron.
- (glomerulus): Steve Gschmeissner/Science Source; (proximal convoluted tubule): Science Photo Library/Getty Images
*
Reversed
Review this slide and take notes
What are the urinary system organs/glands and locate them
Reversed
The renal pelvis
is a central space, or cavity, continuous with the ureter (Fig. 11.3c, d).
Reversed
Nephrons are
Microscopically, the kidney is composed of over 1 million nephrons, sometimes called renal, or kidney, tubules (Fig. 11.3e). Page 221The nephrons filter the blood and produce urine. Each nephron is positioned so that the urine flows into a collecting duct. Several nephrons enter the same collecting duct. The collecting ducts eventually enter the renal pelvis.
Anatomy of a Nephron
Each nephron has its own blood supply, including two capillary regions (Fig. 11.4). From the renal artery, an afferent arteriole transports blood to the glomerulus, a knot of capillaries inside the glomerular capsule. Blood leaving the glomerulus is carried away by the efferent arteriole. Blood pressure is higher in the glomerulus, because the efferent arteriole is narrower than the afferent arteriole. The efferent arteriole divides and forms the peritubular capillary network, which surrounds the rest of the nephron. Blood from the efferent arteriole travels through the peritubular capillary network. The blood then goes into a venule that carries blood into the renal vein.
Blood from the efferent arteriole travels through the peritubular capillary network. The blood then goes into a venule that carries blood into the renal vein.
Reversed
Be able to read a visual overview of urine production and trace the flow.
Figure 11.6 provides an overview of these processes.
Figure 11.6 An overview of urine production. The three main processes in urine formation are described in boxes and color-coded to arrows that show the movement of molecules into or out of the nephron at specific locations. In the end, urine is composed of the substances within the collecting duct (see brown arrow).
