Chapter 13: The urinary system

Question 1

Urinary system consists of?

Answer 1

The urinary system is the main excretory system and consists of the following structures:

• 2 kidneys, which secrete urine
• 2 ureters that convey the urine from the kidneys to the urinary bladder
• the urinary bladder, which collects and stores urine
• the urethra through which urine leaves the body

Question 2

Urinary system

Answer 2

•The urinary system plays a vital part in maintaining homeostasis of water and electrolytes within the body. The kidneys produce urine that contains metabolic waste products, including the nitrogenous compounds urea and uric acid, excess ions and some drugs. The main functions of the kidneys are:
-formation of urine, maintaining water, electrolyte and acid–base balance
-excretion of waste products
-production and secretion of erythropoietin, the hormone that stimulates formation of red blood cells
-production and secretion of renin, an important enzyme in the control of blood pressure.
•Urine is stored in the bladder and excreted by the process of micturition.
•The first sections of this chapter explore the structures and functions of the organs of the urinary system and the impact of ageing on kidney function. In the final section the consequences of abnormal functioning of the various parts of the urinary system on body function are considered.

Question 3

Kidneys

Answer 3

• The kidneys lie on the posterior abdominal wall, one on each side of the vertebral column, behind the peritoneum, and below the diaphragm. They extend from the level of the 12th thoracic vertebra to the 3rd lumbar vertebra, receiving some protection from the lower rib cage. The right kidney is usually slightly lower than the left, probably because of the considerable space occupied by the liver.
• Kidneys are bean-shaped organs, about 11 cm long, 6 cm wide, 3 cm thick and weigh 150 g. They are embedded in and held in position by, a mass of fat. A sheath of fibrous connective tissue, the renal fascia, encloses the kidney and the renal fat.

Question 4

Organs associated with the kidneys

Answer 4

As the kidneys lie on either side of the vertebral column, each is associated with different structures.

Question 5

Right kidney

Answer 5

• Superiorly – the right adrenal gland
• Anteriorly – the right lobe of the liver, the duodenum, and the hepatic flexure of the colon
• Posteriorly – the diaphragm, and muscles of the posterior abdominal wall.

Question 6

Left kidney

Answer 6

• Superiorly – the left adrenal gland
• Anteriorly – the spleen, stomach, pancreas, jejunum, and splenic flexure of the colon
• Posteriorly – the diaphragm and muscles of the posterior abdominal wall.

Question 7

Gross structure of the kidney

Answer 7

•There are three areas of tissue that can be distinguished when a longitudinal section of the kidney is viewed with the naked eye:
-an outer fibrous capsule, surrounding the kidney
-the cortex, a reddish-brown layer of tissue immediately below the capsule and outside the renal pyramids
-the medulla, the innermost layer, consisting of pale conical-shaped striations, the renal pyramids.
•The hilum is the concave medial border of the kidney where the renal blood and lymph vessels, the ureter, and nerves enter.
•Urine formed within the kidney passes through a renal papilla at the apex of a pyramid into a minor calyx

Question 8

The microscopic structure of the kidney

Answer 8

The kidney contains about 1–2 million functional units, the nephrons, and a much smaller number of collecting ducts. The collecting ducts transport urine through the pyramids to the calyces, giving the pyramids their striped appearance. The collecting ducts are supported by connective tissue, containing blood vessels, nerves, and lymph vessels.

Question 9

The nephron

Answer 9

Nephron, the functional unit of the kidney, is the structure that produces urine in the process of removing waste and excess substances from the blood. There are about 1,000,000 nephrons in each human kidney.

Question 10

Formation of urine

Answer 10

The kidneys form urine, which passes to the bladder for storage prior to excretion. The composition of urine reflects the exchange of substances between the nephron and the blood in the renal capillaries. Waste products of protein metabolism are excreted, water and electrolyte levels are controlled and pH (acid-base balance) is maintained by the excretion of hydrogen ions. There are three processes involved in the formation of urine:

• filtration
• selective reabsorption
• secretion

Question 11

Filtration

Answer 11

• This takes place through the semipermeable walls of the glomerulus and glomerular capsule. Water and other small molecules readily pass through, although some are reabsorbed later. Blood cells, plasma proteins, and other large molecules are too large to filter through and therefore remain in the capillaries. The filtrate in the glomerulus is very similar in composition to plasma with the important exceptions of plasma proteins and blood cells.
• Filtration takes place because there is a difference between the blood pressure in the glomerulus and the pressure of the filtrate in the glomerular capsule.

Question 12

Autoregulation

Answer 12

• Renal blood flow, and therefore glomerular filtration, is protected by a mechanism called autoregulation, whereby renal blood flow is maintained at a constant pressure across a wide range of systolic blood pressures (from around 80–200 mmHg). Autoregulation operates independently of nervous control, i.e., if the nerve supply to the renal blood vessels is interrupted, autoregulation continues to operate. It is therefore a property inherent in renal blood vessels; it may be stimulated by changes in blood pressure in the renal arteries or by fluctuating levels of certain metabolites, e.g., prostaglandins.
• In severe shock, when the systolic blood pressure falls below 80 mmHg, autoregulation fails and renal blood flow and the hydrostatic pressure decrease, impairing filtration within the glomeruli.

Question 13

Selective reabsorption

Answer 13

• Most reabsorption from the filtrate back into the blood takes place in the proximal convoluted tubule, whose walls are lined with microvilli to increase surface area for absorption. Many substances are reabsorbed here, including some water, electrolytes, and organic nutrients such as glucose. Some reabsorption is passive, but some substances, e.g., glucose, are actively transported.
• Active transport takes place at carrier sites in the epithelial membrane, using chemical energy to trans­port substances against their concentration gradients
• Some ions, e.g., sodium and chloride, can be absorbed by both active and passive mechanisms depending on the site in the nephron.
• Some constituents of glomerular filtrate (e.g., glucose, amino acids) do not normally appear in urine because they are completely reabsorbed unless blood levels are excessive.
• Reabsorption of nitrogenous waste products, such as urea, uric acid, and creatinine is very limited.

Question 14

parathyroid hormone

Answer 14

This is secreted by the parathyroid glands and together with calcitonin from the thyroid gland regulates the reabsorption of calcium and phosphate from the distal collecting tubules, so that normal blood levels are maintained. Parathyroid hormone increases the blood calcium level and calcitonin lowers it.

Question 15

Antidiuretic hormone (ADH)

Answer 15

This is secreted by the posterior pituitary. It increases the permeability of the distal convoluted tubules and collecting tubules, increasing water reabsorption. Secretion of ADH is controlled by a negative feedback system

Question 16

Aldosterone

Answer 16

Secreted by the adrenal cortex, this hormone increases the reabsorption of sodium and water and the excretion of potassium. Secretion is regulated through a negative feedback system.

Question 17

Atrial natriuretic peptide (ANP)

Answer 17

This hormone is secreted by the atria of the heart in response to the stretching of the atrial wall when blood volume is increased. It decreases the reabsorption of sodium and water from the proximal convoluted tubules and collecting ducts. Secretion of ANP is also regulated by a negative feedback system.

Question 18

Tubular secretion

Answer 18

Filtration occurs as blood flows through the glomerulus. Substances are not required and foreign materials, e.g., drugs including penicillin and aspirin, may not be entirely filtered out of the blood because of the short time it remains in the glomerulus. Such substances are cleared by secretion from the peritubular capillaries into the filtrate within the convoluted tubules. Tubular secretion of hydrogen ions (H+) is important in maintaining normal blood ph.

Question 19

Summary of urine formation

Answer 19

The three processes involved – filtration, selective reabsorption, and tubular secretion.

Question 20

Composition of urine

Answer 20

Urine is clear and amber in color due to the presence of urobilin, a bile pigment altered in the intestine, reabsorbed then excreted by the kidneys. The specific gravity is between 1020 and 1030, and the pH is around 6 (normal range 4.5–8). A healthy adult pass from 1000 to 1500 mL per day. The volume of urine produced, and the specific gravity varies according to fluid intake and the amount of solute excreted.

Question 21

Water balance and urine output

Answer 21

• The source of most body water is dietary food and fluid although a small amount (called ‘metabolic water’) is formed by cellular metabolism. Water is excreted as the main constituent of urine, in expired air, feces, and through the skin as sweat. The amount lost in expired air and feces is constant; the amount of sweat produced is associated with environmental and body temperatures.
• Sensory nerve cells in the hypothalamus (osmoreceptors) detect changes in the osmotic pressure of the blood. Nerve impulses from the osmoreceptors stimulate the posterior pituitary to release ADH. When the osmotic pressure is raised, i.e. the blood is becoming more concentrated, ADH output is increased and as a result, water reabsorption by the distal convoluted tubules and collecting ducts is increased, reducing the blood osmotic pressure and ADH output. This negative feedback mechanism maintains the blood osmotic pressure (and therefore sodium and water concentrations) within normal limits
• When blood volume is increased, stretch receptors in the atria of the heart are stimulated and cardiac muscle cells release atrial natriuretic hormone (ANP). This reduces the reabsorption of sodium and water by the proximal convoluted tubules and collecting ducts, meaning that more sodium and water are excreted. In turn, this lowers blood volume and reduces atrial stretching, and through the negative feedback mechanism, ANP secretion is switched off. Raised ANP levels also inhibit the secretion of ADH and aldosterone, further promoting the loss of sodium and water.

Question 22

Electrolyte balance

Answer 22

•Changes in the concentration of electrolytes in the body fluids may be due to changes in:
-the body water content, or
-electrolyte levels.
•Several mechanisms maintain the balance between water and electrolyte concentration.

Question 23

Sodium and potassium balance

Answer 23

• Sodium is the most common cation (positively charged ion) in extracellular fluid and potassium is the most common intracellular cation.
• Sodium is a constituent of almost all foods and, furthermore, salt is often added to food during cooking. This means that intake is usually in excess of the body’s needs. It is excreted mainly in urine and sweat.
• The amount of sodium excreted in sweat is usually insignificant unless sweating is excessive. This may occur when there is pyrexia (fever), a high environmental temperature, or sustained physical exercise. Normally the renin-angiotensin-aldosterone mechanism (described below) maintains the concentration of sodium and potassium within physiological limits. When excessive sweating is sustained, e.g., living in a hot climate or working in a hot environment, acclimatization occurs in approximately 7 to 10 days and electrolyte secretion lost in sweat is reduced.
• Sodium and potassium occur in high concentrations in digestive juices – sodium in gastric juice and potassium in pancreatic and intestinal juice. Normally these ions are reabsorbed by the colon but following acute and prolonged diarrhea they may be excreted in large quantities causing electrolyte imbalance.

Question 24

Renin-angiotensin-aldosterone system

Answer 24

Sodium is a normal constituent of urine, and its excretion is regulated by the hormone aldosterone, secreted by the adrenal cortex. Cells in the afferent arteriole of the nephron release the enzyme renin in response to sympathetic stimulation, low blood volume, or low arterial blood pressure. Renin converts the plasma protein angio­tensinogen, produced by the liver, to angiotensin 1. Angio­tensin converting enzyme (ACE), formed in small quantities in the lungs, proximal convoluted tubules, and other tissues, converts angiotensin 1 into angiotensin 2, which is a very potent vasoconstrictor and increases blood pressure. Renin and raised blood potassium levels also stimulate the adrenal gland to secrete aldosterone. Water is reabsorbed with sodium and together they increase the blood volume, which reduces renin secretion through the negative feedback mechanism. When sodium reabsorption is increased potassium excretion is increased, indirectly reducing intracellular potassium. Profound diuresis may lead to hypokalemia (low blood potassium levels).

Question 25

ANP

Answer 25

This hormone is also involved in the regulation of sodium levels.

Question 26

Calcium balance

Answer 26

Regulation of calcium levels is maintained by secretion of parathyroid hormone and calcitonin.

Question 27

pH balance

Answer 27

•In order to maintain normal blood pH (acid-base balance), the proximal convoluted tubules secrete hydrogen ions into the filtrate where they combine with buffers:
-bicarbonate, forming carbonic acid
-ammonia, forming ammonium ions
-hydrogen phosphate, forming dihydrogen phosphate
•Carbonic acid is converted to carbon dioxide (CO2) and water (H2O), and the CO2 is reabsorbed, maintaining the buffering capacity of the blood. Hydrogen ions are excreted in the urine as ammonium salts and hydrogen phosphate. The normal pH of urine varies from 4.5 to 8 depending on diet, time of day, and other factors. Individuals whose diet contains a large number of animal proteins tend to produce more acidic urine (lower pH) than vegetarians.

Question 28

Ureters

Answer 28

The ureters carry urine from the kidneys to the urinary bladder. They are about 25–30 cm long with a diameter of approximately 3 mm. The ureter is continuous with the funnel-shaped renal pelvis. It passes downwards through the abdominal cavity, behind the peritoneum in front of the psoas muscle into the pelvic cavity and passes obliquely through the posterior wall of the bladder. Because of this arrangement, as urine accumulates and the pressure in the bladder rises, the ureters are compressed and the openings into the bladder are occluded. This prevents backflow (reflux) of urine into the ureters (towards the kidneys) as the bladder fills and during micturition, when pressure increases as the muscular bladder wall contracts.

Question 29

The structure of the ureters

Answer 29

The walls of the ureters consist of three layers of tissue:

• an outer covering of fibrous tissue, continuous with the fibrous capsule of the kidney
• a middle muscular layer consisting of interlacing smooth muscle fibers that form a functional unit around the ureter and an additional outer longitudinal layer in the lower third
• an inner layer, the mucosa, composed of transitional epithelium

Question 30

Functions of the ureters

Answer 30

Peristalsis is an intrinsic property of the smooth muscle layer that propels urine along the ureter. Peristaltic waves occur several times per minute, increasing in frequency with the volume of urine produced, sending little spurts of urine along the ureter towards the bladder.

Question 31

Urinary bladder

Answer 31

The urinary bladder is a reservoir for urine. It lies in the pelvic cavity and its size and position vary, depending on the volume of urine it contains. When distended, the bladder rises into the abdominal cavity.

Question 32

The structure of the urinary bladder

Answer 32

•The bladder is roughly pear-shaped but becomes more balloon-shaped as it fills with urine. The posterior surface is the base. The bladder opens into the urethra at its lowest point, the neck.
•The peritoneum covers only the superior surface before it turns upwards as the parietal peritoneum, lining the anterior abdominal wall. Posteriorly it surrounds the uterus in the female and the rectum in the male. The bladder wall is composed of three layers:
-the outer layer of loose connective tissue, containing blood and lymphatic vessels and nerves, covered on the upper surface by the peritoneum
-the middle layer, consisting of interlacing smooth muscle fibers and elastic tissue loosely arranged in three layers. This is called the detrusor muscle and when it contracts, it empties the bladder
-the inner mucosa, composed of transitional epithelium that readily permits distension of the bladder as it fills.
•When the bladder is empty the inner lining is arranged in folds, or rugae, which gradually disappear as it fills. The bladder is distensible but as it fills, awareness of the need to pass urine is felt. The total capacity is rarely more than about 600 mL.
•The three orifices in the bladder wall form a triangle or trigone. The upper two orifices on the posterior wall are the openings of the ureters; the lower orifice is the opening into the urethra. The internal urethral sphincter, a thickening of the urethral smooth muscle layer in the upper part of the urethra, controls the outflow of urine from the bladder. This sphincter is not under voluntary control.

Question 33

Urethra

Answer 33

• The urethra is a canal extending from the neck of the bladder to the exterior, at the external urethral orifice. It is longer in the male than in the female.
• The male urethra is associated with both the urinary and reproductive systems
• The wall of the female urethra has two main layers: an outer muscle layer and an inner lining of mucosa, which is continuous with that of the bladder. The muscle layer has two parts, an inner layer of smooth muscle that is under autonomic nerve control, and an outer layer of striated (voluntary) muscle surrounding it. The striated muscle forms the external urethral sphincter and is under voluntary control. The mucosa is supported by loose fibroelastic connective tissue containing blood vessels and nerves. Proximally it consists of transitional epithelium while distally it is composed of stratified epithelium.

Question 34

Micturition

Answer 34

• In infants, accumulation of urine in the bladder activates stretch receptors in the bladder wall generating sensory (afferent) impulses that are transmitted to the spinal cord, where a spinal reflex is initiated. This stimulates involuntary contraction of the detrusor muscle and relaxation of the internal urethral sphincter and expels urine from the bladder – this is micturition or voiding of urine.
• When bladder control is established, the micturition reflex is still stimulated but sensory impulses also pass upwards to the brain and there is awareness of the need to pass urine as the bladder fills (around 300–400 mL in adults). By learned and conscious effort, contraction of the external urethral sphincter and muscles of the pelvic floor can inhibit micturition until it is convenient to pass urine.
• Urination can be assisted by increasing the pressure within the pelvic cavity, achieved by lowering the diaphragm and contracting the abdominal muscles. Over­distension of the bladder is extremely painful, and when this occurs there is a tendency for involuntary relaxation of the external sphincter to occur allowing a small amount of urine to escape, provided there is no mechanical obstruction. Incontinence is the involuntary loss of urine after bladder control has been established.

Question 35

The effects of aging of the urinary system

Answer 35

• The kidneys have a substantial functional reserve; the loss of one kidney does not cause problems in an otherwise healthy individual. The number of nephrons declines with age, the glomerular filtration rate falls and the renal tubules function less efficiently; the kidneys become less able to concentrate urine.
• These changes mean that older adults become more sensitive to alterations in fluid balance, and problems associated with fluid overload or dehydration are more prevalent. Elimination of drugs also becomes less efficient with declining kidney function which may lead to accumulation and toxicity.
• The ability to inhibit contraction of the detrusor muscle declines and may result in the urgent need to pass urine and urinary frequency. Nocturia becomes increasingly common in older adults. Incontinence is more prevalent in older adults affecting 15% of women and 10% of men over 65, figures which double by 85 years of age. Enlargement of the prostate gland is common in older men and may cause retention of urine and problems with micturition.