Chapter 18 (mod 6) Flashcards
Functions of the kidneys
- Excretion
- remove waste products from blood
- waste is normally metabolic by-products of cell metabolism and are toxic - Regulation of blood volume and pressure
- major role in controlling the extracellular fluid volume in the body
- kidneys can produce either a large volume of dilute urine or a small volume of concentrated urine (depends on hydration levels)
- through urine production, the kidneys regulate blood volume and blood pressure - Regulation of blood solute concentrations
- kidneys help regulate the concentration of the major molecules and ions, such as glucose, Na+, Cl-, K+, Ca2+, HCO-, and HPO42- - Regulation of extracellular fluid pH
- kidneys excrete variable amounts of H+ to help regulate extracellular fluid pH - Regulation of red blood cell synthesis
- the kidneys secrete a hormone, erythropoietin which regulates the synthesis of red blood cells in bone marrow - Regulation of vitamin D synthesis
- the kidneys play an important role in controlling blood levels of Ca2+ by regulating the synthesis of vitamin D
Location of the kidneys
The kidneys are behind the peritoneum, or retroperitoneal, and are located on each side of the vertebral column
- bean shaped
External anatomy of the kidneys
- a layer of connective tissues called the renal capsule surrounds each kidney
- around the renal capsule is a tick layer of adipose tissue, which protects the kidney from mechanical shock
- on the medials side of each kidney is the hilum, where the renal artery and nerves enter and where the renal vein, ureter, and lymphatic vessels exit the kidney
- the hilum opens into a cavity called the renal sinus, which contains blood vessels, part of the system of collecting urine, and adipose tissue
Internal Anatomy of kidneys
organised into two major regions:
1. outer cortex
- location for the blood-filtering structures of the kidney
2. inner medulla
- surrounds the renal sinus
- composed of many renal pyramids, whose bases project into the cortex
- renal pyramids are a collection of tubes and ducts that transport fluid throughout the kidney and modify it into urine
- once urine is formed, ducts in the renal pyramids transport it toward the renal sinus through the renal papillae
- The renal sinus contains the renal pelvis and calyces. Urine formed in the renal pyramids flows through the renal papillae into the calyces within the sinus.
- Renal papillae release urine into small, funnel-shaped chambers called calyces.
- Urine from multiple calyces empties into a larger chamber called the renal pelvis, which is embedded in the renal sinus.
- At the hilum, the renal pelvis narrows, forming the ureter, which carries urine to the urinary bladder.
structure of a nephron
Four regions:
1. renal corpuscle
- filters blood
2. proximal convoluted tubule
- returns filtered substances to the blood
3. loop of Henle
- conserve water and solutes
4. distal convoluted tubule
- rids the blood of additional wastes
- the fluid in this region then empties into a collecting duct, which carries the newly formed urine from the cortex of the kidney toward the renal papilla deep in the medulla
- near the tip of the renal papilla, several collecting ducts merge into a large tubule called a papillary duct, which empties into a calyx
Types of nephrons
- juxtamedullary
- have renal corpuscles that are found deep in the cortex near the medulla
- have long loops of Henle, which extend deep into the medulla
- about 15% of nephrons are this type - cortical
- have renal corpuscles that are distributed throughout the cortex
- loops of Henle are shorter in this nephron and are closer to the outer edge of the cortex
The renal corpuscle
Filtration portion of nephron
consists of:
1. glomerulus
- network of capillaries twisted around like a ball of yarn
2. bowman capsule
- an indented, double-walled chamber surround the glomerulus
- from this capsule, the filtered fluid flows into the proximal convoluted tubule region of the renal tubule
Bowman capsule
two layers:
1. an outer layer
- constructed of simple squamous epithelial cells
2. an inner layer
- constructed of specialized cells called podocytes which wrap around the glomerular capillaries
renal corpuscle - characteristics for efficient filtration
- Porous capillaries
- the glomerular capillaries are highly permeable due to the pores and level of permeable depends of pore sizes
- large proteins nor blood cells can fit through these capillaries - Porous inner layer of bowman capsule
- there are gaps between the cell processes of the podocytes of the inner layer
- membrane lies between the endothelial cells of the glomerular capillaries and the podocytes of the bowman capsule - High pressure
- An afferent (toward) arteriole supplies blood to the glomerulus for filtration
- efferent (away) arteriole transport the filtered blood away from the glomerulus
- glomerular capillaries have much higher pressure than other capillaries due to the smaller diameter of the efferent arteriole compared to the afferent arteriole
Juxtaglomerular apparatus
located next to the glomerulus and is an important regulatory structure
- secretes the enzyme renin which is important for filtration formation and regulation of blood pressure
- consists of afferent arteriole cells and specialized cells in the distal convoluted tubule
specialised cells:
1. juxtaglomerular cells
- at the point where the afferent arteriole enters the renal corpuscle, it has a cuff of specialised smooth muscle cells around it
2. macula densa
- a part of the distal convoluted tubule of the nephron lies between the afferent and efferent arterioles next to the renal corpuscle and in this section of the distal convoluted tubule is where macula densa lies
The renal tubule
Once the blood is filtered the resulting fluid is modified to form urine as it passes through each section of the renal tubule
1. first section is the proximal convoluted tubule
- microvilli to increase the surface area
- this tubule descends towards the medulla and the cell type begins to change
2. Loop of henle
- two limbs: descending and ascending limb
3. distal convoluted tubule
- connect to a single collecting duct (extends through the medulla toward the tips of the renal pryamids)
Renal arteries
- renal artery extends deep into the kidney and branches into smaller blood vessels which are:
1. interlobar arteries - pass between the renal pyramids
2. arcuate arteries - branch from the interlobar arteries and arch between the cortex and the medulla
3. Interlobular arteries - branch off the arcuate arteries and project into the cortex
4. Afferent arteriole - arise from branches of the interlobular arteries and cary blood to the glomerular capillaries
5. Efferent arterioles - carry blood FROM the glomerular capillaries
6. the peritubular capillaries - branch from the efferent arterioles. They surround the PCT, DCT, and the loops of Henle.
7. vasa recta - specialised portions of the peritbular capillaries that extend deep into the medulla and surround the loops of Henle and collecting ducts - blood from the peritubular capillaries including the vasa recta, will return to the general circulation through the veins of the kidneys
Urine formation in 3 major processes
- Filtration
- tubular reabsorption
- tubular secretion
Filtration
Blood pressure in the glomerular capillaries forces fluid and small molecules out of the blood to create filtrate. Filtration is nonselective and separates based only on size or charge of molecules. Filtration does not remove everything in the blood only removes substances small enough to fit through the filtration membrane
Tubular reabsorption
Cells in the renal tubules contain many transport proteins that move water and some filtered molecules from the filtrate back into the blood in the peritubular capillaries. This prevents them from being lost from the body as components of urine. Most of the filtered water and useful solutes have been returned to the blood by the time the filtrate has been modified to urine, whereas the remaining waste or excess substances, and a small amount of water, form urine
- critical to ensure the body does not go completely dehydrated and deficient from important materials
Tubular secretion
The movement of non filtered substances from the blood into the filtrate.
- Certain tubule cells transport additional solutes from the blood into the filtrate. Some of these solutes may not have been filtered by the filtration membrane
- can be either passive or active
Filtration membrane
The renal corpuscles in the renal cortex contain filtration membranes, which regulate the movement of substances from the blood.
- The filtration membrane acts as a selective barrier, allowing water and small molecules to pass while preventing blood cells and most proteins from leaving the bloodstream.
- It filters substances based on size and charge.
Structures that make up the membrane:
1. Glomerular Capillaries – Porous capillaries that allow fluid and small solutes to pass.
2. Basement Membrane – Located between the capillary wall and the visceral layer of the Bowman’s capsule, it acts as a secondary filter.
3. Podocytes – Specialized cells in the inner layer of the Bowman’s capsule with filtration slits that further regulate what enters the filtrate.
Filtration pressure
Filtration Pressure = Glomerular Capillary Pressure - Capsular Pressure - Colloid Osmotic Pressure
The glomerulus is a network of capillaries where blood pressure forces water and small solutes out of the blood and into the Bowman’s capsule, forming filtrate.
- This process is driven by filtration pressure
Glomerular Capillary Pressure (GCP) – Outward pressure from blood pushing fluid out of the capillaries, and forcing fluid and solutes into the Bowman’s capsule. This pressure is higher than in other capillaries due to the narrow efferent arteriole, which increases resistance.
Capsular Pressure (CP) – Inward pressure that opposes filtration, caused by the fluid buildup already present in the capsular space.
Colloid Osmotic Pressure (COP) – Inward pressure opposing filtration. Pulls fluid back into the capillaries due to plasma proteins. This pressure increases toward the end of the glomerular capillary as protein concentration rises.
Filtration pressure ensures that waste, excess water, and small molecules enter the nephron while blood cells and large proteins remain in the bloodstream.
Regulation of filtration
Blood pressure is tightly regulated in the glomerular capillaries because the efferent and afferent arterioles can dilate and constrict
- filtration pressure changes dramatically under intense sympathetic stimulation and causes constriction of the kidneys arteries
- this occurs during circulatory shock or vigorous exercise and may decrease filtrate formation and urine volume
- one danger of circulatory shock is that the renal blood flow can be so low that kidneys suffer from a lack of O2 and this can lead to kidney damage or failure
Reabsorption in the proximal convoluted tubule
Proximal is the site of majority reabsorption and 65% of filtrate is reabsorbed.
Process:
1. Sodium reabsorption:
- Na⁺ moves from the filtrate into PCT cells due to a steep concentration gradient created by active transport (Na⁺-K⁺ pump).
- The movement of Na⁺ helps transport other molecules like glucose and amino acids.
2. Solute Transport:
- Carrier proteins in PCT cells cotransport Na⁺ with glucose, amino acids, and other solutes from the filtrate into the cells.
- These molecules then exit the PCT cells into the interstitial fluid and enter the bloodstream by facilitated diffusion.
3. Water Reabsorption:
- As solutes leave the lumen, water follows by osmosis, maintaining fluid balance.
Reabsorption in the loop of Henle
The two limbs differ by epithelial tissue and this creates a difference in permeability properties which create a concentration gradient essential for water conservation.
1. Descending Limb:
- Made of squamous epithelium, highly permeable to water.
- Water exits by osmosis into the surrounding interstitial fluid.
- Some solutes move into the limb by diffusion, increasing filtrate concentration.
2. Ascending Limb:
- Initially permeable to solutes but not water, allowing solutes to diffuse out, reducing filtrate concentration.
- In the thick segment, both water and solutes are impermeable, but solutes are actively transported out by ATP-powered pumps and carrier proteins.
- Na+, K+, and Cl- are cotransported, with Cl- and K+ exiting via facilitated diffusion.
- This active transport helps the kidney conserve water by maintaining the concentration gradient in the medulla.
Reabsorption in the distal convoluted tubule and collecting duct
- some solutes (K+ and H+) are not reabsorbed until it reaches this tubule/collecting duct
- reabsorption of these solutes is generally under hormone control and depends on the current conditions of the body
- hormone regulation changes the tubule and duct permeability to water
- reabsorption of water occurs through osmosis across the wall of the distal convoluted tubule and the collecting duct when the hormone ADH is present
The kidneys ability to control the volume and concentration of the urine depends on 3 factors:
- countercurrent mechanisms
- a medullary concentration gradient
- hormonal mechanisms
countercurrent mechanism
The countercurrent mechanism is a process in the kidneys that helps maintain the medullary concentration gradient, allowing the body to conserve water and produce concentrated urine.
Loop of Henle (Countercurrent Multiplier)
- Descending Limb: Permeable to water, which moves out by osmosis due to the high solute concentration in the medullary interstitial fluid.
- Ascending Limb: Impermeable to water, but actively pumps solutes out, increasing the solute concentration in the medulla.
Vasa Recta (Countercurrent Exchanger)
- Descending Vasa Recta: Water moves out, and solutes enter, preventing dilution of the medullary gradient.
- Ascending Vasa Recta: Water moves in, and solutes exit, ensuring the medullary concentration remains stable.
Purpose:
- Maintains a high solute concentration in the medulla.
- Helps reabsorb water in the collecting ducts under the influence of ADH.
- Allows the kidneys to produce concentrated urine when needed to prevent dehydration.
