Kidney Function - Ultrafiltration and Selective Reabsorption PDF

# Kidney function - ultrafiltration and selective reabsorption - Describe how the processes of ultrafiltration and selective reabsorption are involved in the formation of urine in the nephron ## Functions of the kidneys - Regulating the composition of the blood and maintaining a constant water po...

# Kidney function - ultrafiltration and selective reabsorption - Describe how the processes of ultrafiltration and selective reabsorption are involved in the formation of urine in the nephron ## Functions of the kidneys - Regulating the composition of the blood and maintaining a constant water potential by: - Maintaining a constant volume of water - Removing wastes such as urea - Maintaining the concentration of mineral ions and other substances constant - Regulating blood pressure - Maintaining the body's calcium level - Stimulating the production of red blood cells ## Ultrafiltration - Blood enters the kidney through the renal artery, which branches frequently to give around one million tiny arterioles, each of which enters a renal (Bowman's) capsule of a nephron. - This is called the afferent arteriole and it divides to give a complex of capillaries known as the glomerulus. - The glomerular capillaries later merge to form the efferent arteriole, which then sub-divides again into capillaries (the peritubular capillaries), which wind their way around the various tubules of the nephron before combining to form the renal vein. - The walls of the glomerular capillaries are made up of endothelial cells with pores between them. - As the diameter of the afferent arteriole is greater than that of the efferent arteriole, there is a build up of hydrostatic pressure within the glomerulus. - As a result, water, glucose, mineral ions and other substances up to a relative molecular mass of up to 68000 are squeezed out of the capillary to form the glomerular filtrate. - The movement of this filtrate out of the glomerulus is resisted by the: - Capillary endothelium - Basement membrane of the epithelial layer of the renal (Bowman's) capsule - Epithelial cells of the renal (Bowman's) capsule - The hydrostatic pressure of the fluid in the renal capsule space (the intracapsular pressure) - The low water potential of the blood in the glomerulus. ## The glomerulus - a unique capillary bed - In mammals, the glomerulus is the only capillary bed in which an arteriole (the afferent arteriole) supplies it with blood and an arteriole (the efferent arteriole) also drains blood away. - In all other mammalian capillary beds it is a venule that drains away the blood. - The glomerular capillaries need to merge into an efferent arteriole because this increases the hydrostatic pressure within the glomerulus and allows ultrafiltration to occur. ## Selective reabsorption - In the proximal convoluted tubule nearly 85% of the filtrate is reabsorbed back into the blood. - About 180 dm³ of water enters the nephrons each day. Of this volume, only about 1 dm³ leaves the body as urine. - Eighty-five per cent of the reabsorption of water occurs in the proximal convoluted tubule. ## The loop of Henlé - The loop of Henlé is a hairpin-shaped tubule that extends into the medulla of the kidney. - The cells of the proximal convoluted tubules are adapted to reabsorb substances into the blood by having microvilli which give them a large surface area and many mitochondria to provide ATP for active transport of sodium ions. - The loop of Henlé has two regions: - The descending limb, which is narrow, with thin walls that are highly permeable to water. - The ascending limb, which after a short distance is wider, with thick walls that are impermeable to water. - The loop of Henlé acts as a counter-current multiplier. ## The distal (second) convoluted tubule - The cells that make up the walls of the distal (second) convoluted tubule have microvilli and many mitochondria that allow them to reabsorb material rapidly from the filtrate, by either diffusion or active transport. - The main role of the distal tubule is to make final adjustments to the water and salts that are reabsorbed and to control the pH of the blood by selecting which ions to reabsorb. ## Counter-current multiplier - When two liquids flow in opposite directions past one another, the exchange of substances (or heat) between them is greater than if they flowed in the same direction next to each other. - In the case of the loop of Henlé, the counter-current flow means that the filtrate in the collecting duct with a lower water potential meets interstitial fluid that has an even lower water potential. - This means that, although the water potential gradient between the collecting duct and interstitial fluid is small, it exists for the whole length of the collecting duct. - There is therefore a steady flow of water into the interstitial fluid, so that around 80% of the water enters the interstitial fluid and hence the blood. ## Image descriptions **Figure 1: Podocyte and ultrafiltration** - This figure shows a diagram of a glomerulus and the surrounding structures involved in filtration. - It highlights the podocytes, which are specialized cells that wrap around the capillaries of the glomerulus. - The figure also illustrates the basement membrane, the endothelial cells of the capillary, and the foot-like processes of the podocytes. - The arrows show the direction of fluid movement during ultrafiltration, indicating the passage of filtrate from the blood in the glomerulus through the basement membrane and between the foot-like processes of the podocytes into the renal capsule. **Figure 2: Colourised scanning electron micrograph of podocyte cells around a glomerulus in a human kidney** - This shows a close-up image of podocytes surrounding a glomerulus. - It captures the intricate structure of these cells, which are essential for the filtration process. - Features like the microvilli, pinocytic vesicle, nuclear envelope, and intercellular space are visible. **Figure 3: Details of cells from the wall of the proximal convoluted tubule** - This figure illustrates the cellular components of the proximal convoluted tubule, the region where most reabsorption occurs. - It highlights the microvilli, which increase the surface area for reabsorption, the pinocytic vesicles that are involved in endocytosis, and the various organelles present within the cells, including the nucleus, mitochondria, and endoplasmic reticulum. - The image also showcases the basement membrane and the lumen of the blood capillary, demonstrating the close proximity of these structures to the tubular cells involved in reabsorption. **Figure 1: Counter-current multiplier of the loop of Henlé** - This figure illustrates a simplified diagram of the loop of Henlé. - Shows the descending and ascending limbs of the loop, highlighting the different regions of the loop's permeability to water. - The arrows indicate the movement of water and solutes within the loop. - Features the concentrations of various molecules, such as sodium ions and water, at different points along the loop. **Figure 2: Relative concentrations of three substances in the filtrate as it passes along a nephron** - This graph illustrates the changing concentrations of urea, glucose, and sodium ions as the filtrate travels through the nephron. - The y-axis represents the concentration of each substance relative to the concentration in the renal capsule. - The x-axis represents the different regions of the nephron, starting from the proximal convoluted tubule and ending with the collecting duct. - The graph shows that the concentration of urea remains relatively constant as the filtrate progresses, but its concentration increases relative to water as water is reabsorbed. - This indicates that urea is not reabsorbed as readily as water. - Glucose is rapidly reabsorbed from the proximal convoluted tubule. - The concentration of sodium ions increases in the descending limb as water is reabsorbed, but decreases in the ascending limb due to active transport out of the tubule. - The graph also shows that the concentration of sodium ions increases again in the distal convoluted tubule and collecting duct due to water loss and further concentration of the filtrate. 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