The kidneys produce a small volume of urine when the plasma concentration of vasopressin is high. Under these conditions, the urine is concentrated (hyperosmotic) relative to plasma. The ability of the kidneys to produce hyperosmotic urine is a major determinant of the ability to survive with limited water intake. Urinary concentration takes place as tubular fluid flows through the medullary collecting ducts. The interstitial fluid surrounding these ducts is very hyperosmotic. In the presence of vasopressin, water diffuses out of the ducts into the interstitial fluid of the medulla and then enters the blood vessels of the medulla to be carried away.
The proximal tubule always reabsorbs Na+ and water in the same proportions. Along the entire length of the ascending limb, sodium and chloride are reabsorbed from the lumen into the medullary interstitial fluid. In the upper (thick) portion of the ascending limb, this reabsorption is achieved by transporters. However , in lower (thin) limb, this is a passive process.
The ascending limb is relatively impermeable to water. The net result is that the interstitial fluid of the medulla becomes hyperosmotic compared to the fluid in the ascending limb because solute is reabsorbed without water.
The descending limb does not reabsorb NaCl and is highly permeable to water. Therefore, a net diffusion of water occurs out of the descending limb into the more concentrated interstitial fluid until the osmolarities are equal again.
The loop countercurrent multiplier causes the interstitial fluid of the medulla to become concentrated. It is this hyperosmolarity that will draw water out of the collecting ducts and concentrate the urine. The countercurrent multiplier system concentrates the descending- loop fluid, but then lowers the osmolarity in the ascending loop so that the fluid entering the distal convoluted tubule is actually more dilute (hypoosmotic) - 100 mOsmol/L than the plasma. The fluid becomes even more dilute during its passage through the distal convoluted tubule. This hypoosmotic fluid then enters the cortical collecting duct. In the presence of vasopressin, water reabsorption occurs by diffusion from the hypoosmotic fluid in the cortical collecting duct until the fluid in this segment becomes isosmotic to the interstitial fluid and peritubular plasma of the cortex i.e., until it is once again at 300 mOsmol/L. The isosmotic tubular fluid then enters and flow through the medullary collecting ducts. In the presence of high plasma concentration of vasopressin, water diffuses out of the ducts into the medullary interstitial fluid as a result of the high osmolarity that the loop countercurrent multiplier system and urea trapping establish there. This water then enters the medullary capillaries and is carried out of the kidneys by the venous blood. Water reabsorption occurs all along the lengths of the medullary collecting ducts so that, in the presence of vasopressin, the fluid at the end of these ducts has essentially the same osmolarity as the interstitial fluid surrounding the bend in the loops - that is, at the bottom of the medulla. By this means, the final urine is hyperosmotic.
The proximal tubule always reabsorbs Na+ and water in the same proportions. Along the entire length of the ascending limb, sodium and chloride are reabsorbed from the lumen into the medullary interstitial fluid. In the upper (thick) portion of the ascending limb, this reabsorption is achieved by transporters. However , in lower (thin) limb, this is a passive process.
The ascending limb is relatively impermeable to water. The net result is that the interstitial fluid of the medulla becomes hyperosmotic compared to the fluid in the ascending limb because solute is reabsorbed without water.
The descending limb does not reabsorb NaCl and is highly permeable to water. Therefore, a net diffusion of water occurs out of the descending limb into the more concentrated interstitial fluid until the osmolarities are equal again.
The loop countercurrent multiplier causes the interstitial fluid of the medulla to become concentrated. It is this hyperosmolarity that will draw water out of the collecting ducts and concentrate the urine. The countercurrent multiplier system concentrates the descending- loop fluid, but then lowers the osmolarity in the ascending loop so that the fluid entering the distal convoluted tubule is actually more dilute (hypoosmotic) - 100 mOsmol/L than the plasma. The fluid becomes even more dilute during its passage through the distal convoluted tubule. This hypoosmotic fluid then enters the cortical collecting duct. In the presence of vasopressin, water reabsorption occurs by diffusion from the hypoosmotic fluid in the cortical collecting duct until the fluid in this segment becomes isosmotic to the interstitial fluid and peritubular plasma of the cortex i.e., until it is once again at 300 mOsmol/L. The isosmotic tubular fluid then enters and flow through the medullary collecting ducts. In the presence of high plasma concentration of vasopressin, water diffuses out of the ducts into the medullary interstitial fluid as a result of the high osmolarity that the loop countercurrent multiplier system and urea trapping establish there. This water then enters the medullary capillaries and is carried out of the kidneys by the venous blood. Water reabsorption occurs all along the lengths of the medullary collecting ducts so that, in the presence of vasopressin, the fluid at the end of these ducts has essentially the same osmolarity as the interstitial fluid surrounding the bend in the loops - that is, at the bottom of the medulla. By this means, the final urine is hyperosmotic.
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