Formation of Dilute Urine
Glomerular filtrate has the same ratio of water and solute particles as blood; its osmolarity is 300mOsm/litre. When dilute urine is being formed, the osmolarity of the fluid in the tubular lumen INCREASES (~ 600mOsm/litre) as it flows down the descending limb of the loop of Henle, and DECREASES (~150) as is ascends up the ascending limb and DECREASES (~70) still more as it flows through the rest of the nephron and collecting duct. These changes result from the following: 1. When osmolarity of the interstitial fluid becomes progressively greater, more and more water is reabsorbed by osmosis as tubular fluid flows along the descending limb toward the tip of the loop . As a result, the remaining fluid in the lumen becomes progressively more concentrated. 2. Cells lining the thick ascending limb have symporters that actively reabsorb Na+, K+ and Cl- from tubular fluid. The ions pass from the tubular fluid into thick ascending limb cells, then into interstitial fluid, and finally some diffuse into the blood inside the vasa recta. 3. In the thick ascending limb, solutes are being reabsorbed but water permeability of this portion of the nephron is always quite low, so water cannot follow by osmosis. As these solutes (and not H20) leave tubular fluid, its osmolarity drops to about 150. The fluid entering the DCT is thus more dilute than the plasma. 4. Fluid continues to flow along the DCT, and additional solutes but only a few H20 molecules are reabsorbed. The early DCT cells are not very permeable to water and are not regulated by ADH at this time. 5. The principal cells of the late DCT and collecting ducts are impermeable to water when ADH level is very low . Thus, tubular fluid becomes progressively more dilute as it flows onward. By the time the tubular fluid drains into the renal pelvis, it concentration can be as low as 70 mOsm/litre . This is 4x more dilute then blood plasma or glomerular filtrate.
Formation of Concentrated Urine
When water intake is low or water loss is high (heavy sweating), the kidneys
must conserve water while still eliminating wastes and excess ions. Under the influence of ADH, the kidneys will produce a small volume of concentrated urine which can be 4x more concentrated then blood plasma or glomerular filtrate.
The ability of ADH to cause this excretion of concentrated urine depends upon the presence of an Osmotic Gradient of solutes in the interstitial fluid of the renal medulla.
Glomerular filtrate has the same ratio of water and solute particles as blood; its osmolarity is 300mOsm/litre. When concentrated urine is being formed, the osmolarity of the fluid in the tubular lumen INCREASES (~ 1200mOsm/litre) as it flows down the descending limb of the loop of Henle, and DECREASES (~150) as it ascends up the ascending limb. With the presence of ADH
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