The volume, composition, and osmolality of the glomerular filtrate all change during its flow through the renal tubules. Approximately 80% of the filtrate is reabsorbed at the early stage, in the proximal tubule. Sodium is reabsorbed in the tubule by several mechanisms: through specific ion channels, in exchange for the hydrogen ion, and in cotransport with glucose , amino acids, phosphate, and other anions. The entry of sodium into the proximal tubular cells is passive which is possible because of its low concentration in the cytoplasm of tubular cells which is maintained by the Na+/K+-ATPase localized on the 'blood side' (basolateral membrane) of the tubular cells. Na+/K+-ATPase catalyzes active extrusion of sodium to the interstitial fluid. The sodium transport causes reabsorption of water from the tubule lumen to the surrounding extracellular space.
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| Because of the equal reabsorption of sodium and water, the fluid leaving the proximal tubule is isotonic. Further along, in the descending limb of the loop of Henle, water diffuses from the tubules to the hyperosmotic renal medulla, and some sodium and chloride diffuse back into the tubular lumen. In contrast, the ascending limb is impermeable to water, but actively reabsorbs sodium and chloride. This generates hyperosmolality in the surrounding medulla. The different permeability characteristics of the descending and ascending limbs of the loop of Henle maintain the high osmolality of the medulla essential for the efficient reabsorption of water later; this is known as the countercurrent system (Fig. 22.9). Countercurrent multiplication builds up an osmotic gradient in the medulla through the different permeability characteristics of descending and ascending loop of Henle.
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| Because of the relatively greater absorption of sodium than water, the tubular fluid that leaves the loop of Henle is hypotonic. In the distal tubule and in the collecting duct, still more sodium is reabsorbed in exchange for potassium or hydrogen ion. This is controlled by the hormone aldosterone (Fig. 22.10). Further in the distal tubule and in the collecting duct water is reabsorbed along the osmotic gradient created by countercurrent multiplication; this is known as the countercurrent exchange. The water reabsorption in the collecting duct is controlled by vasopressin (see below).
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