The ProximaITubuIe Reabsorbs More Than 60% of Filtered Water
The proximal tubule reabsorbs the majority of the glomerular filtrate. It takes up solutes from the tubule fluid by both active and passive means. The sodium-potassiumadenosinetriphosphatase (Na\K’-ATPase) pump in the baso- Iateral plasma membrane actively transports Na1 and drives carrier-mediated secondary active transport and passive uptake of solutes and water.
As Na’ is actively transported from the cell into the interstitial fluid, Na’ and other solutes are removed from the tubule fluid by secondary active transport or passive diffusion. Removal of solute from the tubule fluid creates a slight gradient favoring the movement of water into the cells and the intercellular spaces. The proximal tubule apical brush border and complex basolateral plasma membrane create a large surface area for reabsorption that is highly permeable to water because of abundant water channels, aquaporin-1 and aquaporin-7, in the membranes. Thus the small chemical gradient results in rapid movement of water from the tubule fluid to the interstitial fluid. The high oncotic pressure and low hydrostatic pressure in the peritubular capillaries favor the movement of reabsorbed water and solute from the interstitial fluid to the blood.During the course of a day, 32 to 37 I. of water are reabsorbed by the proximal tubules in the kidneys of our 10-kg beagle. However, because the water is reabsorbed nearly isotonically with salt, the osmolality of the tubule fluid remains similar from Bowman s space to the beginning of the thin descending limb of Henle s loop.
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