Urine, creation

Uric acid kidney stones, 475, 478,730 Uridine, chemical structure, 14 Urine, Creation of, 113, 205, 711... 

An essential requirement for diffusion of Na+ ions is the creation of a concentration gradient for sodium between the filtrate and intracellular fluid of the epithelial cells. This is accomplished by the active transport ofNa+ ions through the basolateral membrane of the epithelial cells (see Figure 19.4). Sodium is moved across this basolateral membrane and into the interstitial fluid surrounding the tubule by the Na+, K+-ATPase pump. As a result, the concentration of Na+ ions within the epithelial cells is reduced, facilitating the diffusion of Na+ ions into the cells across the luminal membrane. Potassium ions transported into the epithelial cells as a result of this pump diffuse back into the interstitial fluid (proximal tubule and Loop of Henle) or into the tubular lumen for excretion in the urine (distal tubule and collecting duct). 

Q4 An important process in concentrating urine is the creation of an osmotic gradient in the renal medulla. This is produced by active pumping of salts out of the ascending limb of the loop of Henle, without accompanying water, since the ascending limb is impermeable to water. The pump involves a Na+/K+/ 2C1 coupled co-transporter located in the ascending limb cells, which moves... 

Active transport takes place in the proximal and distal tubules, and most notably in the ascending limb of the Loop of Henle. This transport is almost exactly balanced by back-diffusion of Na" from the tissue into the descending limb. The net result is the creation of a standing external concentration gradient, which allows water to pass from the tubular urine into the surrounding tissue by osmosis (i.e., to be "reabsorbed"). 

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