kidneys' ability to excrete water in excess of solute. C^o is the difference between the urine flow (UF) and Cosm.
The loop of Henle and the early distal convoluted tubule dilute urine to about one-third the osmolality of the plasma. Because the fluid entering the loop from the proximal tubule is nearly isosmotic, the loop reabsorbs a hyper-osmotic fluid into the medulla, thus raising its interstitial osmolality.
The slow medullary blood flow and the arrangement of the capillaries in the long hairpin loops (vasa recta) allow for counter-current exchange, which minimizes dilution of the medullary interstitium by equilibration with the blood plasma.
Although the ascending limb of the loop of Henle can generate at most a 200 mOsm/kg
H2O transepithelial osmolality difference, countercurrent multiplication establishes the 900 mOsm/kg H2O gradient of interstitial osmolality from the corticomedullary junction to the tip of the papilla.
Urea provides half the osmolality of the medullary interstitium. The nephron is impermeable to urea from the tip of the loop of Henle to the inner medullary collecting duct. Thus, during antidiuresis the urea concentration in the urine in the inner medullary collecting duct exceeds 600 mmol/L, producing a favorable gradient for passive urea diffusion from the urine into the medulla. The urea permeability of the inner medullary collecting duct is increased by vasopressin, thus allowing urea reabsorption to occur.
one of the most important functions of the kidney is the regulation of the total body water content (volume) and the osmolality of the body fluid compartments. Because most cell membranes are highly water permeable, any water is rapidly distributed among the various body fluid compartments, and a water excess dilutes them equally. Similarly, in dehydration, the osmolality of all body fluid compartments rises equally. Changes in the total body water content are reflected by changes in the osmolality of the plasma. The osmolality of the body fluid compartments is determined by the total body solute content divided by the total body water volume. In response to changes in the plasma osmolality, the kidney regulates the osmolality of the urine to restore the normal plasma osmolality.
The volume of water excreted by the kidneys depends on the amount that is reabsorbed in the segments of the nephron from connecting tubule to the ducts of Bellini, which are the same nephron segments that constitute the aldosterone-responsive distal nephron (ARDN). Water reabsorption in these segments is controlled by vasopressin (more precisely, arginine vasopressin in man and most mammals; also called antidiuretic hormone or ADH), which regulates the water permeability of the luminal membranes of the CNT and principal cells. In addition, thirst regulates the intake of water. The same stimuli that produce changes in plasma vasopressin levels also regulate the thirst drive that increases water intake when the plasma osmolality rises. This chapter considers those mechanisms involved in regulating thirst and vasopressin secretion, the action of vasopressin on the kidney, and the mechanism by which the kidney concentrates and dilutes the final urine.
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