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Proximal tubule {1)—rapid active reabsorption Post Date: Mon, 13 Oct 2008 5:29:13 +0000 One of these is shown diagrammatically. All numbers refer to the osmotic pressure (milliosmoles per kilogram of H20) of tubular or interstitial fluids during maximal ADH-induced antidiuresis in primates. Proximal tubule {1)—rapid active reabsorption of Na+ (with Cl~), high permeability to H20 and urea; descending thin limb (2)—no active ion transport, permeability to some solutes (including urea); ascending thin limb (3)—increasing passive permeability to Na+ and Cl~, decreasing permeability to H20 and urea; ascending thick limb (4)—active reabsorption of Cl~ (with Na+), impermeability to H20 and urea; distal convolution (5)—active reabsorption of Na+ (with CI"), very low permeability to H20 and urea; cortical and medullary collecting system (6)—active reabsorption of Na+ (with Cl~ or by ion exchange), high permeability to H20 and, in papillary portion, to urea in the presence of ADH (maximal at plasma levels of approximately 4 to 5 /xU/ml). Autor of the post: Undefined It should be noted Post Date: Mon, 13 Oct 2008 5:10:39 +0000 and capillary are approximately equal; that of the fluid in the chloride-pumping portion of the ascending limb is believed to be about 200 milli-osmoles per liter less. In this way a papillary osmotic pressure of 1000 milliosmoles per liter or more above that ofplasma can be achieved, although the greatest transepithelial concentration gradient required at any one point probably need not exceed 100 mEq of sodium chloride per liter, a gradient consistent with those observed in the distal convolution and collecting ducts. It should be noted that the re-absorption of electrolyte by the collecting ducts also adds solute to the medullary interstitium, as does thepassive outward diffusion of urea as the tubular urine is reduced to small volume. Autor of the post: Undefined It has been found Post Date: Mon, 13 Oct 2008 4:58:25 +0000 The latter has been shown to be facilitated by ADH. Indeed, during maximal antidiuresis, urea accounts for a consid-erable portion of the osmotically active solute pres-ent in tissue near the tip of the papilla. It has been found that the reduction of urea excretion by a protein-free diet results in a sharp reduction of maximal renal concentrating ability, which is revers-ible by the administration of urea. Autor of the post: Undefined When such a loop possesses Post Date: Mon, 13 Oct 2008 4:43:24 +0000 Thus, urea plays a passive but important role in the concentrating mechanism.The exchange of solute, water, or heat that occurs between fluids flowing in opposite directions along closely opposed limbs of a folded loop is known as countercurrent exchange. When such a loop possesses an intrinsic mechanism for active transfer between the limbs (e g, chloride transport by the epithelium of the loop of Henle), the system is known as a countercurrent multiplier. Autor of the post: Undefined " In desert rats, for example Post Date: Mon, 13 Oct 2008 4:32:04 +0000 The existence and charac-teristics of such systems have long been familiar to the physicist, but they have only recently been rec-ognized in physiological systems. For physical rea-sons, the achievable gradient increases with increas-ing length of the loop as well as the rate of active "transfer." In desert rats, for example, the papilla is greatly elongated and the maximal concentrating power of the kidney is thereby greatly increased. Autor of the post: Undefined Conversely, the high flow rate Post Date: Mon, 13 Oct 2008 4:13:50 +0000 A low rate of luminal flow also favors the development of a high gradient between the beginning and the tip of the loop. The low rate of luminal flow prevailing during dehydration therefore favors the es-tablishment of maximal papillary osmotic pressure. Conversely, the high flow rate occurring during osmotic diuresis sharply curtails the efficiency of the countercurrent multiplier. Autor of the post: Undefined The microperfusion of isolated nephron segments Post Date: Mon, 13 Oct 2008 4:00:18 +0000 The effectiveness of the system ultimately rests on the rate of sodium and chloride reabsorption in the ascending limb. The effects of aldosterone on this segment await clarifica-tion, although it is known that maximal concen-trating ability is impaired in the absence of the glucocorticoids.The microperfusion of isolated nephron segments is now elucidating the pharmacological as well as physiological characteristics of previously inaccessi-ble portions. Autor of the post: Undefined Were this area per-fused Post Date: Mon, 13 Oct 2008 3:45:00 +0000 It has been shown, for example, that active chloride reabsorption in the thick ascending limb is susceptible to inhibition by potent "natri-uretic" agents such as furosemide and ethacrynic acid {see Symposium, 1974).The vascular architecture of the medulla and papilla also deserves comment. Were this area per-fused by "straight-through" capillaries, randomly associated with the countercurrent mechanism, os-motic gradients would be rapidly dissipated. Autor of the post: Undefined It again approaches iso-tonicity Post Date: Mon, 13 Oct 2008 3:33:22 +0000 How-ever, blood perfuses the area by way of elongated capillary loops (vasa recta) that parallel the loops of Henle and constitute a countercurrent exchange system. Plasma in the capillary loops equilibrates passively as it passes through the zone of high inter-stitial osmotic pressure. It again approaches iso-tonicity as ascending capillaries emerge from the medulla, carrying away only small amounts of the accumulated solutes. Autor of the post: Undefined (For details of the evolution Post Date: Mon, 13 Oct 2008 3:19:58 +0000 It is believed that elevated rates of medullary blood flow, however, may partiallydissipate the osmotic gradient generated by the countercurrent multiplier.Observation of the high solute content of renal papillary tissue first led to the proposal of the coun-tercurrent mechanism by Wirz and coworkers (1951) as well as by Hargitay and Kuhn (1951). (For details of the evolution of present concepts, see Berliner and Bennett, 1967; Symposium, 1974. 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