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Richards and Homer Smith Post Date: Sun, 12 Oct 2008 9:38:18 +0000 , that unequivocally demonstrated the secretion of this dye by the normal kidney. The secretion of other organic acids such as iodopyracet and para-amino-hippuric acid (PAH) was then confirmed by Marshall and others, notably by A. Richards and Homer Smith. Autor of the post: Undefined The importance of the renal tubular Post Date: Sun, 12 Oct 2008 9:23:00 +0000 The reabsorption of organic as well as inorganic solutes by the renal tubule, although naively defined and unmeasured, had necessarily been invoked in all the proposed theories of renal function that included the formation of a glomerular filtrate. The development of technics for tubular micropuncture and accurate clearance measurements and the study of isolated renal tissue have now allowed detailed description of the functional characteristics of the transport of organic solutes out of, as well as into, the tubular lumen.The importance of the renal tubular reabsorption of organic substances is immediately apparent when one considers that most of the nutrients and vitamins in solution in extracellular fluid gain access to the glomerular filtrate. Autor of the post: Undefined One of the most familiar Post Date: Sun, 12 Oct 2008 9:12:23 +0000 Resources of glucose, amino acids, vitamins, and other essentials would soon be dissipated if these substances were not returned from the tubular urine to body fluids. The efficiency of the kidney in this respect is evident in the absence of body essentials in the urine unless presented to the tubules in unusually large amounts. One of the most familiar examples is the tubular reabsorption of glucose. Autor of the post: Undefined These can gain access Post Date: Sun, 12 Oct 2008 9:00:14 +0000 This process is so efficient that virtually complete removal of the sugar from the tubular urine occurs unless the concentration in the glomerular filtrate, and hence the load delivered to the tubules, is so high that the transport capacity is exceeded.On the other hand, many organic compounds that reach the kidney are destined to be excreted. These can gain access to the urine either by filtration and incomplete reabsorption or by tubular secretion. Autor of the post: Undefined Many substances gain egress Post Date: Sun, 12 Oct 2008 8:44:56 +0000 The most casual reflection leads to the conclusion that the mechanisms of renal transport must indeed be numerous and complex if substances necessary for the body economy are to be retained and foreign substances plus those end products of metabolism that no longer serve a useful purpose are to be rejected. It is not to be inferred that aU excretory functions of the kidney depend upon active tubular transport systems. Many substances gain egress from the body by filtration alone. Autor of the post: Undefined Since the secretion of PSP was Post Date: Sun, 12 Oct 2008 8:34:34 +0000 Their rate of excretion depends upon the extent to which passive back diffusion occurs. At the other extreme are com-pounds that not only reach the urine by means of glomerular filtration but also are secreted by the renal tubular cells with such efficiency that a single passage through the kidney eliminates them from the renal blood.Since the secretion of PSP was first proven, little has been learned of the nature of the intracellular mechanisms that accomplish the unidirectional movement of such substances across the renal tubu-lar epithelium. Autor of the post: Undefined Secretion has been shown Post Date: Sun, 12 Oct 2008 8:23:36 +0000 However, a monumental accumula-tion of experimental and clinical studies has de-scribed the active transport of many organic substances. It has been found that reabsorption is accomplished by numerous independent transport systems that tend to be selective for one substance or for a closely related group of substances, such as the amino acids. Secretion has been shown to be attributable largely to two systems. Autor of the post: Undefined These processes occur throughout Post Date: Sun, 12 Oct 2008 8:10:36 +0000 One transports the organic acids already mentioned as well as salic-ylates, penicillins, many diuretics, sulfonamides, and certain metabolites (such as sulfate esters and glucuronides) of drugs and naturally occurring sub-stances. The other transports bases such as hista-mine, thiamine, hexamethonium, tolazoline, and many other naturally occurring and synthetic amines. These processes occur throughout the proximal tubule but are believed not to occur in the distal tubule. Autor of the post: Undefined It is completely de-pendent Post Date: Sun, 12 Oct 2008 7:54:17 +0000 The clearance data for some substances, for example, uric acid, suggest bidirectional active transport. Organic solute transport proceeds quite independently of the simultaneously occurring re-absorption of inorganic solutes. It is completely de-pendent on energy from oxidative metabolism. Autor of the post: Undefined Weiner and associates (1964) Post Date: Sun, 12 Oct 2008 7:42:47 +0000 In addition, the secretion of organic solutes is specifi-cally dependent on the availability of adenosine triphosphate (ATP). It has been shown that the rate of transport of otherwise-independent active trans-port systems may be affected by changes in the amount of substrate presented for transport by other systems. Weiner and associates (1964) have investi-gated the structural features of organic acids that favor active transport. 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