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Measurement of Glomerular Filtration Post Date: Fri, 17 Oct 2008 8:40:37 +0000 Finally, there are a few agents, such as dopamine, that significantly increase renal blood flow and filtration rate, but these have only limited therapeutic use. Experience has demonstrated that one can alter the rate of excretion of many substances much more effectively by drugs that alter tubular function than by those that change filtration rate.Measurement of Glomerular Filtration. Autor of the post: Undefined It is only necessary Post Date: Fri, 17 Oct 2008 8:30:20 +0000 The rate of glomerular filtration can be determined very accurately in animals and man by measuring the renal clearance of substances that are freely filtered at the glomerulus and are neither secreted nor reabsorbed by the renal tubule. Thus, the amount of the substance excreted in the urine over a given period of time is a measure of the amount filtered by the glomeruli. It is only necessary to divide this by the concentration of the substance in the filtrate to determine the volume of filtrate from which the urine was derived. Autor of the post: Undefined There is an increasing number Post Date: Fri, 17 Oct 2008 8:16:39 +0000 Inasmuch as the glomerular filtrate is an ultrafiltrate of plasma, the concentration of such a substance in glomerular fluid can be ascertained by determining the concentration in plasma.The polysaccharide inulin is accepted as the most valid substance for measuring the filtration rate under almost all circumstances. There is an increasing number of other agents that behave in a similar manner but which, under certain conditions, may or may not undergo tubular transport. Autor of the post: Undefined Certain substances at low plasma Post Date: Fri, 17 Oct 2008 8:06:32 +0000 Thus, with the introduction of a new compound for measuring filtration rate it is essential that the identity of its clearance with that of inulin be established for each particular experimental condition.Renal Plasma Flow. Certain substances at low plasma levels are secreted by the renal tubules so efficiently, and undergo so little tubular reabsorption, that essentially all of the substance is removed from the blood in a single passage through the kidney. Autor of the post: Undefined Tubular Reabsorption Post Date: Fri, 17 Oct 2008 7:49:21 +0000 It therefore follows that the total amount of such a substance appearing in the urine over a given time divided by its plasma concentration is an expression of renal plasma flow. The outstanding example of this type is para-aminohippuric acid.Tubular Reabsorption. Autor of the post: Undefined In such an individual Post Date: Fri, 17 Oct 2008 7:34:04 +0000 The importance of the reabsorptive function of the renal tubules to the body economy cannot be overemphasized and can best be illustrated by a few numerical considerations. The rate of glomerular filtration in the average adult is approximately 125 ml per minute. In such an individual, the total extracellular fluid volume is approximately 12,5 liters. Autor of the post: Undefined Therefore, the tubules normally reabsorb Post Date: Fri, 17 Oct 2008 7:21:07 +0000 Thus, a volume equivalent to that of the extracellular fluid is filtered across the glomerular capillary bed within a period of 100 minutes. During this time approximately 100 ml of urine reaches the bladder. Therefore, the tubules normally reabsorb over 99% of the glomerular filtrate. Autor of the post: Undefined The former is a physical Post Date: Fri, 17 Oct 2008 7:04:04 +0000 Obviously the composition of the tubular reabsorbate must closely approximate that of the extracellular fluid; otherwise, extreme distortions in the composition of the extracellular fluid would soon result.The mechanisms of filtration and reabsorption have little in common. The former is a physical process in which the hydrostatic energy is provided by the heart for the transport of water and permeative solute ions and molecules across a permeable membrane. Autor of the post: Undefined This involves the expenditure Post Date: Fri, 17 Oct 2008 6:47:38 +0000 Tubular reabsorption, on the other hand, bears little relationship to the mechanism by which fluid and solutes are returned to the circulation after their egress from other capillary beds. Such reabsorption is largely achieved by active transport of electrolyte and other solutes from tubular fluid to tubular cell and thence to the extracellular fluid. This involves the expenditure of energy derived from metabolic activity. Autor of the post: Undefined Although many of the intimate mechanisms Post Date: Fri, 17 Oct 2008 6:30:38 +0000 Physical forces involving the oncotic pressure of the peritubular plasma may also contribute to the reabsorption of water. The magnitude of this component is relatively small and uncertain.Although many of the intimate mechanisms of electrolyte transport are incompletely understood, for operational purposes it is possible to describe them in terms of the scheme summarized in Figure VIII-1, Considered primarily from the point of view of the action of pharmacological agents, the most important tubular mechanisms of electrolyte transport are (1) reabsorption of sodium and chloride, (2) secretion of hydrogen ion, and (3) secretion of potassium. 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