And secretion occur in the renal tubules glomerular

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and secretion occur in the renal tubules.Glomerular FiltrationFiltration in the glomerulus takes place across the very porous membrane that lies between thecapillaries and the interior of the glomerular capsule. This is mechanical filtration, not energy-requiring, in which fluids and solutes (such as water, glucose, amino acids, and nitrogenouswastes) are forced through the membrane by the high hydrostatic blood pressure in the capillarybed. The capillary pores prevent passage of blood cells and most blood proteins are alsoprevented from moving across the filter. The loss of all of the water from the blood plasma isprevented by retaining the plasma proteins in the capillaries since they maintain the osmoticpressure of the glomerular blood.
The normal glomerular filtration rate (GFR) is 120-125 ml/min or 180 L/day due to the hugesurface area of glomerular capillaries, the large degree of filtration membrane permeability andthe moderate net filtration pressure. The GFR is increased by an increase in the arterial (andglomerular) blood pressure in the kidneys and is decreased by an increase in glomerular osmoticpressure most often caused by dehydration. Maintenance of a relatively constant GFR isimportant for adequate reabsorption of water and other needed substances from the filtrate andfiltration of wastes since if flow is very rapid, needed substances cannot be adequatelyreabsorbed and if flow is too slow nearly all of it is reabsorbed, including most of the wastes thatare normally disposed of.There are three mechanisms that regulate renal flow and thereby regulate the GFR: renalautoregulation, nervous system control, and hormone control.Under normal conditions, the GFR is controlled by regulation of the diameter of the afferent andthe efferent arterioles allowing the kidney, through its autoregulatory system, to determine itsown rate of blood flow. By means of this autoregulatory system, the kidney can maintain a fairlyconstant GFR despite variations in the arterial blood pressure of the rest of the body.Occasionally it becomes necessary to divert blood to the heart, brain and skeletal muscles andduring these times the renal autoregulatory system may be superseded by nervous systemcontrols. In this event, a narrowing of the afferent arterioles is caused by sympathetic nervefibers and release of epinephrine by the adrenal medulla with a subsequent decrease renal flowand the GFR.A third mechanism, one that is hormonal in nature, controls the renal flow. This is the renin-angiotensin mechanism in which the enzyme renin is released by the juxtaglomerular cells inresponse to a drop in the body's blood pressure or direct stimulation of the JG cells by the renalsympathetic nerves. Renin causes constriction of the afferent and efferent arterioles, a rise in thebody's blood pressure and also stimulates the release of aldosterone thereby causing the renaltubules to reclaim more sodium ions (and subsequently water) from the filtrate. All of these

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Term
Fall
Professor
Rebekah Stepp
Tags
renal corpuscle, peritubular capillaries, renal pelvis, loop of Henle, renal medulla

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