Kidney Function Figs and Transport & Excretion Figs

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Kidney Function Figs and Transport & Excretion Figs

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 37 Current Lecture Overview of Kidney Function At broadest level, the kidney performs four processes: filtration, reabsorption, secretion, and excretion. Keep in mind that the kidney tubule is just a tubule, the walls of which are made up of a single layer of epithelial cells. Afferent arteriole brings blood in, efferent arterioles carry it out, and these capillary beds form a portal system. Filtration is leakage of fluid out of the glomerular capillaries into the lumen of the tubule. The fluid that comes out of the tubule is similar to plasma, minus the proteins. Reabsorption is shown by the green arrow pointing from the lumen of the tubule back into capillaries, and it refers to movement of material from the tubule back into the blood were it originally came from. That happens to most of the salt, water, all of the important organic molecules, like glucose and A.a’s. Reabsorption occurs back into the same blood from which the material was filtered in the first place. Secretion works in the opposite direction, going from the blood plasma into the lumen of the tubule. In general, reabsorption is a nonselective process and a lot of solutes get reabsorbed. Secretion, however, is much more selective. Now imagine if the material in these capillaries was not reabsorbed, but a lot of the water was reabsorbed. And imagine that a lot of molecules get secreted into the tubule - the concentration in the tubule can go very high. Excretion is the secretion of fluid out to the bladder and the external environment. Simple equation is shown above: amount filtered- amount reabsorbed + amount secreted = amount of solute excreted. To the left is a more realistic drawing of the nephron. It has all the parts. Filtration happens out of the glomerular capillaries of the bowman’s capsule. You can see from the green and lavender arrows that reabsorption and secretion occur in the proximal, distal tubules and the collecting duct. But in the loop of henle only reabsorption occurs. 180L /d are filtered out of plasma into capsule. The total volume of plasma in your body is around 3L. So the entire plasma volume is filtered 60x a day. You cannot afford to lose 60x your plasma volume a day. So most of it has to be recovered. You pee out 1.5L/d, and you filter 180L a day, so very little is being excreted per cycle. The concentration of fluid leaking out of the capillaries is 300 mOsM in the proximal tubule, which is the same concentration as plasma without proteins. And if you look at the total volume coming out of the bowman’s capsule and compare it with the 54L/d hitting the loop of henle, you can see that the proximal tubule is doing the bulk of the reabsorption. That fluid at the end of the proximal tubule enters the loop of henle where only reabsorption occurs. At the top of the loop of henle, you are left with 18L ...

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