BISC 307L 2nd Edition Lecture 37 Current LectureOverview of Kidney FunctionAt broadest level, the kidney performs four processes: filtration, reabsorption, secretion, and excretion.Keep in mind that thekidney tubule is just atubule, the walls of whichare made up of a singlelayer of epithelial cells.Afferent arteriole bringsblood in, efferent arteriolescarry it out, and thesecapillary beds form a portalsystem.Filtration is leakage of fluidout of the glomerularcapillaries into the lumenof the tubule. The fluid thatcomes 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 thebowman’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. Butin 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 volumecoming 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 from 54L. And if you look at the osmolarity, it has changed to 100 mOsM. This is because so much salt got reabsorbed, the osmolarity of the fluid actually went down. And this is the only part of the nephron that can produce a hpyoosmotic tubular content. It can get as low as 50 mOsM. And this is important because there are times when your body has to get rid of excess water. But you can’t afford to wash all the salt out of your body along with it. So the ability of the nephron to producehypoosmotic urine, with high water content, is a safe way to get rid of water without getting rid of all your solutes. More reabsorption occurs in the distal tubule and the upper part of the collecting duct, and you end up with the 1.5 L/d leaving the body. If the fluid passes through the collecting duct without being changed, then the osmolarity stays at 100. But reabsorption can occur such that levels up to 1200 mOsM can bereached. This isimportant for creatingsmall amounts ofhighly concentratedurine, in order to preserve water in the body. Renal CorpuscleThe Renal Corpuscle forms starting with the blind ended tubule at number 1 with an expanded end. The finger of evolution comes in and invaginates one side of the expanded end, and you end up with number four, a double walled cup. The outer wall is pretty nonporous. The inner wall is highly porous though. It consists of podocytes with extensions coming out the sides, and it looks like the teeth of a comb. And adjacent podocytesinterdigitate theirprocesses to formfiltration slits. If youlook in detail at thecross section on thebottom left, andfocus even more onthe filtration slits,you would end upwith the crosssection on thebottom right andsee that there arethree barriers thatthe fluid goesthrough. The slits inpores in theendothelium – these are fenestrated capillaries where there are pores going right through theendothelium. Then it goes through the basal lamina, the ECM protein, and then through the spaces between the filtration slit between the podocytes processes to, enter the yellow which isthe lumen of the bowman capsule. The podocytes have contractile protein in them so they can change their shape. The mesanglial cell, which connects adjacent capillaries, is also contratile and also can change shape. Movement of the mesanglial cells and podocytes can change the width of the filtration slits, making them bigger or smaller and therefore adjusting the filtration that is occurring. Filtration FractionIf the total plasma volume entering the afferent arteriole is 100%, not all of that plasma leaks into the bowman’s capsule. Typically, about 20% of the plasma volume enters the lumen, and the other 80% stays in the blood. Andsince most of that fluid is reabsorbed,only <1% of volume is actually excretedto the external environment. Filtration PressureWhere is the pressure gradient forfiltration coming from? It comes fromthe same forces that describe howfluids leak out of capillarieseverywhere. Blood enters the afferent arteriole andis delivered to capillaries underpressure. The blood pressure is PH, and it is 55 mmHg. These are the highest-pressure capillaries in the body. The total pressure forcing fluid out of the capillaries is due to the blood pressure pushing it out, the colloid osmotic pressure due to proteins in plasma puling it back, and the back pressure in the bowman’s capsule due to the fact that the filtration is occurring into a closed space. That back pressure is equivalent to another 15mmHg. So you are left with 10mmHg aas the pressure gradient pushing the fluid and making it move. It is not a lot, but it is enough. One of the