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 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 produce hypoosmotic 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 be reached This is important for creating small amounts of highly concentrated urine in order to preserve water in the body Renal Corpuscle The 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 podocytes interdigitate their processes to form filtration slits If you look in detail at the cross section on the bottom left and focus even more on the filtration slits you would end up with the cross section on the bottom right and see that there are three barriers that the fluid goes through The slits in pores in the endothelium these are fenestrated capillaries where there are pores going right through the endothelium 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 is the 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 Fraction If 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 And since most of that fluid is reabsorbed only 1 of volume is actually excreted to the external environment Filtration Pressure Where is the pressure gradient for filtration coming from It comes from the same forces that describe how fluids leak out of capillaries everywhere Blood enters the afferent arteriole and is delivered to capillaries under pressure 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 reasons that the blood pressure in the capillary has to be so high in the kidney is