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USC BISC 307L - Kidney Function
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Overview of Kidney Function4 jobs1. Filtrationleakage of fluid out of the capillaries into the glomerular tubulelike plasma minus the proteins2. Reabsorptionfrom glomerular tubule back into the capillarymost of the saltmost of the waterglucose, amino acidsgeneral process3. Secretionfrom blood into the lumen of the tubulemore of a selective processconcentration can change due to this. Excretionwhat comes out of the end of the collecting duct into the renal pelvis unaltered and is excretedIn loop of henle only reabsorption occurs not secretion180 L/day are filtered out of the plasma into bowmans capsule – all of your plasma is filtered 60 times a day so most of it has to be recoveredover 99% of the fluid that is being filtered is reabsorbed300mOsM is the same as plasmaover 70% of the fluid is reabsorbed by the end of the proximal tubule by active transport of solutes and water follows by osmosisbecause of the way the fluid is reabsorbed (transport of mostly salt) the osmolarity should stay the same and it doesby the end of the loop of henle the osmolarity is 100mOsM because so much salt was reabsorbed that the osmolarity went down- this is the only part of the nephron that can produce a hypoosmotic system- because sometimes you need to get rid of excess waterthe osmolarity at the collecting duct varies due to reabsorption of water (can be as high as 1200)Renal CorpuscleIn order to understand look at the poking finger analogyThe outer wall is pretty non porous but the inner wall is very porousPodocytes form filtration slits in the inner wall3 barriers that the fluid goes through1.pores in the endothelium2. Basal lamina3. Spaces in between the podocytes to enter the lumen of bowmans capsulepodocytes can change their shape and a masangial cell can also change shape and this movement can change the width of the filtration slitsFiltration FractionIf the total plasma volume entering is 100% , not all of that plasma leaks into bowmans capsuleOnly about 20% of plasma volume entering gets filtered and the other 80% stays in the bloodSo 20% of the volume that gets filtered over 19% gets reabsorbedFiltration PressurePressure gradientPH= 55mmHg which is very high pressure for a capillaryThe total pressure is the blood pressure driving it out minus the osmotic pressure exerted by protein and minus the back pressure in bowmans capsule due to the fact that this is happening in a closed space (55-30-15= 10- net filtration pressure)This is what is causing the fluid to move out of the capillariesOne of the reasons that blood pressure has to be so high in the kidney capillaries is because the glomerular capillaries are all about filtration not reabsorption need it high enough so that even at the downstream end it will be pushing blood outBy the time we get to the peritubular capillaries, the pressure is so low that it strongly favors reabsorption of fluid when it gets to the proximal tubuleThe net filtration pressure is going to drive a glomerualr filtration rate of 180L/day and this is an important measurement of kidney function- this is maintained at about this level over a wide range of blood pressureHigh blood pressure can cause kidney damageImportant that these structures are no overwhelmed by too much flowRegulation of filtration rate1. Movement of podocytes to change the resistance by which the net filtration pressure pushes out2. Constriction of afferent and efferent arteriolesTop: renal blood flow is coming from the same vessels that supply the rest of the body.When you have vasoconstriction of the afferent arterioleincreases resistance to blood flow decreases total blood flowdecrease blood pressurefiltration rate downVasoconstriction of efferent arteryblood dams up upstreamcapillary blood pressure is higher than normalincreased filtration rate3. Extrinsic and intrinsic mechanisms1.Extrinsic: outside the kidneysympathetic nervescauses vasoconstriction and shut down of filtration rate (Hemorrhage and dehydrationreduces fluid loss)Also circulating hormones that control this2. Intrinsic:1. Myogenic response to stretch of smooth musclewhen they are stretched by high blood pressure they stretchvasocconstrictionincreases resistance to flowflow rate maintained.2. Juxtaglomerular apparatusnext slideJuxtaglomerular ApparatusIntrinsic regulation of filtration rateAt the top of the ascending limb of the loop of henle passes in between the fork of the afferent and efferent arterioleThe epithelial cells that touch the arterioles are modified and the smooth muscle cells that surround the arterioles are also modified- called granular cells or JG cellsThe macula densa cells are sensory tissue and the JG cells are hormone tissueThe macula densa is sensing the flow rate of salt through the tubule and in response they release paracrine hormones (ATP, prostaglandin E2 and NO) and the target cells of these are the smooth muscle cells (contractile-vasoconstriction and granular cells- secrete renin) Renin is an enzyme and activates a hormone activates the angiotensin pathwayTransport and ExcretionGeneral Mechanisms of ReabsorptionReabsorption of most stuff1. Na+ reabsorbed by active transport to ECF2. Creates electrical gradient that drives the movement of anions into the ECF (paracellular route or transcellular)3. Also all of this Na+ and CL- being moved creates an osmotic gradient which drives the reabsorbtion of water4. K+ and Ca+ and urea are left behind which creates a concentration gradient that drives their movement back into the ECFUrea is about 50% reabsorbedReabsorption of Na+, Glucose, HCO3- etcMechanisms for the reabsorption of Na+Every place in the proximal tubule, what is driving this is Na+/K+ pumps in the basolateral membrane pumping sodium out of the basolateral membraneThey are replaced by: depends on what part of the tubule you are inIn the proximal tubule a Na+/H+ antiport pumps Na+ into the cell and there are also Na+ dependent glucose and amino acid transporters that pump Na+ into the cellThere is also a Na+ inorganic phosphate (PO4, H2PO4) cotransport which moves both into the cellBrings with is phosphate so it is a phosphate reabsorbing transporter and this is inhibited strongly by PTHBISC 307L 1st Edition Lecture 36 Current Lecture- Overview of Kidney Functionoo 4 jobs 1. Filtration leakage of fluid out of the capillaries into the glomerular tubule like plasma minus the proteins  2. Reabsorption from glomerular tubule back


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