The Mammalian KidneyKidneys maintain salt and water balance -maintain overall concentration of solutesmaintain concentration of specific solutesKidneys filter blood to remove unwanted solutesremoval of solutes requires some loss of water - the mammalian kidney allows solute removal with minimal water lossPaired organs located inlower back regionReceives blood from renalartery, produces urineUrine drains through ureterto urinary bladderAdrenal gland producesseveral hormonesInternal structure of kidneyMouth or ureter forms funnel-like renal pelvisCup-shaped extensions receive urine from renal tissueRenal tissue composed of outer renal cortex and inner renal medullaFunctional unit is the nephroneach kidney contains one million nephronsjuxtamedullary nephrons have long loops that descend deep intomedullacortical nephrons have shorter loopsNephron has tubular and vascular componentsThe tubular components include Bowman’s capsule, the proximalconvoluted tubule, the loop of Henle, the distal convoluted tubule,and the collecting duct.The vascularcomponentincludes theafferent arteriole,glomerulus,peritubulecapillaries, andefferent arteriole.Afferent arteriole carries blood to capillary tuft called glomerulusAt the glomerulus the blood is filtered - pressure forces fluid and small solutes blood through capillary walls into Bowman’s capsule - cells and large plasma proteins cannot pass throughFiltrate travelsthrough tubules ofnephroneach region plays arole in recovery ofvaluable solutesand waterrecovered waterand solutesreturned tocirculatory systemLarge amounts of water and molecules form glomerular filtrateFiltrate enters first part of tubular component, Bowman's capsule Capsule surrounds glomerulus Capsule has pores through which fluid passes to nephron tubules Humans produce 180 l of glomerular filtrate each day - nearly all of the water is recovered Much of the water is recovered through the creation of an osmoticgradientBowman’scapsuleProximalconvoluted tubuleDescending loopof HenleAscending loop ofHenleDistal convolutedtubuleCollecting ductUreterPath of filtrate and tubule regionsFiltrate first enters proximal convoluted tubule (PCT)2/3 NaCl and water in capsule reabsorbed immediatelymost valuable solutes reabsorbed by active transport or diffusionNa+ actively transported out of filtrate into blood vesselsCl- passively follows Na+ by electrical attractionWater follows both because of osmosisFiltrate is still isotonic to blood plasma1/3 of volume remains - ~60 l of fluid dailyadditional water must be reabsorbedoccurs mostly across wall of collecting duct through osmosisrenal medula is made hypertonic to filtrate in collecting duct by active transport of Na+ out of ascending Loop of Henlewater drawn outof collectingduct by osmosisremainingfiltrate (urine) ishypertonic tobloodReabsorption dependent on hypertonic renal medullaSteeper gradient will cause more water reabsorptionLoop of Henle creates hypertonic conditions in renal medullaAscending limb of loop actively pumps Na+ out, Cl- followsAscending limb not permeable to water so as Na+ exits, fluid in ascending limb gets more dilute,Surrounding tissue becomes more concentrated, hypertonicNaCl remains in medulla because ofvasa recta - an arrangement ofcapillaries that creates a countercurrentmultiplier systemAs blood moves through capillaries, descending into medulla, theconcentration of Na and Cl increases -As blood ascends out of medulla - the Na and Cl it holds diffusesinto blood descending into the medullaThe blood leavesthe renal medullawith the same Naand Clconcentration ithas when itentered.Because of fluid in tissues of the medulla is hypertonic to the fluid inthe collecting duct and in the descending loop of Henle, water can bedrawn out of the filtrate and taken away by the circulatory systemSome urealeaves thecollecting ductby diffusion andcontributes toosmoticgradient inmedullaRemaining ureaand othersolutes leave inurine
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