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Mizzou MPP 3202 - Chapter 20

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Chapter 20: Fluid and Electrolyte BalanceFluid and Electrolyte HomeostasisWater BalancePowerPoint PresentationUrine FormationUrine Dilution/ConcentrationTubular Fluid OsmolaritySlide 8Dilute UrineConcentrated UrineAntidiuretic HormoneADHConcentrated Urine: Distal Tubules & Collecting DuctsMedullary Osmotic GradientCountercurrent Exchanger: Maintains GradientSlide 16Chapter 20: Fluid and Electrolyte Balance•Fluid and electrolyte homeostasis•Water balanceNa+ and water ECF volume and osmolarityK+Cardiac and muscle functionCa2+Exocytosis, muscle contractions, and other functionsH+ and HCO3–pH balanceBody must maintain mass balance Excretion routes: kidney, feces, sweat, and lungsFluid and Electrolyte HomeostasisWater BalanceWATER BALANCE IN THE BODYWater gain2.2 L/dayFood and drinkSkinLungsWater lossInsensiblewater loss0.9 L/day0.3 L/dayMetabolismUrine1.5 L/dayFeces0.1 L/day2.5 L/dayTotals2.5 L/dayIntake2.2 L/dayMetabolic production0.3 L/dayOutput2.5 L/day 0THE KIDNEYS CONSERVE VOLUMEKidneys cannot restore lost volume. They only conserve fluid.Volume loss can bereplaced only by volumeinput from outsidethe body.VolumegainGFR canbe adjusted.Glomerularfiltration rate(GFR)If volume fallstoo low,GFR stops.Kidneysrecyclefluid.Body fluidvolumeCan beoffset byVolumeloss inthe urineKidneysconservevolume.Regulated H2OreabsorptionResponse to decreased blood pressure and volumeBlood volumeBlood pressureVolume receptors in atria andcarotid and aortic baroreceptorstrigger homeostatic reflexesCardiovascularsystemBehaviorKidneys Cardiac output,vasoconstrictionThirst causeswater intakeECF and ICFvolumeBloodpressureConserve H2Oto minimizefurther volumelossKEYStimulusSensorTargetTissue responseSystemic responseKEYStimulusSensorTargetTissue responseSystemic responseResponse to elevated blood pressure and volumeBlood volumeBlood pressureVolume receptors in atria,endocrine cells in atria, andcarotid and aortic baroreceptorstrigger homeostatic reflexesCardiovascularsystemKidneysCardiac output,vasodilationExcrete saltsand H2O in urineECF and ICFvolumeBloodpressureSystemic Response: Volume ChangeUrine FormationGeneralizations:Reabsorption of Na+ is an active, process driven by Na-K-ATPase.Reabsorption of Cl- is both passive and active. It is directly/indirectly coupled to Na+; parallel reabsorption of Na+ and Cl-.Reabsorption of water is by osmosis and secondary to solute reabsorption, particularly Na+. Renal excretion of water independently of solute – ADH (vasopressin)Urine Dilution/Concentration•Dilute urine is 100 mOsm; concentrated urine is 1400 mOsm.•Controlled by water and Na+ reabsorption in the distal tubules and collecting duct.•Dilute urine: reabsorb solute without water•Concentrated urine: reabsorb water without solute. The sum of waste products is 600 mOsm/day; 600 mmol/1400 mOsm/L=.43 L/dayTubular Fluid OsmolarityTubular Fluid Osmolarity•Reabsorption in the proximal is isosmotic.•Fluid entering loop of Henle is about 300 mOsm.•Filtrate at tip of loop of Henle (long loops) is about 1200 mOsm; approximately the same as the interstitium.•In the ascending limb, solute is reabsorbed, impermeable to water; the filtrate is then hypoosmotic (100 mOsm). •Fluid exiting the early distal tubule is hypoosmotic, regardless of ADH “status”.•In the collecting duct, water reabsorption is under hormonal regulation.–In the absence of ADH, no water is reabsorbed. Some solutes are reabsorbed, thus final concentration can be as low as 50-100 mOsm.–In presence of ADH, water is reabsorbed; at maximal permeability concentration can be 1200-1400 mOsm.Dilute UrineNo ADH = no water reabsorption in distal segments of tubule; elimination of dilute, hypoosmolar urine.Concentrated Urine1. High level of ADH to increase water permeability in the distal tubules and collecting ducts. 2. High osmolarity of renal medullary interstitial fluid to provide the necessary osmotic gradient for water reabsorption to occur (in presence of ADH). The countercurrent mechanism depends on the anatomical arrangement of the loops of Henle and the vasa recta.Antidiuretic HormoneADHThe effect of plasma osmolarityon vasopressin secretion105280 290 300Plasma osmolarity (mOsM)Plasma vasopressin (picomol/L)Concentrated Urine: Distal Tubules & Collecting Ducts•As tubular fluid flows into the cortical collecting tubule, the fluid is hypoosmotic. •In presence of ADH, water is reabsorped into cortical interstitium and reabsorbed into peritubular capillaries. •In the medullary collecting ducts, water is reabsorbed (but relatively less than cortical interstitium) and carried away by vasa recta.Medullary Osmotic Gradient1. Active Na+ transport and co-transport of K+, Cl- into interstitium without corresponding water movement in the thick ascending limb.2. Active transport of ions from the collecting ducts into the medullary interstitium.3. Facilitated diffusion of urea from intermedullary collecting ducts into the intersititium.4. Diffusion of small amounts of water from medullary tubules into interstitium – solutes >>> water. Medullary interstitial fluid osmolarity sets the upper limit on urine osmolarity at ~1600 mOsm. Na+ and Cl- reabsorption in the distal nephron sets the lower limit at ~ 100 mOsm. This is not static; the gradient can change in terms of low and high water intake.Countercurrent Exchanger: Maintains Gradient•Low blood flow in the vasa recta. •Blood flow moves in the opposite direction from the filtrate flow.•Descending: water moves out of vasa recta, solutes move in.•Ascending: solutes move out; water moves in.•The descending vasa recta express aquaporins, i.e. move water without solute. •Although there is a fluid and solute exchange across the vasa recta, there is little net dilution of interstitial


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Mizzou MPP 3202 - Chapter 20

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