AHS 1221Urine ConcentrationDr. Suzanah Abdul RahmanKulliyyah of Allied Health SciencesInternational Islamic University MalaysiaKuantan

ObjectivesTo describe the processes in the formation of urineTo describe the involvement of renal tubule and collecting ducts in the production of dilute and concentrated urine

Function of kidneysThe kidneys play an important role in homeostasis through regulation of water lossRate of GFR controlled according to the water requirements of the bodyThe role of ADH in the reabsorption of water and the formation of concentrated urine.

Formation of dilute urineGlomerular filtrate the same osmolarity as blood plasma (300 mOsm/liter)Tubular fluid leaving proximal convoluted tubule still isotonic to plasmaOsmolarity changes along the rest of the renal tubule usually resulting in a decrease of osmolarity as low as 65 70 mOsm/liter

Formation of dilute urineDecrease in osmolarity due to:

Water reabsorption along descending Loop of Henle due to increasing osmolarity of the interstitial fluid of the renal medullaReabsorption of ions from the thick ascending Loop of HenleAscending Loop of Henle is relatively impermeable to water blocking water reabsorption Distal convoluted tubule cells are relatively impermeable to water and not regulated by ADHPrincipal cells of collecting duct impermeable to water when ADH level is low

Formation of dilute urine

Formation of concentrated urineKidney has to conserve water at low water intake or high water loss while at the same time excrete wastes and excess ionsADH role depends on osmotic gradient of solutes in interstitial fluid of renal medullaOsmotic gradient 300 mOsm/liter at cortex but 1200 mOsm/liter deep in medullaFactors contributing to building & maintaining of this osmotic gradientMaintained by differences in solute and water permeability and reabsorption at different sections of LOH and collecting ductCountercurrent flow

Formation of concentrated urineConcentrated urine production involves:Establishment of osmotic gradient in the renal medullaFluid in the interstitial fluid, ascending LOH & vasa recta plasma may be of the same osmolarityCollecting duct cells reabsorb more water & urea in the presence of ADHIncreased reabsorption of water causes urea to concentrate in tubular fluid. Permeability of duct cells deep in the medulla allows diffusion of urea into interstitial fluid

Formation of concentrated urineConcentrated urine production involves (cont):Urea recyclingDiffusion from interstitial fluid into descending & thin part of ascending LOHUrea remain in tubular fluid passing through thick ascending limb, DCT & cortical portion of collecting ductWater reabsorption via osmosis in the presence of ADH further increases tubular urea concentrationMore urea diffuses into interstitial fluid of inner renal medullaCycle repeats leading to small volume of concentrated urine

Formation of concentrated urine

Formation of concentrated urineCountercurrent mechanism:Contributor to the osmotic gradientAt the long loops of Henle of juxtamedullary nephronsCountercurrent flow b/w fluid in descending & ascending limbs of LOHOsmolarity highest at the hairpin turn of the LOH as descending limb is very permeable to water but impermeable to solutes except ureaReabsorption of Na+ & Cl- at ascending limb dilutes tubular fluid

Formation of concentrated urineCountercurrent mechanism (cont):At the vasa rectaSimilar countercurrent flow in parallel descending & ascending limbsOsmolarity of blood entering vasa recta is 300 mOsm/literNa+, Cl- & urea diffuses from interstitial fluid into blood in descending limb increasing blood osmolarityBlood then flows up the ascending limb in the region where interstitial fluid is less concentrated resulting in either:diffusion of ions & urea from blood into interstitial fluidreabsorbed water diffusion via osmosis from interstitial fluid into the vasa recta

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