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Making up a .3 OsM NaCl solutionSodium chloride = Na+Cl-1 Na+ = 231 Cl- = 35 58 58g Na+Cl-/L water = 1M Na+Cl-Therefore: 0.3M Na+Cl-= 58g * 0.3 = 17.4g Na+Cl- /L = 0.3M Na+Cl-Since Na+Cl- ionizes in solution into 2 particles (ions): 0.3M Na+ Cl- = 0.6O Osm = 600 mOsM. Therefore to make up a 300 mOsm Na+ Cl- solution one should start with 58g/2 =29g Na+ Cl-/LVertical Osmotic GradientVariable water reabsorption is made possible by a vertical osmotic gradient in the medullary interstitial fluidGradient is established by the long loops of Henle of the juxtamedullary nephrons via countercurrentmultiplicationGradient preserved by the vasa recta of these nephrons via countercurrent exchangeMedullary osmotic gradientLoop of Henle:Descending limb is permeable to water but not NaClAscending limb is impermeable to water but actively transports NaCl into the surrounding interstitial fluidThere is a countercurrent flow produced by the close proximity of the two limbsIn the face of water deficit (need to preserve water in circulatory system)Loop of Henle establishes a vertical osmotic gradient in the medullary interstitial fluid – used by collecting ducts to be able to excrete a more concentrated urine than normal body fluidsVasopressin present: distal and collecting tubules become permeable to H2OIn the face of waterexcess
Since the fluid is hypotonic as it enters the distal tubule, the kidneys without the presence of vasopressin can excrete a urine ore dilute than normal body fluidsNo vasopressin present: distal and collecting tubules become impermeable to H2OExchange within the vasa recta PRESERVES the medullary vertical osmotic gradient counter-currentThe hairpin (counter-current) construction of the vasa recta allows the blood to leave the medulla and enter the renal vein essentially isotonic to incoming arterial bloodIf blood supply to renal medulla flowed straight through from the cortex to the inner medulla, theblood would be isotonic on entering but very hypertonic on leaving having picked up salt and lost H2O as it equilibrated with the surrounding interstitial fluid at each incremental horizontal levelWould be impossible to maintain the vertical osmotic gradient because the salt pumped out by the ascending limb of Henle’s loop would be continuously flushed away by blood flowing through the medullaBlood equilibrates withthe interstitial fluid ateach incrementalhorizontal level in boththe descending limb andthe ascending limb of