Lecture 29 – Renal Physiology IV

You can make the collecting duct very permeable to

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you can make the collecting duct very permeable to water or not permeable to water at  all meduallry osmotic gradient exists because of  countercurrent multiplier in loop of henle  and facilitated diffusion of urea from the lumen of the CD into meduallyry lumen Establishment of medually osmotic gradient Start out at 300 milliosmolar There is no gradient at this point Tubulr fluid flows through When you get the the thichk portion o the ascending limb of loop of henle, Na, K and Cl  are transported into the medually fluid It is hyperosmotoci (only a little) compared to the tubular fluid Therefore bulk flow continues and water flows out Transport more sodium out now Jumps to about 350-500 Keep doing this until the system is at EQ EQ is established by bulk flow, flow of water (passive from descending limb) and active  transport of ions in the thick part of the ascending limb Steady-state gradient Role of Urea in the medullary osmotic gradient Countercurrent multiplier requires urea to maintain it Urea is formed in the lier during protein catabolism Makes up about 40% of the solutes in glomerular filtrate Only about 40% of the urea stays in the tubules (critical in preventing massive water  loss via the osmotic effect of urea on water) Urea also prevents excessive water loss by strengthening the medullary osmotic  gradient Water would flow in if you lost all the urea, therefore it is transported out with specific  transporters
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Highly water soluble but cannot penetrate membranes without tansporters UT Transporters Located in the distal part of the collecting tubules UTB transporters in descending limb of vaso recta UTA transporters of severeal types in the epithelim of cells of the CD deep within the  medulla Facilitates diffusion of urea from the filtrate to the peritubular fluid Vasa recta in maintainsing osmotic gradient As blood flows intoe the renal medualla to supply it with oxygen and nutrueents, solutes  tend to diffuse into the capillaries and water tends to difuse out Encoutners the osmotic gradient and water leaves the lasma Increases the osmolarity of plasma This is because it is hypoosmotic Solutes that can diffuse in do Once it starts ascending, it encounters outside hypotonic fluid Therefore water flows back into the blood When blood reaches the top, it comes back pretty much the same as it came in The vasa recta does not destry the osmotic gradient because it has its own ocmostic  gradient Role of MOG in water reabsorption from the distal tubules (DT) and collecting tubules  (CT) 70% of water filtered from the plasma is reabsorbed in the proximal tubule 20% of the filtered water is reabsorbed in the DT and most od the remaning 10% is 
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