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LSS Urinary System      

Alexandra Burke-Smith

The loop of Henle The thin descending limb is permeable to water, and the cells are involved in a lot of water reabsorption (large brush border with few mitochondria) The thick ascending limb is permeable to salt but not water, and the cells are involved in a lot of ion pumping (tight junctions with many mitochondria) As tubular fluid flows through the ascending limb, it generates a salt gradient of approx. 200mosmol/l  and interstitial fluid of ~400mosmol/l To equilibriate that, water is reabsorbed from the descending limb, decreasing the interstitial fluid osmolarity restoring the equilibrium However at the bottom of the loop + the beginning of the ascending limb, more salt is pumped out generating a further difference of ~200mosmol/l – thus the tubular fluid leaving the loop of Henle is hypoosmolar (the water reabsorption in the descending limb is not sufficient to fully equilibriate the IF conc) The collecting duct is then permeable to water under presence of ADH, therefore as the collecting duct transverses the renal medulla urine is concentrated as water moves out of the tubular fluid to maintain the IF osmolarity

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Role of urea the concentration gradient generated in the loop of Henle cannot be created by salt alone Urea has 2 regions of the kidney nephron which are permeable to it, and the movement of urea continues in a cycle: o The bottom of the loop/beginning of the ascending limb of Henle is permeable to urea, therefore it enters the tubular fluid thus resulting in a more hyposmolar tubular fluid o The renal collecting duct is then also permeable to urea, thus it is removed and reabsorbed

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Renal medullary blood flow All the tubular cells require oxygen + nutrients in order to maintain function. However an ordinary blood supply would result in a loss of the hyperosmotic IF gradient as excess salt would move into the capillaries This gives a need for a specialised blood supply – the vasa recta The vasa recta is a looped blood system which follows the tubules, but is permeable to water and solutes This means that in the descending limb, water diffuses out/solutes diffuse into the limb from the vasa recta. But in the ascending limb the reverse happens; therefore the concentration gradient is maintained on leaving the loop of Henle

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The Distal Convoluted tubule + the collecting duct Vasopressin  Also known as antidiuretic hormone (ADH)  Peptide hormone (9 aa long) – synthesis in hypothalamus, packaged into granules then secreted from neurohypophysis (PPG)  Binds to specific receptors on basolateral membrane of the principal cells  This causes insertion of aquaporins into the cells luminal membrane, Aquaporins are an active water uptake system which are stored in vesicles unless under the influence of ADH (therefore water uptake in the tubule is regulated). This increases the permeability of the duct to water.  ADH also stimulates urea transport from the inner medullary collecting duct into the thin ascending limb of loop of Henle What triggers ADH release?  Release is regulated by hypothalamic osmoreceptors which respond to an increase in plasma osmolarity >300Mos  Baroreceptors also act as a secondary signal, stimulated by a fall in BP/blood volume  Ethanol inhibits ADH  increase in urine volume + dehydration 18

Alex's Urinary  

Alex's Urinary

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