Lecture 29 – Renal Physiology IV

Reabsorbed in the ct pretty much all water is

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reabsorbed in the CT pretty much all water is reabsorbed in the initial portion of the distal tubule the renal filtrate is hypo-osmotic to the peritubular  fluid
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as the tubular fluid moves down the collecting duct, the osmolarity of the tubular fluid is  alwaus less than that of the peritubular fluid increasing the osmotic force for water to  move from the CD to the intersititial fluid osmotic force is for water to move out of the duct and into the medually osmotic gradient whenever water can leave, it does (and is reabsorbed) epithelial cells lining the late DT and all of the CD re connected via tight junctions which  are impermeable to water lipid bilayers are also impermeable to water ability of water to cross the plasma membrane and diffuse across the epithelial lining of  the late DT and CD depends on the presence of water cannels called aquaporins in the  plasma membranes of principal cells aquaporin 3 is present in the basolateral membrane at all times aquaporin 2 is present in the apical membrane only in response to ADH presence Regulating permeability of dital part of distal tubule
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ADH regulation Secreted in the hypothalamus in response to osmoreceptors located in the brain where  there is an incomplete blood brain barrier In a particular nucleus, and change frequency with which they generate action potentials Anywhere there is an AD receptor, a change in the cel will occur Steps in the principal cells lining the distal DCT and CD AQ2 is stored in the membrane of cytoplasmic vesicles ADH binds to receptors on the basolateral membrane Activation of g-protein activates adenylyl cycles which catalyzes the formation of cAMP  Activates PKA  Stimulates insertion of new water channels into the apical membrane by exocytosis,  increasing water permeability Plasma concentration of ADH determines the number of AQP2 pores and thus water  permeability of the apical membrane AQP2 is present in the apical membrane only in the presence of ADH In the absence of ADH, the apical membrane is impermeable to water and reabsorption  of water does not occur ADH is secreted from the posterior pituitaty by neurosectreoy cells that originate in the  hypothalamus Two mechanisms stimulate release ECF concentration  Changes in the osmolarity of ECF is the strongest stimulus for ADH release Osmoreceor cells located in the hypothalamus monitor the osmolarity of ECF An increase in ECF mosmolarity stimulates ADH ecretion
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Baroreceptors  Plasma levels of ADH are also influenced bt signals arsing in baroreceptors that detect  changes in blood pressure and volume Baroreceptors ain the atria respond to changes in to blood volume Baoreceptos in the aortic arch and carotid sinuses respond to changes in blood  pressure
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