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| | * Without them producing concentrated urine would be impossible | | * Without them producing concentrated urine would be impossible |
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| − | ==[[Endocrine System - Pituitary Gland - Anatomy & Physiology #Posterior Pituitary Gland| ADH]]== | + | ==[[Endocrine System - Pituitary Gland - Anatomy & Physiology #Posterior Pituitary Gland| ADH]] and the [[Collecting Duct - Anatomy & Physiology|Collecting Duct]]== |
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| − | * AQP2 is stored in vesicle membranes within the [[Collecting Duct - Anatomy & Physiology #Principal Cells| principal cells]] of the [[Collecting Duct - Anatomy & Physiology| collecting ducts]]
| + | The membrane of the [[Collecting Duct - Anatomy & Physiology|Collecting Duct]] is practically impermeable to water. Therefore in order for water reabsorbtion from this part of the nephron to occur it is vital that aquaporins are inserted. This is the role of [[Endocrine System - Pituitary Gland - Anatomy & Physiology #Posterior Pituitary Gland| ADH]]. These channels are always present on the basolateral membrane of the epithelial cells but their presence and concentration on the apical membrane is reliant on the concentration of ADH in the blood. ADH works by binding to V<sub>2</sub> receptors on the basolateral membranes of the cells and activates adenyl cyclase creating cyclicAMP (cAMP). This causes vesicles with ready made glycoprotein '''AQP2''' aquaporins in their membranes to move to the apical cell membrane and insert the aquaporins via exocytosis. This rapidly increases the water permeability and allowing reabsorption |
| − | * [[Endocrine System - Pituitary Gland - Anatomy & Physiology #Posterior Pituitary Gland| ADH]] causes these vesicles to attach to the principal cell membranes.
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| − | ** The higher its concentration the more AQP2 bind to principal cell membranes therefore the more AQP2 available to water within the collecting duct
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| − | ** The more water is reabsorbed
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| − | ** Thanks to the gradient created by the hypertonic medulla which is present due to the countercurrent multiplier in the [[Loop Of Henle - Anatomy & Physiology| Loop of Henle]]
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| − | * This system allows both concentrated and dilute urine to be produced depending on ADH secretion and therefore the density of aquaporins in the principal cell membranes
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| − | * Without ADH the collecting duct would remain impermeable to water and as a result water and salt would be lost and wasted.
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| − | * This would result in an increase in plasma osmolarity
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| − | ===The Concentrating Mechanism, Aquaporins and ADH===
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| − | Water is drawn from the lumen of the tubule by the increasing hypertonicity of the surrounding tissue as the duct makes its way deeper into the medulla. However the rate is limited by the concetration of water channels called aquaporins on the cell membranes. These channels are always present on the basolateral membrane of the epithelial cells but their presence and concentration on the apical membrane is reliant on the concentration of ADH in the blood. If their was no ADH in the blood then no aquaporins would be found on the apical membrane and therefore very little water would be reabsorbed despite the gradient. ADH works by binding to receptors on the basolateral membranes of the cells and activates adenyl cyclase creating cyclicAMP (cAMP). This causes vesicles with ready made glycoprotein aquaporins in their membranes to move to the apical cell membrane and insert the aquaporins via exocytosis. This rapidly increases water permeability. This would not be possible if the urine did not go back up the [[Loop Of Henle - Anatomy & Physiology #Thick ascending limb| thick ascending limb]] of the loop of henle, where its concentration was decreased by the reabsorption of salt, but instead went straight into the collecting ducts. Although this would mean a very concentrated urine is would result in massive salt losses. Thus the collecting duct allows for a very concentrated urine with minimal salt loss. It also allows the concentration of the urine to vary under the control of the hypothalamus and ADH concentrations.
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| | ==Species Variation of the Ability to Conserve Water== | | ==Species Variation of the Ability to Conserve Water== |