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Water Balance - Coggle Diagram
Water Balance
Mammalian system that allows production of Concentrated or Diluted Urine
consists of:
Generation of a hypertonic medullary interstitium which allows excretion of concentrated urine
maintained by:
the reabsorption of osmotically active substances by tubules in the medulla
via Countercurrent mechanism of loop of Henle
Basically because of nature of permeability of the thin limbs (water permeable at descending, Na permeable at ascending) and their arrangement (parallel to each other) interstitial osmolality is increased without much effort
at descending thin limbs, it is permeable to water but not to Na at least at outer medullary part, and due to hypertonic interstitial fluid contributed by thick ascending limb actively reabsorbing Na water moves out into the interstitium.
Interstitial fluid osmolality increases as descending thin limbs goes deeper in the medulla
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the removal of water from the medullary interstitium by the vasa recta
via Countercurrent Exchange in the Vasa Recta
Vasa recta are permeable to water, salts, and urea
Vasa recta entering the medulla has high oncotic pressure, so water can get into the capillary lumen
Luminal (of vasa recta) NaCl and urea concentration equilibrate with the interstitial concentration thus as vasa recta goes deeper into the inner medulla, the plasma osmolality and urea concentration increases until the hairpin turn (cause interstitial fluid at this point also has high osmolality and urea concentration) and then falls as the vessels ascends out of medulla.
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Dilution of the tubule fluid by the thick ascending limb and the distal convoluted tubule, which allows excretion of dilute urine
TAL and DCT actively reabsorb Na which drives Cl reabsorption, but these segments are impermeable to water so tubule fluid became dilute
variability in the water permeability of the collecting duct in response to antidiuretic hormone (ADH, vasopressin), which determines the final urine concentration
During water overload
ADH is absent so CD is impermeable to water, tubule fluid remains hypotonic
If ADH is absent, AQP is located in cytoplasmic vesicles in principal cells and IMCD cells
During dehydration, hypotension or volume depletion
ADH is released from neurohypophysis
If ADH is present, APQ2 inserts into apical p.m. of the said cells and allow water to pass through
AQP3 and AQP4 exists on basolateral p.m. regardless of ADH thus allows movement of water from cells to interstitium
CD is now permeable to water, as it travels down the inner medulla, tubular fluid equilibrates with the interstitium by movement of water out of the tubule producing highly concentrated urine
ADH also contributes to urea reabsorption, thus contributing to medullary hypertonicity
triggered by rise in plasma osmolality as little as 3-5 mOsm/kg
Cells in the Inner Medulla Adapt to Interstitial Hyperosmolality by Accumulation of Organic Osmolytes
organic osmolytes maintain intracellular osmotic pressure and prevent cell shrinkage
Concetrations of osmolytes varies depending of diuretic states of animals
, increased during periods of urine concentration when medullary interstitial osmolality is maximised
decreased during diuresis when medullary interstitial osmolality decreases