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Water Balance (Antidiuretic Hormone Regulates Collecting Duct Water…
Water Balance
- Antidiuretic Hormone Regulates Collecting Duct Water Permeability to Determine the Final Urine Osmolality
Water overload: ADH present, collecting duct impermeable to water. Tubule fluid delivered by distal convoluted tubule remains hypotonic because water is retained in collecting duct lumen. Absence of ADH - dilute urine, excess water excreted.
Dehydration/hypotension/volume depletion: ADH released from pituitary triggered by a rise in plasma osmolality resulting from dehydration/sakt overload & by decreased blood pressure due to heart failure, etc.
ADH present - water flows from dilute tubule fluid into cell into interstitium down the concentration gradient, producing structural alteration.
Birds: Salt & water reabsorption occurs distal to collecting ducts. Lacks of urinary bladder, urine travels from kidney via ureters to cloaca where both salt and water reabsorbed.
- The Countercurrent Mechanism Increases Medullary Interstitial Osmolality with Minimal Energy Expenditure
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Passive - thin limbs have reabsorbed both water & salt. Water reabsorbed from descending thin limb & salt reabsorbed from ascending thin limb.
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- Countercurrent Exchange in the Vasa Recta Removes Water from the Medullary Interstitium Without Reducing Medullary Interstitial Hypertonicity
Ascending vasa recta leave medulla, have net movement of fluid into capillary
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High initial plasma oncotic pressure, countercurrent arrangement of vasa recta, passive equilibration of plasma with changing interstitial osmolalities in different regions of medulla allowing removal of water & solute from medullary interstitium without dissipating medullary hypertonicity.
Vasa recta permeable to water, salts & urea.
- The Kidney Maintains Water Balance
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- The Proximal Tubule Reabsorbs More Than 60% of Filtered Water
Takes up solutes from tubule fluid by both active & passive means. Sodium-potassium-adenosine triphosphatase(Na+,K+-ATPase) pump in basolateral plasma membrane actively transports Na+ & drives carrier-mediated secondary active transport & passive transport uptake of solutes.
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Water is reabsorbed nearly isotonically with salt, osmolality of tubule fluid remains similar from Bowman's space to beginning of thin descending limb of Henle's loop.
- A Hypertonic Medullary Interstitium Is Needed to Form Concentrated Urine
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- Shoot-Loop & Long-Loop Nephrons Have Different Roles in Urine Concentration
Superficial mephrons: Have short loops of Henle that extend only into inner stripe of outer medulla. Short-loop nephrons: Have a descending thin limb that parallels thick ascending limb, but do not have an ascending thin limb.
Juxtamedullary nephrons: Have long loops of Henle extend deep into inner medulla. Long-loop nephrons: Have several segments of descending & ascending thin limbs with specific urea & water transporter expression.
- Urea Reabsorption by Inner Medullary Collecting Duct & Urea Recycling Enhance Medullary Hypertonicity
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Birds: Urea nearly absent in medullary interstitium, urates do not contribute appreciably to osmotic pressure because they have low water solubility. Medullary hypertonicity: depend on single-solute (NaCl) recycling.
- The Kidney Can Produce Either Concentrated or Diluted Urine
Have 3 components
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Dilution of tubule fluid by thick ascending limb & distal convoluted tubule allowing excretion of dilute urine
Variability in water permeability of collecting duct in response to antidiuretic hormone (ADH,vasopressin) determining final urine concentration.
- Sodium Chloride Reabsorption by the Medullary Thick Ascending Limb Generates Medullary Hypertonicity
Hypertonic interstitium allows water to be abstracted from water-permeable descending thin limbs & returned to circulation.
- Active Sodium Chloride Reabsorption in the Thick Ascending Limb & Distal Convoluted Tubule Dilutes the Tubule Fluid
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- Cells in the Inner Medulla Adapt to Interstitial Hyperosmolality by Accumulation of Organic Osmolytes
Intracellular concentrations of osmolytes vary with diuretic state of animal, increasing during periods of urine concentration, when medullary interstitial osmolality is maximized, and decreasing during diuresis when medullary interstitial osmolality decreases.