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Establishing Concentration Gradient

• Fluid enters duct, pumping sodium ions across membranes.

• Juxtamedullary nephrons create a concentration gradient from low to high concentration.

Countercurrent Exchange

• Blood vessels (capillaries) parallel to nephron loops facilitate substance exchange:

  • Water and sodium chloride move from high to low concentration.

  • Mechanism involves the countercurrent exchange principle.

Role of Aquaporins

• Aquaporins in collecting duct membranes control water permeability:

  • Not present when the membrane is impermeable to water.

  • Hormonal action (e.g., vasopressin) stimulates vesicles to merge with the outer cell membrane, inserting aquaporins.

Urine Concentration Mechanism

• Collecting duct's permeability to water affects urine concentration:

  • Permeable: water reabsorbed, forming concentrated urine.

  • Not permeable: allows for the secretion of dilute urine.

• Loop of Henle establishes the gradient, enabling effective reabsorption in the collecting ducts.

Importance of Concentration Gradient

• Concentration gradient necessary for water reuptake in collecting duct:

  • Gradient allows water to leave the duct towards the higher concentration outside.

  • Higher concentrations in the renal medulla (up to 1,200 mOsm) assist in water conservation and urine concentration.