Regulation of Urine Concentration & Volume

1. Osmolarity – Number of Solute particles in 1 Liter water

• independent of size of solute (Na+, glucose)

• 1 osmol = 6.02 X 10^23 particle in 1 Liter

• milliosmol (mosm) = 0.001 osmol

• normal body fluids = 300 mosm
2. Countercurrent Multiplier Mechanism for Maintenance of Blood/Urine Osmolarity

• Water moves out along Descending Limb of the Loop of Henle, creating 1200 mosm urine at the base

• Na+Cl- moves out along the Ascending Limb of the Loop of Henle, creating 100 mosm urine at distal end. This salt helps pull more water out of the Descending Limb inpositive feedback mechanism.

• In times of dehydration, Collecting Tubules leak urea to interstitial space, further increasing water retention by increasing osmolarity.

• Vasa recta (capillaries around Loop of Henle) have no Net Effect on water/salt balance
3. Formation of Dilute Urine

• When water removal is needed, no ADH is released, so that the Distal and Collecting Tubules will not actively transport Na+ out; no water moves out

• Urine may be as low as 50 mosm
4. Formation of Concentrated Urine (Water Conservation)

• antidiuretic hormone (ADH) – stimulates reabsorption of water in the Distal and Collecting Tubules
• Diuretics (Stimulate Water Loss)

• alcohol – inhibits action of ADH

• caffeine – causes renal vasodilation; increases GFR

• Na+ reabsorption blockers – block Na+ movement

Category: Physiology Notes