Water balance pg 1 2 4

Water Balance

• Body fluid osmolarity is maintained at roughly 290 mOsm/L (or 300 mOsm/L for simplicity) through osmoregulation.
• Small deviations in body fluid osmolarity trigger hormonal responses to adjust water reabsorption in the kidneys to restore normal osmolarity.
• Renal water reabsorption mechanisms maintain constant body fluid osmolarity, controlled in the late distal tubule and collecting duct.
• Variations in water reabsorption lead to variations in urine osmolarity, which ranges from 50 mOsm/L to 1200 mOsm/L.
• Isosmotic urine has equal osmolarity to blood, hyperosmotic urine has higher osmolarity, and hyposmotic urine has lower osmolarity than blood.

Regulation of Body Fluid Osmolarity

• Body fluid osmolarity is regulated in response to water deprivation and drinking water.

Response to Water Deprivation

• Example: A person is stranded in the desert without water for 12 hours
• Water is continuously lost through insensible water loss (sweat, mouth, and nose vapor), increasing plasma osmolarity when not replaced.
• Increased osmolarity stimulates osmoreceptors in the anterior hypothalamus, which are sensitive to changes of less than 1 mOsm/L.
• Stimulation of hypothalamic osmoreceptors increases thirst, driving drinking behavior, and stimulates ADH secretion from the posterior pituitary gland.
• ADH from the posterior pituitary gland increases water permeability in the principal cells of the late distal tubule and collecting duct.
• Increased water permeability boosts water reabsorption in the late distal tubule and collecting ducts, increasing urine osmolarity and decreasing urine volume.
• Increased reabsorption returns more water to body fluids, decreasing plasma osmolarity back to normal along with increased thirst and drinking, representing a negative feedback loop.

Response to Water Drinking

• Ingested water distributes throughout body fluids, decreasing plasma osmolarity since the amount of solute is unchanged.
• Decreased plasma osmolarity inhibits osmoreceptors in the anterior hypothalamus.
• Inhibition of osmoreceptors reduces thirst and water drinking behavior and suppresses ADH secretion from the posterior pituitary gland.
• Reduced ADH levels decrease water permeability in the principal cells of the late distal tubule and collecting ducts.
• Decreased water permeability reduces water reabsorption in the late distal tubule and collecting ducts, increasing urine volume and decreasing osmolarity, as water is excreted.
• Less water is reabsorbed, so less water returns to the circulation, increasing plasma osmolarity back toward normal, coupled with suppressed drinking and thirst.

Corticopapillary Osmotic Gradient

• The corticopapillary osmotic gradient is a gradient of osmolarity in the kidney's interstitial fluid, from the cortex to the papilla.
• The cortex has an osmolarity of approximately 300 mOsm/L, similar to other body fluids.
• Osmolarity progressively increases from the cortex to the outer medulla, inner medulla, and papilla, reaching up to 1200 mOsm/L at the papilla's tip.
• Countercurrent multiplication via the loop of Henle deposits NaCl in deeper kidney regions.
• Urea recycling via the inner medullary collecting ducts deposits urea, which contributes to the osmotic gradient.

Countercurrent Multiplication

• Countercurrent multiplication, a function of the loop of Henle, forms the corticopapillary osmotic gradient by depositing NaCl in the deeper kidney regions.
• The loop of Henle itself initially has no gradient of osmolarity; osmolarity is initially 300 mOsm/L throughout the loop and surrounding interstitial fluid
• The loop of Henle enhances the gradient of osmolarity in the interstitial fluid through a repeating two-step process which is the single effect and the flow of tubular fluid.