Lecture Notes: Hormonal Regulation of Water Balance and Urine Concentration

Key Hormones

• ADH (Antidiuretic Hormone)
  • Major regulator of water balance.
  • High levels concentrate urine; low levels result in dilute urine.
  • Involved in conditions like diabetes insipidus (low ADH, large amounts of dilute urine).
• Aldosterone
  • Regulates sodium and potassium levels.

Urine Concentration Mechanisms

• ADH's Role
  • Concentrates urine by acting on the collecting ducts and renal medulla.
  • Inserts aquaporins in the collecting duct, facilitating water reabsorption.
  • Adapts urine concentration from very dilute (65 mOsm) to very concentrated (up to 1200 mOsm) depending on fluid balance.

Nephron and Counter Current Mechanism

• Nephron Structure

  • Cortex: Solute concentration approx. 300 mOsm.
  • Medulla: Gradient increasing from 350 mOsm to 1200 mOsm.
  • Descending Loop of Henle: Water leaves to concentrate urine.
  • Ascending Loop of Henle: Impermeable to water; solutes actively transported out.

• Counter Current Mechanism

  • Vasa recta maintains osmolarity without disrupting the gradient.
  • Blood in vasa recta flows opposite to filtrate, maintaining solute and water balance.

Urine Formation

• Dilute Urine

  • Low ADH levels lead to less water reabsorption.
  • Overhydration reduces ADH release, decreased aquaporins in collecting duct.

• Concentrated Urine

  • High ADH levels in response to high osmolarity detected by the hypothalamus.
  • Increased water reabsorption, producing small volume of concentrated urine.

Reabsorption Sites

• Proximal Convoluted Tubule
  • 65% reabsorption of water and solutes.
• Loop of Henle
  • 15% water reabsorption (descending part).
• Distal Convoluted Tubule
  • 10-15% reabsorption.
• Collecting Duct
  • 5-10% with ADH.

Diuretics and Substances Influencing Urine Flow

• Loop Diuretics (e.g., Furosemide/Lasix)
  • Potent; inhibit sodium transporters in the ascending loop, reducing water reabsorption.
• Caffeine
  • Inhibits sodium reabsorption, increasing urine flow.
• Alcohol
  • Inhibits ADH secretion, leading to increased urine output.
• Other Prescription Medications
  • Act on different nephron areas (proximal and distal convoluted tubule).

Learning Goals

• Understand the effects of ADH, atrial natriuretic peptide, and the RAA system on fluid balance and urine production.
• Identify the origin and action sites of each hormone within the nephron.
• Comprehend the formation of dilute and concentrated urine and the counter current mechanism.