Renal Physiology: Production of Urine and Hormonal Regulation

Main Hormonal Systems in Fluid and Electrolyte Balance

1. Renin-Angiotensin-Aldosterone System (RAAS)
2. Antidiuretic Hormone (ADH/Vasopressin)
3. Atrial Natriuretic Peptide (ANP)

Renin-Angiotensin-Aldosterone System (RAAS)

• Angiotensinogen
  • Synthesized by the liver; inactive form in the blood.
  • Activated when cleaved by enzymes.
• Renin
  • Released from the kidney when high volume of water and solutes detected in the urine.
  • Cleaves angiotensinogen to form Angiotensin I.
• Angiotensin-Converting Enzyme (ACE)
  • Cleaves Angiotensin I to form Angiotensin II.
  • Angiotensin II is active and acts as a vasoconstrictor, reducing glomerular filtration rate (GFR).
• Aldosterone
  • A steroid hormone produced in the adrenal cortex.
  • Increases sodium reabsorption, leading to increased water reabsorption, blood volume, and blood pressure.
• ACE Inhibitors
  • Prevent ACE from converting Angiotensin I to Angiotensin II, thus reducing blood pressure.

Antidiuretic Hormone (ADH/Vasopressin)

• Released from the posterior pituitary when high osmolarity (high solute concentration) is detected by the hypothalamus.
• Acts on principal cells in the collecting duct to insert more aquaporin-2 channels, increasing water reabsorption.
• Results in increased blood volume and blood pressure.

Atrial Natriuretic Peptide (ANP)

• Released from the atria of the heart in response to high blood volume or pressure.
• Increases GFR by relaxing mesangial cells in the glomerulus.
• Inhibits ADH and aldosterone.
• Decreases sodium and water reabsorption, leading to decreased blood volume and pressure.

Summary

• RAAS: Balances blood pressure by regulating sodium and water reabsorption.
• ADH: Major regulator of water reabsorption in response to dehydration.
• ANP: Opposes RAAS and ADH by decreasing blood volume and pressure when elevated.