Which process primarily regulates the excretion of water from the kidneys?

• A. Glomerular filtration rate (GFR) autoregulation
• B. Renin-angiotensin-aldosterone system activation
• C. Antidiuretic hormone (ADH) secretion
• D. Tubular secretion of potassium

Answer: (C)

Verified step by step guidance

1. Understand the role of the kidneys in water balance: The kidneys regulate water excretion primarily through processes that control the reabsorption of water in the nephron, particularly in the collecting ducts.
2. Learn about Antidiuretic Hormone (ADH): ADH, also known as vasopressin, is a hormone secreted by the posterior pituitary gland. It increases the permeability of the collecting ducts to water, allowing more water to be reabsorbed into the bloodstream and reducing water excretion.
3. Compare ADH secretion to other processes: While GFR autoregulation, renin-angiotensin-aldosterone system activation, and tubular secretion of potassium are important for kidney function, they primarily regulate blood pressure, electrolyte balance, and filtration rate rather than directly controlling water excretion.
4. Focus on the mechanism of ADH: ADH binds to receptors on the cells of the collecting ducts, triggering the insertion of aquaporin-2 channels into the cell membranes. These channels facilitate water reabsorption from the filtrate back into the bloodstream.
5. Conclude that ADH secretion is the primary regulator of water excretion: By increasing water reabsorption, ADH plays a central role in maintaining the body's water balance, especially during dehydration or high plasma osmolarity.