Which processes do the kidneys use to regulate fluid and electrolyte balance?

• A. Absorption in the intestine.
• B. Secretion in the liver.
• C. Filtration, reabsorption, and secretion.
• D. Filtration and excretion only.

Answer: (C)

The kidneys regulate fluid and electrolyte balance through three interconnected nephron processes: filtration, reabsorption, and secretion. In the glomerulus, plasma is filtered to form filtrate. As this filtrate travels through the tubules, essential substances such as water and electrolytes are selectively reabsorbed back into the bloodstream, fine-tuned by hormonal signals to meet the body's needs. Substances can also be secreted from the blood into the tubular fluid in different tubule segments, helping to adjust pH and balance of ions like hydrogen, potassium, and various drugs or wastes. The final urine excreted reflects the combined effects of filtration, reabsorption, and secretion, with excretion being the ultimate outcome rather than the primary regulatory steps. Choices involving intestinal absorption or liver secretion aren’t kidney processes, and a plan that includes only filtration and excretion misses the crucial reabsorption and secretion steps that actively regulate fluid and electrolyte levels.