How do the kidneys help maintain homeostasis?

• A. By filtering plasma and raising metabolic rate
• B. By conserving water, producing urine that is concentrated relative to blood plasma or by producing urine that is dilute relative to plasma
• C. By increasing thirst sensation
• D. By secreting aldosterone only

Answer: (B)

Maintaining fluid balance and plasma composition is what the kidneys are mainly doing. They adjust how much water is reabsorbed back into the bloodstream versus how much is excreted, which changes the concentration of urine. When the body needs to conserve water, urine becomes more concentrated than blood plasma; when there’s excess water, the urine is more dilute than plasma. This ability to produce either concentrated or dilute urine helps keep blood volume, osmolality, and electrolyte balance stable.

Aldosterone is produced by the adrenal glands, not the kidneys, and while the kidneys respond to aldosterone to reabsorb sodium and water, they do not secrete it. Thirst is controlled by the brain, not kidney secretion, and although kidneys filter plasma, raising metabolic rate is not a kidney function.