What is homeostasis and how do the kidneys contribute to maintaining it?

• A. Homeostasis is the body's immune defense against infection.
• B. Homeostasis is the steady internal state of balance, and the kidneys conserve or excrete excess fluid to keep the internal environment balanced.
• C. Homeostasis refers to the body's ability to produce energy from nutrients.
• D. Homeostasis is the process of digesting food in the stomach.

Answer: (B)

Homeostasis is the body's ability to keep the internal environment stable and balanced despite changes in the outside world. The kidneys are key players in this process because they control the volume and composition of body fluids. They adjust how much water and electrolytes are reabsorbed back into the bloodstream or excreted as urine, in response to signals about hydration, salt intake, and overall fluid status. This helps maintain steady levels of fluids, electrolytes like sodium and potassium, and the blood’s acid-base balance.

For example, when you’re dehydrated, the kidneys conserve water and concentrate urine; when there’s excess fluid or salt, they excrete more into the urine. They also help keep the body’s pH balanced by reabsorbing bicarbonate and excreting hydrogen ions as needed, and they play a role in regulating blood pressure through hormonal systems. So, the kidneys contribute to staying in balance by conserving or eliminating fluid to keep the internal environment stable.

Other options describe different body functions (immune defense, energy production, digestion) and do not capture the balancing role the kidneys play in maintaining internal stability.