How do the kidneys primarily respond to metabolic acidosis to maintain acid-base balance?

- The kidneys play a crucial role in maintaining acid-base balance, especially during conditions of metabolic acidosis, where there is an excess of hydrogen ions (H⁺) in the body.
- The primary response of the kidneys to metabolic acidosis is to excrete H⁺ ions into the urine and simultaneously reabsorb bicarbonate (HCO₃⁻) back into the bloodstream.

- This process helps to reduce the acidity of the blood by removing excess hydrogen ions and restoring the levels of bicarbonate, which acts as a key buffer that neutralizes acid.
- By increasing tubular hydrogen ion secretion, the kidneys acidify the urine and prevent systemic acidosis.

- Additionally, the kidneys enhance ammoniagenesis, which produces ammonia (NH₃) that binds to excess hydrogen ions to form ammonium (NH₄⁺), further facilitating acid excretion.
- However, the fundamental mechanism remains the excretion of H⁺ along with reabsorption of HCO₃⁻ to correct the disturbance.

Therefore, the correct answer is: Excreting H⁺ ions and reabsorbing HCO₃⁻. This response is essential for restoring the normal pH and maintaining a stable internal environment.

Reference: Ganong’s Review of Medical Physiology, 26th Edition, Chapter 48: Renal Regulation of Acid-Base Balance