The kidneys respond to acid-base disturbances by:

The correct answer is D because the kidneys play a crucial role in maintaining acid-base balance by excreting or reabsorbing hydrogen or bicarbonate ions. When the body is acidic, the kidneys excrete excess hydrogen ions and reabsorb bicarbonate ions to help neutralize the acidity. Conversely, when the body is alkaline, the kidneys excrete excess bicarbonate ions and retain hydrogen ions to restore balance. Choices A (producing phosphate buffers) and C (producing protein buffers) are incorrect because the kidneys primarily regulate acid-base balance through ion excretion/reabsorption. Choice B (adjusting PaCO2 levels) is incorrect as this is primarily controlled by the respiratory system.