How do bicarbonate buffering, respiratory CO2 changes, and renal bicarbonate handling maintain pH homeostasis?

• A. The bicarbonate buffer system buffers H+; the lungs regulate CO2 via respiration; the kidneys reabsorb or generate bicarbonate and excrete H+ to maintain pH.
• B. Bicarbonate buffering is not involved in pH; only lungs regulate pH.
• C. pH is regulated solely by diet; kidneys do nothing.
• D. The bicarbonate system acidifies the blood.

Answer: (A)

pH homeostasis relies on three coordinated defenses: chemical buffering with bicarbonate, respiratory control of carbon dioxide, and renal handling of bicarbonate and hydrogen ions. The bicarbonate buffer system acts quickly: bicarbonate ions neutralize added hydrogen ions to form carbonic acid, which then rapidly dissociates into water and CO2. This dampens sharp pH changes right away. The lungs then regulate CO2 levels through breathing. When acid loads or CO2 rise, ventilation increases to blow off more CO2, shifting the balance toward less H+ and a higher pH; when CO2 accumulates, ventilation decreases, CO2 builds up, and H+ increases, lowering pH. Over the longer term, the kidneys fine-tune pH by reabsorbing filtered bicarbonate, generating new bicarbonate, and excreting hydrogen ions in the urine. This renal adjustment replenishes bicarbonate and removes acid, sustaining pH near normal despite ongoing metabolic or respiratory demands. Together, these systems explain why pH remains tightly controlled: buffering provides immediate protection, respiration adjusts gas-derived influences, and the kidneys provide durable correction.

The other statements miss essential parts: buffering and renal contributions are real and important, not just lung control; diet does not set blood pH in a way that fixes it, and the bicarbonate system does not acidify the blood but helps prevent acidification.