Fig. 2.

The human respiratory and urinary systems and their role in regulating extracellular pH. The respiratory system (main panel) is responsible for removing CO₂ (produced as the result of metabolism) from the body, and in the process controls [CO₂] in the arterial blood. The urinary system (lower right) excretes H⁺, acidic buffers (HA ⇌ A⁻ + H⁺), and ammonium (NH₄⁺), and thereby controls [HCO₃⁻] in the arterial blood. Together, the kidney (by controlling [HCO₃⁻]) and the lungs (by controlling [CO₂]) control pH, as described by the Henderson-Hasselbalch equation (upper right). Modified from Fig. 26–4 and Fig. 33–7 in Medical Physiology, 2nd Edition Updated Edition, edited by WF Boron and EL Boulpaep, Philadelphia: Elsevier, 2012.