Figure 5
From: Bench-to-bedside review: Fundamental principles of acid-base physiology
The Stewart model. pH is regulated through manipulation of the three Stewart variables: [SID+], [ATOT] and PCO2. These variables are in turn 'upset', 'regulated', or 'modified' by the gastrointestinal (GI) tract, the liver, the kidneys, the tissue circulation, and the intracellular buffers. [ATOT], total concentration of weak acids; PCO2, partial CO2 tension; SID+, strong ion difference.