What is the primary variable that affects blood pH?

Blood pH is primarily determined by the balance between carbon dioxide (PCO2) and bicarbonate (HCO3-) in the blood, which are key components of the bicarbonate buffer system. The concentration of PCO2 plays a critical role in this system because it affects the formation of carbonic acid when carbon dioxide dissolves in blood.

When PCO2 levels rise, more carbonic acid is produced, which dissociates to release hydrogen ions, leading to a decrease in pH (more acidic). Conversely, when PCO2 levels fall, there is less carbonic acid, resulting in fewer hydrogen ions and an increase in pH (more alkaline). This relationship is fundamental to maintaining acid-base homeostasis in the body and is regulated by respiratory function.

While HCO3- concentration also plays a crucial role in buffering blood pH, the immediate and more significant influence on pH changes is often due to alterations in PCO2 levels, particularly in acute conditions. Temperature and O2 saturation do have effects on various physiological processes, but they do not directly influence blood pH in the same primary manner as the concentrations of carbon dioxide and bicarbonate.