Which substance carried by venous blood contributes to its lower pH compared to arterial blood?

Venous blood has a lower pH compared to arterial blood primarily due to the higher concentration of carbon dioxide (CO2) it carries. When tissues metabolize nutrients, they produce CO2 as a waste product, which then diffuses into the blood.

In the bloodstream, CO2 reacts with water to form carbonic acid (H2CO3), leading to the release of hydrogen ions (H+). This reaction is represented by the following equilibrium:

CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3^-

The increase in hydrogen ions results in a decrease in pH, making the blood more acidic. This is especially pronounced in venous blood, which has already circulated through the tissues and thus has collected more CO2 than arterial blood, which is reoxygenated in the lungs.

The other substances noted in the question—oxygen (O2), nitrogen (N2), and sodium (Na+)—do not significantly alter blood pH in this manner. Oxygen is essential for aerobic metabolism but does not contribute to acidity. Nitrogen is an inert gas under physiological conditions and does not react in a way that affects pH. Sodium primarily affects osmotic balance and nerve function, rather