Which of the following is not a function of sodium in the human body?

Sodium plays several critical roles in the human body, but its functions are not related to oxygen transport. The primary functions of sodium include regulating blood pressure by influencing the volume of blood through its effect on fluid balance, participating in nerve impulse transmission by maintaining the electrochemical gradients necessary for nerve signal conduction, and balancing fluid levels by controlling the distribution of water in and out of cells.

Oxygen transport is primarily the function of hemoglobin within red blood cells, which binds to oxygen and carries it from the lungs to the tissues. Sodium does not have a role in this process, which is why the choice highlighting the function of sodium in oxygen transport is the correct answer to the question.