Which ion is primarily responsible for regulating blood and tissue fluid acidity?

The ion primarily responsible for regulating blood and tissue fluid acidity is ammonium. Ammonium ions play a crucial role in the acid-base balance of the body, particularly in the kidneys. They are formed during the metabolism of amino acids and can accept protons (H+), a process that helps to neutralize acidity in the blood and maintain a stable pH level.

The kidneys manage the levels of ammonium by excreting it into the urine when there is excess acid, thereby helping to adjust and regulate the overall acidity of blood and tissues. This function is essential for maintaining homeostasis, as even slight deviations from the normal pH range can significantly affect biochemical processes.

In contrast, while potassium, sodium, and calcium ions have important roles in other physiological processes, they do not primarily regulate the acidity of blood and tissue fluids like ammonium does. Potassium is mainly involved in nerve and muscle function, sodium is essential for fluid balance and blood pressure regulation, and calcium is critical for bone health and muscle contraction. Therefore, ammonium's specific role in adjusting acidity makes it the key ion for this function.