Which of the following is NOT a key ion in fluid regulation?

Calcium is not considered a key ion in fluid regulation. Instead, it plays a critical role in various physiological processes, including muscle contractions, neurotransmitter release, and blood coagulation. While calcium does influence some aspects of cell signaling and can affect fluid balance indirectly, it is not directly involved in maintaining the balance of fluids in the body's compartments as some other ions are.

In contrast, sodium is a primary regulator of extracellular fluid volume and helps maintain osmotic pressure. Chloride, often associated with sodium, also helps in fluid balance and maintaining acid-base homeostasis. Phosphate plays a crucial role in energy transfer and is important in bone health, but it is not primarily responsible for fluid regulation. Therefore, given these roles, calcium does not fit into the category of key ions specifically involved in the regulation of body fluid balance.