Which of the following is NOT a process that calcium is involved in?

Calcium plays a crucial role in several physiological processes within the body, particularly in the fields of muscle function and blood coagulation. Muscle contraction is significantly dependent on calcium ions, as they facilitate the interaction between actin and myosin filaments in muscle fibers. Similarly, muscle tone involves a continuous and passive partial contraction of muscles, which is also regulated by calcium levels in the body.

In the coagulation cascade, calcium is essential for various steps in the blood clotting process, acting as a cofactor in the activation of certain coagulation factors, which ultimately leads to the formation of a stable clot.

Water retention, however, is primarily regulated by the kidneys through mechanisms involving hormones such as aldosterone and the antidiuretic hormone (ADH), rather than calcium. While electrolyte balance, including calcium, may influence overall fluid homeostasis, calcium itself is not directly involved in the process of water retention. This distinguishes it from the other options listed, which are integral roles of calcium in bodily functions.