calcium homeostasis
Calcium homeostasis is the body's stubborn insistence on keeping blood calcium almost perfectly constant, like a thermostat that refuses to let the temperature drift. Even though bones store huge amounts of calcium, the fraction circulating in blood is held within a remarkably tight window because it is too important to fluctuate.
Three hormones run the system. PTH raises calcium by mobilizing bone, conserving it in the kidney, and (via vitamin D) increasing gut absorption. Calcitriol, the active vitamin D, mainly increases intestinal calcium uptake. Calcitonin can nudge calcium down by inhibiting osteoclasts, though its role in adult humans is minor. The kidney, gut, and skeleton are the three organs these hormones act on to balance how much calcium enters, leaves, and is stored.
Why so strict? Calcium ions trigger muscle contraction, nerve firing, blood clotting, and countless cell signals. Too little blood calcium makes nerves hyperexcitable and muscles cramp or spasm; too much makes them sluggish, causing confusion, constipation, and kidney stones. The free, ionized fraction is what is regulated, while a large part of blood calcium rides bound to the protein albumin and is not directly active.
When a vegan athlete sweats out calcium during a long race, PTH and vitamin D quietly ramp up gut absorption and kidney retention to keep blood calcium steady.
The system defends blood calcium even when intake or loss varies.
Because about half of blood calcium is bound to albumin, a low albumin level can make total calcium look falsely low; clinicians correct for albumin or measure ionized calcium directly.