What is Calcium homeostasis?

Ionised calcium in plasma is held at $1.2\,\mathrm{mmol}/\mathrm{L}$ within a few per cent, because it sets the excitability of every nerve and muscle: too low and they fire spontaneously (tetany), too high and they are sluggish. The four parathyroid glands sense it with a surface receptor and secrete parathyroid hormone (PTH) along a steep inverse sigmoid: PTH mobilises calcium from bone, makes the kidney reabsorb it and excrete phosphate, and activates vitamin D — made in skin by ultraviolet light or eaten, hydroxylated in liver, then in kidney to calcitriol — which makes the gut absorb calcium. Bone is the reservoir, a kilogram of calcium, remodelled continuously by osteoclasts and osteoblasts under PTH, calcitriol, oestrogen and load. Deficiency of vitamin D gives rickets in children and soft bones in adults; the fall of oestrogen at the menopause tilts remodelling toward resorption and gives osteoporosis; a parathyroid adenoma raises calcium, dissolves bone and forms kidney stones — “bones, stones, groans and moans.”