Calcium Homeostasis: Introduction and Physiological Basis
Calcium homeostasis is the dynamic regulation of extracellular ionised calcium concentration by coordinated actions of bone, kidney, intestine and parathyroid glands. It maintains neuromuscular excitability, cardiac conduction, blood coagulation, intracellular signalling and skeletal mineralisation. In orthopaedics, disturbance of this system is relevant to rickets, osteomalacia, osteoporosis, hyperparathyroid bone disease, renal osteodystrophy, fracture healing and perioperative metabolic complications.
1. Distribution and forms of calcium in the body
The adult human body contains approximately 1–1.2 kg of calcium. About 99% is stored in bone and teeth, mainly as hydroxyapatite crystals associated with type I collagen. The remaining calcium is present in extracellular fluid and soft tissues, where even small changes have major physiological effects.
| Form of serum calcium | Approximate proportion | Clinical significance |
|---|---|---|
| Ionised or free calcium | Approximately 45–50% | Biologically active fraction; determines neuromuscular and cellular effects. |
| Protein-bound calcium | Approximately 40% | Predominantly bound to albumin; not immediately diffusible. |
| Complexed calcium | Approximately 10% | Complexed with phosphate, citrate, bicarbonate and other anions. |
The ionised calcium concentration, rather than total serum calcium alone, is the principal regulated variable. Alkalosis increases calcium binding to albumin and therefore reduces ionised calcium, potentially producing paraesthesia, carpopedal spasm or tetany despite a normal…