The skeleton is more than a structural framework. During constant cycles of bone formation and resorption, it plays a vital role in calcium homeostasis. Calcium is the most abundant mineral in the body and 99% of it is contained in bone. Half of plasma calcium is bound to albumin and is therefore inactive. Calcium results must be adjusted to account for albumin levels by adding or subtracting 0.02 mmol/l for each g/l by which the albumin is below or above 40 g/l, respectively.

Calcium homeostasis and bone metabolism are principally governed by vitamin D and parathyroid hormone. Bone metabolism is also modulated by calcitonin, glucocorticoids, sex hormones, growth hormone and thyroxine. See Chapter 31 ‘Disorders of bone metabolism’ for a description of osteomalacia, rickets and Paget’s disease; also see Chapter 30 ‘Osteoporosis’.

This fat-soluble vitamin is found in the diet and its precursors are also generated in the skin in response to sunlight. Following renal and hepatic hydroxylation, the active component 1,25-dihydroxy-D₃ is released. Its actions are:

• Gut: increases calcium absorption from the small bowel.
  
• Bone: increases mineralisation and resorption.