After C,H,O and N, Calcium is the most abundant body constituent, making up about 2 percent of body weihgt: 1-5 kg in an adult. Over 99% of this is stored in bones, the rest being distributed in plasma and all tissues and cells.
1 Calcium controls excitability of nerves and muscle and regulates permeability of cell membranes. It also maintains integrity of cell membranes and regulates cell adhesions.
2 Ca2+ ions are essential for excitation contraction coupling in all types of muscle and excitation secretion coupling in exocrine and endocrine glands, release of transmitters from nerve ending and other release reactions.
3 Intracellular messenger for hormones, autacoids and transmitters.
4 Impulse generation in heart determines level of automaticity and A-V conduction.
5 Coagulation of blood.
Plasma Calcium Level
Plasma Calcium Level is precisely regulated by 3 hormones almost exclusively devoted to this function
- Parathormone (PTH)
- Calcitriol (active form of vitamin D).
Normal plasma calcium is 9-11 mg/dl.
40 percent is bound to plasma proteins-chiefly albumin; 10 percent is complexed with citrate, phosphate and carbonate in an undissociable form; the remaining (about 50 percent) is ionized and physiologically important.
Acidosis favours and alkalosis disfavours ionization of calcium: hyperventilation precipitates tetany and laryngospasm in calcium deficiency by reducing ionization.
Absorption and Excretion
Calcium is absorbed by facilitated diffusion from the entire small intestine as well as from duodenum by a carrier mediated active transport under the influence of vitamin D.
Phytates, phosphates, oxalates and tetracyclines and interfere with absorption.
Glucocorticoids and phenytoin also reduce calcium absorption.
All ionized calcium is filtered at glomerulus and most of it is reabsorbed in the tubules. Vitamin D increases and calcitonin decreases proximal tubular reabsorption, while parathormone increases distal tubular reabsorption of Ca2+. About 300 mg of endogenous calcium is excreted daily: half in urine and half in faeces.
To maintain calcium balance the same amount has to be absorbed in the small intenstine from diet. Because normally only 1/3 of ingested calcium is absorbed, the dietary allowance for calcium is 0.8-1.5 g per day. However, calcium deficiency and low dietary calcium increases fractional calcium absorption.
Thiazide diuretics impede calcium excretion by facilitating tubular reabsorption.
Calcium supplements are usually well tolerated; only gastrointenstinal side effects like constipation, bloating have been reported.