FUNCTIONS OF CALCIUM

Amount of calcium absorbed from the GIT Parathyroid hormone Like increasing intestinal absorption of calcium, also mobilizes bone calcium and causes marked hypercalcemia. It also increases calcium absorption from the distal convoluted tubules of the kidney.

Inverse relationship between plasma calcium and inorganic phosphate (Pi) level When plasma calcium rises, the plasma inorganic phosphorus level falls and vice versa. The product of plasma calcium and inorganic phosphorus (both in mg/dl) remains constant at 30 to 40 in the adult and 40 to 55 in children. 1

Plasma calcium level falls in kidney diseases due to retention of phosphate, which leads to a high plasma Pi level and hypocalcemia. The other contributory factor is non-activation of vitamin D. The hypocalcemia leads to secondary hyper parathyroidism .

Plasma protein level Each gram of plasma/serum protein binds 0.84 mg of calcium. Thus a decrease in serum protein level (i.e. hypoproteinemia) will result in a decreased level of bound calcium and will also result in a lowered total serum calcium.

Plasma protein level As the ionized serum calcium remains normal, therefore such a person will not suffer tetany because it is only the ionized calcium which is physiologically active.

Renal excretion Normally only a small part of calcium lost from the body is excreted in urine. If excessive amount of calcium is administered, more Na+ is being excreted in urine, Renal loss of calcium is also increased if more Na+ is being excreted e.g. by diuretics.

Calcitonin Produced by the para follicular cells (clear or C cells) of the thyroid gland. It lowers the plasma Ca++ level, (it antagonizes the action of PTH ) It decreases bone re-sorption and increases the deposition of Ca++ in bones. It increases urinary loss of Ca ++ by inhibiting the Ca++ re-absorption in the proximal convoluted tubules.

FUNCTIONS OF CALCIUM

1- Function Calcium salts take part in bone and tooth development . Deficient supply of calcium leads to rickets in children and osteomalacia in adults. Sufficient calcium intake must be ensured in early life to buildup the skeletal reserves.

If this is not done then there occurs an increased incidence of osteoporosis in old age because at that time deficiency of sex hormones especially in females results in calcium mobilization from bones leading to osteoporosis

2- Function The clotting of blood needs calcium ions. Calcium is known as factor iv in blood coagulation cascade. Prothrombin contains gamma carboxyl glutamate residue which are chelated by calcium during the thrombin formation.

3-Function By regulating the membrane permeability Ca++ control the excitability of nerves. If plasma Ca++ level falls markedly, tetany results in which spasms of various muscle groups occur. Death may occur from convulsions or from laryngospasm. Excess of plasma Ca++ depresses nervous activity.

4-Function Calcium ions act as a cofactor or activator of certain enzymes. A protein namely calmodulin is present within cells which can bind calcium.

calmodulin-calcium complex Becomes attached to certain enzymes which are activated. Such enzymes include adenylate cyclase. Ca++ ATPase phosphorylase kinase, myosin light chain kinase, phosphodi-esterase and phospholipase A 2. This mechanism is also required for the release of acetylcholine at the neuro-muscular junctions.

5-Functions Calcium ions take part in the contraction of muscle including heart muscle and are involved in the excitation- contraction coupling mechanism. With an increased plasma calcium, heart stops in systole (calcium rigor)

In addition a high plasma calcium decreases conduction of cardiac impulses and thus can produce heart block. Calcium ions potentiate the effects of cardiac glycosides and therefore should not be given to patients who are using these drugs.

Functions Calcium ions are responsible for initiating contraction in vascular and other smooth muscles. Calcium ions enter through specific channels just as is the case with cardiac muscle. Drugs which block these channel (Ca++ channel blockers) have profound effect on the contractility of cardiac and smooth muscles as well as on the conduction of impulses within the heart. These drugs find use in treatment angina pectoris, cardiac arrhythmias and hypertension.

6-Functions Calcium is essential for maintaining the integrity of capillary wall. in its deficiency, capillary walls become fragile and there is increased permeability of capillaries .In a similar manner it has a role maintaining the integrity of mucosal membranes and cell adhesion.

7-Function Calcium ions are involved in exocytosis and thus have an important role in stimuls-secretion coupling in most exocrine and endocrine glands e.g the release of catecholamines from the adrenal medulla, and histamine from mast cell is dependent upon Ca++ .

Some hormones exert their influence through Ca++ . For example, the effect of adrenaline on the liver cells to increase glycogenolysis is partly due to an increased Ca++ within these cells.

Resent studies Calcium may play a substantial contributing role in reducing the incidence of obesity and prevalence of the insulin resistance syndrome

Resent studies High calcium intake is associated with a plasma lipoprotein-lipid profile predictive of a lower risk of coronary heart disease compared with a low calcium intake

Resent studies Dairy product intake (with recommended calcium levels) protect women consuming oral contraceptives from spine and hip bone loss Children who avoid drinking cow milk have low dietary calcium intakes and poor bone health

Toxicity of calcium Excessive intake of calcium can be dangerous because calcium gets deposited in various body tissues resulting in their mal-functioning . This effect of hypercalcemia is seen more frequently if serum Pi level is normal or raised.

Calcium excretion Daily 1240 mg calcium enters the gut, 1000 mg from food and 240 mg from the blood; of this 480 mg is absorbed showing a net absorption of 240 mg. Feces eliminate 760 mg calcium day which represents the unabsorbed dietary calcium along with calcium coming from the blood.

Calcium excretion Of the 6000 mg calcium filtered by the glomeruli most of it is reabsorbed and the kidneys excrete only 240 mg calcium /day which represents the real excretion of calcium from the body.

Calcium excretion Normally only a negligible amount is lost from the skin but excessive sweating result in an appreciable loss of body calcium.

Calcium excretion Pregnancy and lactation result in a significant loss of mother’s calcium

Calcium balance It is the difference between the quantity of calcium ingested and lost from the body over a certain period of time. If the quantity of calcium lost from the body is less than that ingested, the balance in called positive. If more calcium is lost than ingested, it is called a negative calcium balance.

Conditions associated with a positive calcium balance. 1. Growth 2. Hyperpituitarism involving growth hormone 3.Pregnancy and lactation 4.Calcium intake after a long period of calcium deprivation

Conditions associated with a negative calcium balance. 1.Rickets and osteomalacia 2. Hyperparathyroidism 3. Malabsorption 4. Hyperthyroidism 5. Starvation 6. Decreased calcium intake 7. Deficiency of sex hormones in both sexes (eg .old age)

Body need Vitamin D PTH Protein Lactose Acid medium Absorption increased by: Absorption decreased by: Vitamin D deficiency Cal-phos imbalance Oxalic acid Phosphorous Dietary fiber Excessive fat High alkalinity Stresses and Lack of exercise Body need Vitamin D PTH Protein Lactose Acid medium

Low PTH High ECF Vol. High BP Low plasma PO=4 Vitamin D3 Excretion increased by Excretion decreased by High PTH Low ECF Vol Low BP High plasma PO=4 Metabolic acidosis Vitamin D3 Low PTH High ECF Vol. High BP Low plasma PO=4 Metabolic alkalosis

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