Calcium Metabolism: Calcium Function, Distribution, Metabolism by NJE

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Calcium Metabolism: The normal plasma calcium concentration is 9–11 mg% (average 10 mg%). This is among the most tightly regulated physiological parameters of the body. The regulation of plasma calcium concentration within a narrow range of 1–2% indicates the importance of this ion in controlling critical body functions.

Functions of Calcium

Calcium ions are involved in many key physiological processes of the body. These are:

1. Genesis and maintenance of action potentials, especially in cardiac and smooth muscles.

2. Genesis of pacemaker potential in various pacemaking tissues.

3. Excitation-contraction coupling during muscle contractions.

4. Excitability of nerve and muscle: Calcium influences sodium permeability; therefore, influences the ease with which action potentials are triggered. Especially in nerves, low calcium can cause generation of spontaneous action potentials. This causes the characteristic muscle spasm in hypocalcemic tetany.

5. Cell division.

6. Bone formation (mineralization of bone).

7. Secretion of endocrine and exocrine glands (calcium-mediated exocytosis causes release of hormones and enzymes from the gland cells).

8. Neurotransmitter release from nerve terminals.

9. Blood coagulation: Calcium is coagulation factor IV and 

is highly essential for clotting of blood. Many anticoagulants chelate calcium to prevent clotting.

10. Modulation of various enzyme activities (enzymes use calcium as a cofactor).

11. Calcium acts as a second or third messenger in various intracellular signaling pathways for hormone actions.

12. GI motility and motility of many other structures.

Therefore, it is essential to maintain the calcium concentration within its normal range. A minor deviation in calcium level results in alteration in many physiological functions of the body.

Distribution of Calcium in the Body

In the cell, calcium is stored in mitochondria and the endoplasmic reticulum. A transient alteration in plasma calcium is balanced by entry or extrusion of calcium from the intracellular or extracellular reservoirs. The total intracellular free calcium is about 0.2 mg, whereas about 9 g is present in the bound form or in the storage sites such as in mitochondria and endoplasmic reticulum. This intracellular calcium provides an immediate source of calcium for cell functions.

Distribution of Calcium in the Body
Total body content: 1200 g (average)
In bones and teeth: 99% of total
In intracellular fluid: 0.9% (11 g approx.)
In extracellular fluid: 0.1% (1 g approx.)

1. Normally, a healthy adult contains 1 to 1.5 kg of calcium in his body. 

2. About 99% of the total calcium is present in bones and teeth. 

3. The total extracellular pool of calcium is about 1 to 1.5 g and intracellular pool is about 10 – 15 g. 

4. The half of total plasma calcium (about 50%) is ionized and present in the biologically active form. 

5. About 10% of the plasma calcium is in the non-ionized 

form like calcium bicarbonate and about 40% is bound to albumin. 

6. The pH of blood affects ionization of calcium. In alkalosis, ionized calcium concentration decreases, and in acidosis, it increases.

Metabolism of Calcium (Calcium Metabolism)

Daily dietary intake of calcium ranges from 200 mg to 2 g. In an adult, the recommended daily intake is about 800 mg. 

1. Calcium absorption from the intestine is inversely proportional to its intake. This prevents calcium overload when intake is high or maintains calcium concentration even with low intake. 

2. With a daily intake of 1 g of calcium, generally 30% (300 mg) of it is absorbed in the intestine and about 70% (700 mg) is excreted in the stool.

3. About 150 mg of calcium is secreted into the intestine, which makes a total of about 850 mg excreted daily in the stool. 

4. About 150 mg is excreted in the urine. 

5. Thus, about 1 g of calcium is excreted daily from the body to maintain the balance between input and output.

 6. Calcium absorption from the intestine is diminished with advancement of age, which contributes to the development of osteoporosis.

Calcium absorbed from intestine enters the ECF pool of calcium which is about 1000 mg. This is in equilibrium with the rapidly exchangeable pool (4000 mg) of the body. From the rapidly exchangeable pool of plasma, 500 mg of calcium enters bones for normal bone remodeling, and from bone, about 500 mg of calcium extrudes back into the plasma.

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