CALCIUM HOMEOSTASIS-PART-I

CALCIUM METABOLISM

Calcium is an important mineral that our body needs for various functions.Unlike iodine which is almost available in our body mostly in the form of thyroid hormone,  plasma protein bound  and as a free iodine  ion or elemental iodine.Iodine may not stay in our body as a free ion unless for elimination.But in the case of calcium our body needed it in all the forms mostly ionic form for the contractions of various smooth muscles like heart,liver,kidney,pancreas,lungs and stomach etc.etc..About 80% of calcium is stored in bone as phosphates.

Calcium is distributed in our body almost in compartments including bones,plasma,intra-cellular (ICF) and extra-cellular(ECF) fluids.

Our bone contains most of the calcium content of our body  approximately 80% as phosphate salts.Approximately our body's total content of calcium is 1 kg.Bone serves as healthy and safety storage for calcium and phosphorus.10% of calcium is exchanged between bone and ECF in almost every day. 

Thus ECF serves as the next storage compartment,followed by blood plasma {2.3 mmol per liter(9.5 mg/dl) out of which the ionized calcium is 1.4mmol per liter(5.5 mg/dl) and ICF (>0.0002 mmol per liter)}

Calcium helps blood clotting factors by activating them at times of need.

The nerve cells are very sensitive to calcium concentration of in plasma as a small decrease in calcium level below the normal range (Hypocalcemia)may cause the opening of the voltage gated sodium channel and sodium is leaked into the cells of nerve axons and causes nerve hyper excitability which results in muscular spasms a disease known as tetany and paraesthesia (a pricking skin sensation around the mouth and extermities)

Conversely if the plasma calcium level rises above the normal range (Hypercalcemia) the sodium exchange is restriced from out side to inside of the nerve and muscular cells and the nerves are hyper polarised,and lost their normal sense of excitability,and become weak and tired leads to smooth muscle weakness which causes,lethergy,anorexia,constipation and emotions.

Calcium gives the structure,shape and strength for bones by forming as phosphate salts (calcium hydroxyapatite)

Pharmacokinetics

A normal daily diet contains 30 mmol calcium out of which 6 mmol is absorbed by our intestine daily.

Calcium absorption is almost regulated by vitamin D-3 a cholesterol derivative prepared by our skin. 

95% of the calcium from the glomerular filterate is reabsorbed by the kidneys leaving a small fraction to be excreted.If this level is disturbed by some pathological defects such as defects in calcium metabolism or bone desorption then there are possibilities of kidney stone formations occurs.

 

  In general calcium is metabolized  by our body in three ways as follows

1.By a hormone secreted by parathyroid gland (PTH)

2.By calcitonin an hormone secreted by paraglobular cells of thyroid glands

3.By viamin D-3.
1.Parathyroid glands are two pairs in number and each pair is positioned at either lobe of the thyroid glands.As these glands are situated at the back of the thyroid glands these glands are not visible during an exhamination of neck.The usual weight of parathyroid glands are  30 gms in men and 35 gms in women.
The major function of this gland is to maintain calcium and phosphate levels in a very narrow ranges in order to regulate the neuro muscular functions properly.Thus the PTH takes active part in the homeostasis of calcium. 
To keep up the plasma calcium ion within almost in a stable range thyroid and parathyroid glands working in an opposite direction to each other.Thus when the plasma concentrations of calcium ion rises above the narrowly fixed set level (Hypercalcemia) the thyroid secretion of calcitonin act on it and bring it down to normal range.
Conversely if the plasma calcium level go down below the normal set level(Hypocalcemia) then Para Thyroid Hormone (PTH) secreted by parathyroid gland act on it to raise its level to normal range.
Pathology
Hypocalcemia
Hypercalcemia