In this article we will discuss about the functions of ionic calcium.
i. Maintenance of RMP:
Because of this, the excitability of neuron and muscle is maintained. A decrease in calcium ion level increases the excitability and vice versa when calcium ion level decreases.
ii. N-M transmission:
It plays an important role in conduction of impulse across neuromuscular junction. The amount of ACh released is directly dependent on the number of calcium ion influx at the presynaptic terminal.
iii. Maintenance of excitability and contractility of cardiac muscle:
The plateau phase of action potential of cardiac muscle and for contraction of cardiac muscle calcium ion is necessary.
iv. Blood coagulation:
In many of the steps of blood coagulation, calcium ion is required. Removal of calcium ion, blood fails to clot.
v. Development of bone and teeth
vi. It is required for activation of certain enzymes.
vii. It is required for release of certain hormones.
Calcium is actively transported from GI tract. The most important factor influencing calcium absorption is 1, 25-dihydroxycholecalciferol. The other factors which can also influence calcium absorption are growth hormone, and acidic pH in duodenum.
Calcium absorption is decreased in old age. Large amount of calcium is filtered in the kidneys and about 99% of filtered calcium is reabsorbed. About 60% of reabsorption occurs in PCT and the rest from DCT.
Calcium present in the bone are two types namely rapidly exchangeable compartment and in a much stable compartment. The calcium that is present in plasma and bone are in equilibrium.
When the plasma calcium level increases suddenly, calcium is removed from plasma to get deposited in the bone along with phosphate. When the plasma calcium level falls, calcium is removed from bone matrix by a process of demineralization and this calcium enters plasma (Figs 6.48 and 6.49).
Total body phosphate is about 500-800 g, of this 85- 90% is present in bones. The total plasma phosphate level is about 12 mg%. About one-third of this is in inorganic form mostly as PO4–, HPO4– and H2PO4. About 3 mg of phosphate enters the bone everyday and an almost equal amount of this is removed from the bone.
Phosphate is filtered into the renal tubules and 85 to 90% of this gets reabsorbed. Most of this reabsorption occurs at PCT and is by an active process. When parathormone acts on the renal tubules, the amount of phosphate reabsorbed is decreased. In the GI tract, phosphate absorption occurs both actively and passively in duodenum. Factors that increase calcium absorption also increase phosphate absorption.
Features of Phosphate Depletion:
i. Skeletal muscle weakness.
ii. Cardiac and respiratory muscle dysfunction.
iii. Loss of red blood cell membrane integrity.
iv. Abnormal formation of bone.
Bone is a special form of connective tissue. On collagen framework, calcium and phosphate is deposited in the form of hydroxyl appatite crystals. Deposition and resorption of calcium and phosphate ions goes on continuously in the bone matrix. In young people, deposition of salts occurs to a greater extent than resorption and in elderly people it is reversed.
The cells responsible for bone formation are osteoblasts. The osteoclasts are responsible for bone resorption. Osteoclastic activity is associated with increase in the alkaline phosphatase.
The third type of cell found in bone matrix is osteocyte. The cells are responsible for transferring large amount of calcium from the interior to the exterior (ECF) is known as osteocytic osteolysis.