Calcium
Total calcium in the human body is 1 to 1.5kg, out of which 99% is seen in bone and 1% in extracellular fluid. The main source of calcium is milk. But in India cereals is major source of calcium. The daily requirement of calcium for child is 1200mg/day and for adult it is 500mg/day. During pregnancy /lactation the calcium requirement is 1500mg/day.
The absorption of calcium takes place in 1st and 2nd part of deuodenum. Calcium absorption requires carrier protein, helped by Ca2+ – dependent ATpase.
Factors responsible for increase in calcium absorption include Vitamin D, Parathyroid hormone, acidity and amino acids. Factors such as phytic acid,oxalates, malabsorption syndromes and Phosphates decreases calcium absorption. Thenormal calcium level in blood is 9-11mg/dl.
Function of Calcium
The major functions of calcium are
(a) Excitation and contraction of muscle fibres needs calcium. The active transport system utilizing calcium binding protein is called Calsequestrin. Calcium decreases neuromuscular irritability.
(b) Calcium is necessary for transmission of nerve impulse from presynaptic to postsynaptic region.
(c) Calcium is used as second messenger in system involving protein and inositol triphosphate.
(d) Secretion of insulin, parathyroid hormone, calcium etc, from the cells requires calcium.
(e) Calcium decrease the passage of serum through capillaries thus, calcium is clinically used to reduce allergic exudates.
(f) Calcium is also required for coagulation factors such as prothrombin.
(g) Calcium prolongs systole.(h) Bone and teeth contains bulk quantity of calcium.
Factors regulating blood calcium level
The factors regulating the blood calcium level includes
(i) Vitamin D
(a) Vitamin D and absorption of calcium:
Active form of calcium is calcitriol. Calcitriol enters intestinal wall and binds to cytoplasmic receptor and then binds with DNA causes depression and consequent transcription of gene code for calbindin. Due to increased availability of calbindin, absorption of calcium increases leading to increased blood calcium level.
(b) Vitamin D and Bone:
Vitamin D activates osteoblast, bone forming cells & also stimulates secretion of alkaline phosphatase. Due to this enzyme, calcium and phosphorus increase.
(c) Vitamin D and Kidney:Calcitriol increase reabsorption of calcium and phosphorus by renal tubules.
(ii) Parathyroid hormone (PTH)Normal PTH level in serum is 10-60ng/l.
(a) PTH and bones:
In bone, PTH causes demineralization. It also causes recreation of collagenase from osteoclast leads to loss of matrix and bone resorption. As a result, mucopolysacharides and hydroxyproline are excreted in urine.
(b) PTH and Kidney:
In kidney, PTH causes increased reabsorption of calcium but decreases reabsorption of phosphorus from kidney tubules.
(iii) Calcitonin
Calcitonin decreases serum calcium level. It inhibits resorption of bone. It decreases the activity of osteoclasts and increases osteoblasts.
Hyper Calcemia
When plasma Ca2+ level is more than 11mg/dl is called Hypercalcemia. It is due to parathyroid adenoma or ectopic PTH secreting tumor. In this condition, calcium excreted in urine decreases excretion of chloride causing hyperchloremic acidosis. The main symptoms of hyperchloremic acidosis are:
(a) Anorexia, nausea,vomiting
(b) polyuria, polydypsia
(c) Confusion, depression, psychosis
(d) renal stones
(e) osteoporosis
Hypocalcemia
Plasma calcium level less than 8mg/dl is called hypocalcemia. Tetany due to accidental surgical removal of parathyroid glands or by autoimmune disease. In tetany, neuromuscular irritability is increased. Increased Q-7 internal in ECG is seen. Main manifestation is carpopedal spasm. Laryngismus and stridor are also observed. Laryngeal spasm may lead is death. The causes of laryngeal spasm includes
(a) Deficiency of vitamin D
(b) Deficiency of parathyroid
(c) Increased calcitonin
(d) Deficiency of calcium intake
Intext Questions
1. The total calcium range in human is ……………..
2. Calcium absorption requires …………….. dependent ATpase.
(a) Ca (b) Mn (c) Mg (d) fe
3. Factors responsible for increase in calcium absorption include ……………..
4. When plasma Ca2+ level is more than 11mg/dl is called ……………..
(a) Hypercalcemia (b) Anemia (c) Fluorosis (d) Selenosis
