Calcium metabolism

1. CALCIUM METABOLISM
Calcium is the most abundant micromineral in the human body. Human calcium in the human body is
about 1 to 1.5kg, 99% of which is the seen in bone together with phosphate. Small amounts in soft tissue
and 1% in extracellular fluid. It is required most required during pregnancy(1500mg/day) followed by
children(1200mg/day) and adult(500mg/day).
DIETARY SOURCES
 Milk is a good source of calcium as well as its dietary products. Egg, fish, cheese, beans, lentils,
nuts, cabbage and green leafy vegetables are all sources of calcium.
FORMS OF CALCIUM IN PLASMA
Calcium exist in three different forms in blood plasma and they are;
 Ionized, free or unbound calcium which is metabolically active
 Bound calcium to a protein, mostly albumin
 Complex calcium. About 10% plasma calcium is complexed with anions including bicarbonate,
phosphate, etc
HORMONAL REGULATION OF PLASMA CALCIUM
Three main hormones regulate plasma level of calcium and they are calcitriol, parathyroid hormone and
calcitonin.
DURING LOW LEVEL OF CALCIUM IN PLASMA
Calcitriol and parathyroid hormones are secreted. Action Of Calcitriol and Parathyroid Hormone (PTH)
 ON BONE
Calcitriol in osteoblast of bone stimulates calcium uptake for deposition as calcium phosphate.
Calcitriol along with parathyroid hormone increases mobilization of calcium and phosphate from
the bone and this causes elevation in the plasma calcium.
 ON KIDNEY
Parathyroid hormone increases calcium reabsorption by kidney tubules. PTH promotes the
production of calcitriol in the kidney by stimulating 1-hydroxylation of 25-
hydroxycholecalciferol. Calcitriol minimizes the excretion of calcium by decreasing their
excretion and enhancing their absorption.
 ON INTESTINE
As production of calcitriol is stimulated by the PTH, calcitriol binds with cytosolic receptor to
form calcitriol-receptor complex. The complex interacts with DNA leading to the synthesis of
specific calcium binding protein. This protein increases calcium uptake by intestine.
DURING HIGH LEVEL OF CALCIUM IN PLASMA
Calcitonin is released. Calcitonin is secreted by parafollicular cells of thyroid gland. Its action is
antagonistic to that of PTH. Calcitonin promotes calcification of bones by increasing the activity of
osteoblasts. Calcitonin decreases resorption and increases excretion of calcium into urine. Calcitonin acts
via increased cAMP concentrations to inhibit osteoblast motility and cell shape and inactivates them