Calcium homeostasis
Calcium homeostasis is a important thing in human and animals body to maintain normal level of calcium in blood as well as in bones. It is done by Calcitonin, Parathyroid hormone and Calcitriol (Vitamin D3 / Cholecalciferol).
About 99% of the body’s Ca2+ is in the bone and a very small amount is in the plasma (1%). In most animals and in non-laying birds the blood Calcium ranges between 7.9 and 11.7 mg/dl except in horses in which it is between 10.4 and 13.4 mg/dl and in layers 17-39mg/dL.
The smallest ECF Ca2+ pool is the most vigorously controlled Ca2+ pool in the body and it is composed of interstitial Ca2+, blood Ca2+ and bone Ca2+ in solution.
The most important regulation of Ca2+ between bone and ECF involves the soluble Ca2+ of bone which is the source of ready exchange of ions with blood and it is located between osteoblasts.
Plasma Ca2+ exists in three forms – (1) free or ionizableCa2+ (2) complexed to HCO3, citrate, PO4 (3) bound to plasma proteins. Free or ionizable Ca2+ is predominant (50%) and is in constant exchange with ECF Ca2+. This is the physiologically active Ca2+ which participates in many biochemical functions.
Bone Ca2+ exists in two forms (1) a readily exchangeable reservoir and (2) larger stable pool which is slowly exchangeable. Two independent homeostatic systems operate on the bone Ca2+ – one is the system that regulates plasma Ca2+ and about 500mmol Ca/day moves into and out of the readily exchangeable pool in bone. The second is concerned with bone remodeling by interplay between bone resorption and deposition and this accounts for bone formation and this pool is about 7.5mmol/day.
Role of Calcium and Phosphorus
Calcium is important for-
- Muscle contraction
- Nerve cell activity
- Release of hormones by exocytosis
- Activation of number of enzymes
- Coagulation of blood
- Maintaining stability of cell membrane
- Maintaining structural integrity of bone and teeth
Phosphate is important for–
- Structure of bone and teeth
- As part of cell membrane organic PO4
- As part of number of intracellular components
Bone cells
In bones there are two types of cells – osteoblasts and osteoclasts.
Osteoblasts and osteoclasts that are responsible for bone formation and bone reabsorption respectively. Osteoblasts are bone forming cells which secrete alkaline phosphatase and collagen for the organic matrix .
The osteoblasts after the formation of organic matrix are embedded into the matrix and now they are termed as osteocytes.
Alkaline phosphatase promotes precipitation of calcium phosphate on the organic matrix and thus helps in bone mineralization.
Osteoclasts are motile, multinucleated, large cells derived from monocytes and they actively reabsorb bone leaving a lacuna in the bone and thus these cells are bone reabsorption cells.
Physiological significance of calcitonin and PTH
Calcitonin and Parathyroid hormone provide a reciprocal inhibitory control mechanism to maintain the concentration of calcium in extracellular fluids within narrow limits.
Parathyroid hormone is the major factor concerned with the minute-to-minute regulation of blood calcium under normal conditions.
Calcitonin is a short-term regulator of blood calcium and it operates more rapidly compared with Parathyroid hormone.
Calcitonin function more as an emergency hormone to prevent the development of hypercalcemia during the rapid postprandial absorption of calcium and to protect against excessive loss of Calcium and phosphates from the maternal skeleton during pregnancy and lactation.