Skeleton and Calcium Homeostasis

Approximately 99% of the body's calcium, totaling up to 2 kg in a typical adult, is stored in the skeletal system. The skeleton serves as the primary calcium reservoir, making it essential for maintaining calcium homeostasis—the regulation of calcium levels in the blood. This storage function allows bones to release calcium when blood levels drop below normal.

When blood calcium drops below 9 to 11 mg/dL, the parathyroid gland releases parathyroid hormone, which stimulates osteoclasts to increase bone resorption and release calcium into the blood. Parathyroid hormone also promotes calcium reabsorption by the kidneys and stimulates vitamin D synthesis to enhance calcium absorption from food in the small intestine.

When blood calcium is too high, the thyroid gland releases calcitonin, which inhibits osteoclast activity and stimulates calcium uptake by bones. Calcitonin also decreases calcium reabsorption by the kidneys, lowering blood calcium levels back to normal and stopping further calcitonin secretion.

Severe calcium deficiency causes brittleness in both old and new bones because osteoclasts resorb the bone matrix equally regardless of age. This uniform resorption weakens bone integrity throughout the skeleton, increasing fracture risk and compromising the structural support the skeletal system provides.

Hypocalcemia, characterized by abnormally low blood calcium, adversely affects multiple body systems. Blood has difficulty coagulating, the heart may skip beats or stop, muscles struggle to contract, nerves may malfunction, and bones become brittle. These effects demonstrate calcium's critical role beyond skeletal health.

Hypercalcemia, or abnormally high blood calcium, causes the nervous system to become underactive, resulting in lethargy, sluggish reflexes, constipation, loss of appetite, and confusion. In severe cases, hypercalcemia can lead to coma, and chronically elevated levels may cause kidney stones.

Calcium is a chemical element that cannot be produced by biological processes, so the only way it enters the body is through diet. The bones act as a calcium storage site, depositing excess dietary calcium when blood levels are high and releasing it when levels are low, maintaining homeostasis.