Calcium and phosphate in extracellular fluid and plasma
Excitable cells are very sensitive to changes in the concentration of calcium ions if increased beyond the normal threshold causes a decrease in the activity of the nervous system; conversely, a decrease in blood calcium levels (hypocalcaemia) causes nerve cells to become more excitable.
The concentration of extracellular fluid calcium is usually maintained steady, it rarely increases or decreases several percent around the normal value of about 9.4 mg/dl, or about 2.4 mmol/l. This stabilization is essential because calcium plays a key role in many physiological processes, including skeletal muscle contraction, cardiac muscle contraction, smooth muscle contraction, blood clotting, and nerve conduction. Excitable cells (such as neurons) are very sensitive to changes in calcium ion concentration is increased beyond the normal threshold (hypercalcemia) causes a decrease in the activity of the nervous system; conversely, a decrease in blood calcium levels (hypocalcaemia) causes nerve cells to become more excitable.
A feature of the extracellular fluid calcium concentration is that only 0.1% of the total body calcium is located here, about 1% is in the cells and its organelles, and the rest is stored in the bones. The bone thus acts as a large reservoir, releasing calcium when the extracellular calcium concentration is low, and storing less when the concentration is exceeded.
About 85% of the body's phosphates are stored in bone, 14-15% in cells, and less than 1% in extracellular fluid. Although extracellular fluid phosphate concentrations are not as controlled as calcium levels, phosphate plays many important functions and is regulated by the same factors that control calcium.