Vitamin D in Health and Disease

Abstract

Calcium is one of the most tightly regulated subtances in the plasma, being held constant at 10 mg/100 ml or 2.5 mM. This remarkable constancy of plasma calcium concentration is of obvious essential importance because of its role in the neuromuscular junction, nerve conduction, muscle contraction, adhesion of one cell to another, membrane permeability, blood clotting, and other functions. Perhaps the most important function is that which occurs at the neuromuscular junction, in which the ambient calcium concentration is critical. In the absence of sufficient amounts of calcium there is a continual excitation at the neuromuscular junction, resulting in the serious state of tetany and convulsions. An important basic concept in calcium homeostasis is that the physiology of the body is so constructed to preserve the plasma calcium concentration at the expense of both the diet and the skeleton.