However, only ionized is under hormonal control. Because calcium is intimately involved in various bodily functions, tight regulation of this ion is required. Increased excitability of nerve and muscle cells is associated with significant decreases in . All segments of the intestine can actively abosrb , and some segments can accumulate against a concentration gradient. Calcium bound to a calcium-binding protein is transported across the intestinal brush border membranes.
Renal excretion equals intestinal absorption.
Thus, renal excretion of Ca parallels changes in intestinal Ca absorption to maintain adequate Ca balance. Na reabsorption since Ca reabsorption in these segments occurs secondaray to Na reabsorption. Bone Ca homeostasis is primarily under hormonal control. Even small increases in plasma will decrease PTH secretion. PTH and low plasma phosphate levels stimulate the conversion of vitamin to the active form.