Define Absorption, Transport and Excretion - Calcium?

Calcium (Ca) in food occurs as calcium salts e.g. calcium phosphate or is associated with other dietary constituents e.g. calcium caseinate in milk. Before absorption, Ca must be released from foods and solubilized. This is achieved by the combined actions of gastric acid, intestinal enzymes, intestinal contractions and peristalsis. When calcium intake is low, calcium is mainly absorbed by active (transcellular) transport, but at higher intakes, an increasing proportion of calcium is absorbed by simple (paracellular) diffusion. Thus, calcium is absorbed from the intestine by transcellular route and paracellular route.