Final answer:
Calcium enters enterocytes through calcium channels induced by PTH, is attracted by an electrochemical gradient created by Na+/K+ ATPase pumps, and is transported within the cell by calcium-binding proteins stimulated by calcitriol before moving into the bloodstream.
Step-by-step explanation:
Calcium crosses the apical border of an enterocyte primarily through calcium channels on the luminal surface that are induced by the action of parathyroid hormone (PTH). Once PTH binds to its receptors on the enterocytes of the distal convoluted tubule (DCT), it promotes the insertion of these calcium channels to increase calcium reabsorption. Furthermore, the Na+/K+ ATPase pumps create an electrochemical gradient that also contributes to the attraction of calcium into the enterocyte. In addition, the active form of vitamin D, calcitriol, stimulates the production of calcium-binding proteins within the enterocytes which aid in the transcellular transport and eventual exocytosis of calcium across the basolateral membrane into the interstitial fluid, and finally into the bloodstream.