Final answer:
Cells of the parathyroid gland release PTH, which stimulates bone resorption, calcium reabsorption by the kidneys, and vitamin D synthesis to enhance calcium absorption.
Step-by-step explanation:
Cells of the parathyroid gland have plasma membrane receptors for calcium. When calcium is not binding to these receptors, the cells release PTH, which stimulates osteoclast proliferation and resorption of bone by osteoclasts. This demineralization process releases calcium into the blood. PTH promotes reabsorption of calcium from the urine by the kidneys, so that the calcium returns to the blood. Finally, PTH stimulates the synthesis of vitamin D, which in turn, stimulates calcium absorption from any digested food in the small intestine.