IMAT Biology Practice Exam

Calcium homeostasis is controlled by hormones, and parathyroid hormone acts to raise blood calcium when it's low. It does this by signaling bone to release calcium into the bloodstream (osteoclast-mediated resorption), increasing calcium reabsorption in the kidneys, and promoting formation of calcitriol (the active vitamin D) to boost intestinal calcium absorption. This combination directly raises blood calcium levels, which is why this hormone is the one that increases calcium by stimulating bones to release it.

Calcitonin lowers blood calcium by inhibiting bone resorption, so it wouldn’t increase calcium. Estrogen helps maintain bone density by reducing bone breakdown rather than triggering a rise in blood calcium. Growth hormone influences bone growth and turnover but doesn’t specifically elevate blood calcium through bone resorption.