Final answer:
The hormone that regulates calcium exchange between bones and blood and increases blood calcium levels is parathyroid hormone (PTH), also known as parathormone. Option A is correct.
Step-by-step explanation:
The hormone that regulates the exchange of calcium between the bones and blood and increases blood calcium when levels are low is the parathyroid hormone (PTH). PTH is produced by the parathyroid glands and serves several functions to elevate blood calcium levels:
It enhances reabsorption of calcium by the kidneys.
It stimulates osteoclast activity and inhibits osteoblast activity in the bones, which results in the release of calcium from the bones into the bloodstream.
It stimulates the synthesis and secretion of calcitriol by the kidneys, which in turn enhances calcium absorption by the digestive system.
Contrastingly, calcitonin is another hormone involved in calcium regulation, but it has the opposite effect; it lowers blood calcium levels by inhibiting osteoclasts and stimulating osteoblasts. Therefore, the correct answer to the question is parathormone, which is another name for parathyroid hormone (PTH).