Final answer:
Calcitonin is the hormone that lowers blood calcium levels by inhibiting osteoclasts and stimulating osteoblasts. It is produced by the parafollicular cells of the thyroid gland and acts to deposit calcium into the bones and reduce its reabsorption in the kidneys. Its action contrasts with that of parathyroid hormone which increases blood calcium levels.
Step-by-step explanation:
The correct option : b
The hormone that inhibits osteoclasts and stimulates osteoblasts to lower blood calcium levels is calcitonin. This hormone is produced by the parafollicular or C cells of the thyroid gland. The primary function of calcitonin is to regulate the amount of calcium and phosphate in the blood, opposing the action of parathyroid hormone (PTH). When blood calcium levels are high, calcitonin works by inhibiting osteoclasts, which are cells that break down bone tissue, thereby preventing the release of calcium from bones. At the same time, it stimulates osteoblasts, which are cells responsible for bone formation, to increase the deposition of calcium into the bones.
Calcitonin importantly also decreases reabsorption of calcium by the kidneys, further contributing to the reduction of blood calcium levels. Together, these effects lower the calcium concentration in the blood. The action of calcitonin is particularly significant in children, during pregnancy, and in periods of prolonged starvation, although its role in healthy nonpregnant, well-fed adults is less well-defined. Contrastingly, when blood calcium levels are low, PTH works to increase them by enhancing reabsorption of calcium in the kidneys, stimulating the activity of osteoclasts, and promoting the production of calcitriol, which enables increased calcium absorption by the intestines.