Explain the interactions of parathyroid hormone and calcitonin.

Parathyroid hormone and calcitonin are antagonistic hormones, which are released in response to blood calcium levels. In other words, PTH acts on the same target organs as calcitonin, but produces the opposite effects. Parathyroid hormone is released from the parathyroid glands, and calcitonin is released from the thyroid gland. If blood calcium concentration is high, calcitonin is released. Calcitonin activates osteoblasts and causes calcium to be deposited in bone and thus removed from the blood, restoring normal concentration levels. Parathyroid hormone is released when blood calcium concentration is too low. PTH activates osteoclasts in bone. This causes reabsorption of calcium from the bone and release into the blood, restoring normal blood levels of calcium.