Final answer:
PTH increases blood calcium levels by stimulating osteoclasts and enhancing calcium absorption and reabsorption, whereas calcitonin decreases blood calcium levels by inhibiting osteoclasts and promoting calcium deposition in bones.
Step-by-step explanation:
Blood calcium levels are tightly regulated by two hormones with opposing actions: parathyroid hormone (PTH) and calcitonin. PTH is secreted by the parathyroid glands in response to low blood calcium levels. It acts by stimulating osteoclasts which resorb bone and release calcium into the blood, thus increasing the blood calcium levels. PTH also enhances renal calcium reabsorption and stimulates the production of calcitriol, an active form of vitamin D, which increases intestinal absorption of calcium. Conversely, calcitonin, released from the C cells of the thyroid gland, lowers blood calcium levels by inhibiting osteoclast activity and stimulating osteoblasts, helping to store calcium in bone tissue, and promoting the excretion of calcium in the urine.