Final answer:
PTH stimulates osteoclasts to break down bone and release calcium, increases kidney reabsorption of calcium, and enhances dietary calcium absorption via vitamin D activation.
Step-by-step explanation:
When blood calcium levels are low, PTH (parathyroid hormone) stimulates the activity of osteoclasts. Osteoclasts are specialized cells that break down bone tissue, releasing calcium into the blood to raise calcium levels. PTH also promotes the reabsorption of calcium from the urine in the kidneys and increases the absorption of dietary calcium in the intestines through the activation of vitamin D into calcitriol.