Final answer:
Parathyroid hormone (PTH) increases blood calcium levels by promoting bone breakdown, reabsorption of calcium by the kidneys, and absorption of dietary calcium in the intestines.
Step-by-step explanation:
Parathyroid hormone (PTH) is released in response to low blood calcium levels. PTH stimulates osteoclasts, which breaks down bone and releases calcium ions into the bloodstream.
PTH also promotes the reabsorption of calcium by the kidneys and increases dietary calcium absorption in the intestines through the production of calcitriol, the active form of vitamin D. This overall effect of PTH raises blood calcium levels.