Final answer:
Parathyroid hormone (PTH) increases osteoclast activity to release more calcium ions into the bloodstream.
Step-by-step explanation:
The hormone that increases osteoclast activity to release more calcium ions into the bloodstream is parathyroid hormone (PTH).
When blood calcium levels are low, PTH stimulates osteoclast proliferation and activity. This results in the release of calcium from bones into the circulation, thereby increasing the calcium ion concentration in the blood.
PTH also promotes the reabsorption of calcium by the kidney tubules, affecting calcium homeostasis.