Final answer:
Parathyroid hormone (PTH) increases blood calcium levels by prompting the release of calcium from bones, reabsorption in the kidneys, and aiding absorption in the digestive system through vitamin D activation.
Step-by-step explanation:
The hormone that increases the blood level of calcium is the parathyroid hormone (PTH). Produced by the parathyroid glands, PTH is released when blood levels of calcium ions fall below normal. It enhances calcium ion concentrations by stimulating the release of calcium from bone matrices, increasing the reabsorption of calcium by the kidneys, and upregulating the activation of vitamin D in the kidneys, which aids in calcium absorption from the digestive system.