Final answer:
The parathyroid glands secrete parathyroid hormone (PTH), which increases blood calcium levels when they are too low, via actions on the kidneys, bones, and digestive system.
Step-by-step explanation:
The parathyroid glands produce a hormone known as parathyroid hormone (PTH), which plays a critical role in calcium homeostasis. PTH is released in response to low calcium levels in the blood and works to increase these levels. It enhances reabsorption of Ca2+ by the kidneys, stimulates osteoclasts while inhibiting osteoblasts to release calcium from the bones, and stimulates the production of calcitriol, which increases the absorption of calcium in the digestive system. Additionally, PTH inhibits the renal reabsorption of phosphate, further ensuring that calcium levels are regulated properly.