Final answer:
Parathyroid hormone (PTH) is secreted in response to low blood calcium levels and stimulates the conversion of vitamin D to its active form, calcitriol, aiding in calcium absorption, while elevated calcium levels feedback to inhibit PTH release.
Step-by-step explanation:
The negative feedback relationship between parathyroid hormone (PTH) and the biologically active form of vitamin D, mainly calcitriol, is a crucial aspect of calcium homeostasis. When blood calcium levels fall, the parathyroid glands release PTH to counteract this decrease.
PTH performs several functions: it stimulates osteoclasts to break down bone and release calcium, increases the reabsorption of calcium in the kidneys, and crucially, it facilitates the conversion of vitamin D to its active form, calcitriol, in the kidneys.
In turn, calcitriol aids the gastrointestinal absorption of dietary calcium. As blood calcium levels rise due to these actions, they exert a negative feedback on the parathyroid glands, inhibiting further release of PTH, thus preventing hypercalcemia.