Final answer:
Parathyroid hormone (PTH) is produced by the parathyroid glands and is essential for calcium regulation in the body. It prompts actions in the kidneys, bones, and gastrointestinal tract to maintain and increase blood calcium levels. This is achieved by transforming vitamin D into its active form, stimulating osteoclasts, and modulating phosphate reabsorption.
Step-by-step explanation:
Parathyroid hormone (PTH) is a critical hormone produced by the parathyroid glands that plays a vital role in calcium regulation within the body. This peptide hormone is made up of 84 amino acids and is released in response to low circulating calcium (Ca++) levels.
PTH acts on various target organs, with one of its main targets being the proximal convoluted tubule (PCT) of the kidney. Here, it stimulates the transformation of calcidiol into calcitriol, the active form of vitamin D. Moreover, it inhibits the reabsorption of phosphate (PO43-), promoting its loss in urine to prevent the formation of calcium phosphate in the plasma, which would otherwise reduce circulating Ca++ levels.
In the bones, PTH enhances osteoclast proliferation and activity, resulting in the breakdown of the bone matrix and the release of inorganic calcium-phosphate salts into the bloodstream. This action effectively increases blood calcium ion concentration. Furthermore, PTH aids in the reabsorption of calcium by the kidney tubules and indirectly increases the gastrointestinal absorption of dietary calcium through the conversion of vitamin D into dihydroxyvitamin D (calcitriol).
In summary, PTH raises blood calcium levels by encouraging the release of calcium from the bones, reducing calcium loss through the kidneys, and increasing the gastrointestinal absorption of calcium.