Optometry Admission Test (OAT) Practice

Parathyroid hormone (PTH) plays a critical role in maintaining the body's calcium levels, which are essential for many physiological processes, including muscle contraction, blood coagulation, and nerve function. PTH is secreted by the parathyroid glands and primarily acts to increase blood calcium levels when they drop below normal.

When released, parathyroid hormone targets various organs: it promotes the release of calcium from bones into the bloodstream by stimulating osteoclast activity, enhances the reabsorption of calcium in the kidneys, and stimulates the conversion of vitamin D into its active form (calcitriol) in the kidneys, which in turn increases intestinal absorption of calcium from the diet. This combined action leads to a rapid increase in serum calcium levels, making it a crucial hormone for calcium regulation in the body.

The other options refer to functions outside of PTH's role. For instance, the decrease of blood glucose levels is primarily regulated by insulin, water absorption is largely managed by antidiuretic hormone (ADH), and metabolic activity is often influenced by other hormones such as thyroid hormones and cortisol. Thus, the effectiveness of parathyroid hormone in increasing blood calcium levels marks it as the correct response in this context.