Biological Systems MCAT Practice Exam

What is the role of parathyroid hormone (PTH)?

1. To increase blood glucose levels

2. To stimulate the synthesis of steroid hormones

3. To break down bone and increase calcium levels in blood

4. To regulate metabolism

The correct answer is: To break down bone and increase calcium levels in blood

Parathyroid hormone (PTH) plays a crucial role in maintaining calcium homeostasis in the body. The primary function of PTH is to regulate calcium levels in the blood by promoting the release of calcium from bones. When blood calcium levels fall below a certain threshold, PTH is secreted by the parathyroid glands. The hormone acts primarily on three target areas: bones, kidneys, and the intestine. In the bones, PTH stimulates osteoclasts, which are cells responsible for bone resorption. This breakdown of bone tissue releases calcium into the bloodstream, thereby increasing blood calcium levels. In the kidneys, PTH increases the reabsorption of calcium, decreasing the amount of calcium excreted in urine. It also promotes the activation of vitamin D, which aids in the intestinal absorption of calcium from dietary sources. This multi-faceted approach ensures that PTH effectively raises and maintains the necessary levels of calcium in the blood, which is vital for various physiological processes, including muscle contraction and nerve function. Other options listed do not align with the specific role of PTH in calcium regulation.