Calcitonin and parathyroid hormone work _ on blood calcium levels. Answers: A – C A antagonistically B permissively C synergistically
The correct answer and explanation is:
The correct answer is A – antagonistically.
Calcitonin and parathyroid hormone (PTH) have opposing effects on blood calcium levels. These two hormones help regulate calcium homeostasis in the body, but they act in opposite directions to maintain a stable concentration of calcium in the bloodstream.
Calcitonin, which is produced by the thyroid gland, lowers blood calcium levels. It achieves this by inhibiting the activity of osteoclasts, the cells responsible for breaking down bone tissue. By inhibiting osteoclasts, calcitonin decreases the release of calcium from bones into the bloodstream. It also enhances the excretion of calcium through the kidneys. As a result, calcitonin helps to reduce high blood calcium levels.
On the other hand, parathyroid hormone (PTH), secreted by the parathyroid glands, increases blood calcium levels. PTH acts primarily on three organs: the bones, kidneys, and intestines. In the bones, it stimulates osteoclast activity, leading to the release of calcium into the blood. In the kidneys, PTH promotes calcium reabsorption, reducing calcium loss in urine. Additionally, PTH enhances the activation of vitamin D in the kidneys, which increases calcium absorption in the intestines. Through these mechanisms, PTH raises blood calcium levels when they are too low.
These two hormones counterbalance each other in a way that helps keep blood calcium concentrations within a narrow, optimal range. When blood calcium levels are high, calcitonin is released to lower them, and when calcium levels are low, PTH is released to raise them. Therefore, their actions are antagonistic, meaning they work against each other to maintain calcium homeostasis.