Correct Option
The correct option is
Both A and R are true but R is NOT the correct explanation of A
.Explanation
Calcium homeostasis in the body is primarily regulated by the antagonistic actions of two hormones: Thyrocalcitonin (TCT) secreted by the thyroid gland and Parathyroid Hormone (PTH) secreted by the parathyroid glands. These hormones maintain blood calcium levels within a narrow physiological range.
Statement-wise Analysis
- Assertion (A): Correct
Thyrocalcitonin (TCT) is a peptide hormone secreted by the parafollicular cells (C-cells) of the thyroid gland. It plays a crucial role in regulating blood calcium levels. Specifically, it acts to decrease blood calcium levels when they become elevated, preventing hypercalcemia.
- Reason (R): Correct
Parathyroid hormone (PTH) is a peptide hormone secreted by the parathyroid glands. It acts directly on bone tissue to stimulate the process of bone resorption (dissolution or demineralization). This action releases calcium ions from the bone matrix into the blood, thereby increasing blood calcium levels.
Relationship Analysis
While both statements are factually true, the Reason (R) does not explain the Assertion (A). The Assertion describes the function of TCT, while the Reason describes the mechanism of action of PTH. Although both hormones regulate calcium, they function antagonistically (TCT lowers calcium, PTH raises it). The action of PTH on bones does not provide the causal explanation for why TCT regulates blood calcium levels.
Key Takeaway
Thyrocalcitonin (TCT) is a hypocalcemic hormone (lowers blood calcium), whereas Parathyroid Hormone (PTH) is a hypercalcemic hormone (increases blood calcium). They work together to maintain calcium balance.