The parathyroid glands maintain adequate levels of blood calcium. This is accomplished through ________.

Correct Answer: B

Rationale: The correct answer is B because the parathyroid glands target the bone and activate osteoclasts to release calcium into the bloodstream. This process helps maintain adequate blood calcium levels. By activating osteoclasts, calcium is released from the bone into the blood, ensuring a balance of calcium levels. A: Blocking the action of growth hormone is unrelated to the function of parathyroid glands in maintaining blood calcium levels. C: Antagonizing the synthesis of calcitonin is not the primary mechanism through which parathyroid glands regulate blood calcium levels. D: Slowing the activity of tissues that require calcium for activity would not help maintain adequate blood calcium levels as it does not address the direct release of calcium from bone by activating osteoclasts.

Correct Answer: C

Rationale: The correct answer is C because peptide hormones typically bind to receptor proteins on the plasma membrane. This is because peptide hormones are not able to pass through the cell membrane due to their size and polarity. Once the hormone binds to the receptor on the plasma membrane, it activates a second messenger system within the cell, leading to various cellular responses. Choice A is incorrect because peptide hormones do not bind directly to intracellular components like second messengers inside the cell. Choice B is incorrect because peptide hormones do not directly activate particular genes within the cell. Choice D is incorrect because peptide hormones do not bind directly to genes on the plasma membrane.

Correct Answer: C

Rationale: The correct answer is C because ADH (antidiuretic hormone) increases water reabsorption by the kidneys (increasing blood volume), causes vasoconstriction (narrowing of blood vessels), and ultimately leads to an increase in blood pressure. Choice A is incorrect because ADH does not decrease water reabsorption. Choice B is incorrect because ADH does not cause dilation of blood vessels, which would decrease blood pressure. Choice D is incorrect because ADH does not cause dilation of blood vessels, which would not lead to a decrease in blood pressure.

Correct Answer: A

Rationale: The correct answer is A: thyroid hormone. The thyroid hormone plays a crucial role in regulating metabolism by increasing the body's metabolic rate. It accomplishes this by stimulating the production of proteins that control the rate at which cells use energy. Calcitonin (B) is involved in regulating calcium levels in the blood, not metabolic rate. Aldosterone (C) is a hormone that regulates electrolyte balance and blood pressure. Parathyroid hormone (D) regulates calcium and phosphorus levels in the blood, not metabolic rate. Thus, the correct choice is thyroid hormone as it directly affects metabolic rate.

Correct Answer: A

Rationale: The correct answer is A: atrial natriuretic hormone (ANH). ANH opposes the actions of aldosterone by promoting salt and water excretion, thus reducing blood volume and blood pressure. ANH is released in response to high blood pressure or high blood volume. B: antidiuretic hormone (ADH) is not the antagonistic hormone to aldosterone; it acts on the kidneys to regulate water balance. C: cortisol is a stress hormone and is not directly involved in regulating sodium and water balance. D: glucagon is a hormone that increases blood sugar levels and is not related to the regulation of aldosterone.