The rate of metabolism of all body cells is regulated by _______.

Correct Answer: D

Rationale: The correct answer is D: thyroid hormone. Thyroid hormone plays a crucial role in regulating the rate of metabolism in all body cells by influencing the production of enzymes involved in energy production. It increases the body's metabolic rate, affecting processes such as heart rate, temperature regulation, and energy expenditure. Parathyroid hormone (A) regulates calcium levels, aldosterone (B) regulates electrolyte balance, and calcitonin (C) regulates calcium metabolism, but they do not directly influence overall metabolism rate in all body cells. Thyroid hormone is the primary hormone responsible for controlling metabolism throughout the body.

Correct Answer: A

Rationale: The correct answer is A because oxytocin release is an example of a positive feedback control mechanism. When oxytocin is released during childbirth, it stimulates contractions, which in turn lead to more oxytocin release, creating a positive feedback loop. This helps in the progression of labor. Choice B is incorrect because oxytocin is a neurohypophyseal hormone, not an adenohypophyseal secretion. Choice C is incorrect as oxytocin's main effects are related to labor, delivery, and lactation, not menstruation. Choice D is incorrect because oxytocin primarily controls uterine contractions during labor and milk ejection during breastfeeding, not milk production itself.

Correct Answer: B

Rationale: The correct answer is B: steroid hormones, because both diffuse easily into target cells. Thyroid hormone, being a small iodinated amine, can easily pass through the cell membrane and bind to intracellular receptors, similar to steroid hormones. Unlike insulin (choice A), which binds to cell surface receptors, thyroid hormone and steroid hormones enter the cell to exert their effects. Growth hormone (choice C) and glucagon (choice D) do not share the same mechanism of entry into target cells as thyroid hormone, making them incorrect choices.

Correct Answer: D

Rationale: The correct answer is D because hormones from the hypothalamus first enter the hypophyseal portal system before reaching the pituitary gland. This system allows direct communication between the hypothalamus and the pituitary gland, enabling precise control over hormone release. Choices A, B, and C are incorrect because they do not accurately describe the normal pathway of hormone regulation from the hypothalamus to the pituitary gland. Option A is incorrect as hormones do not travel to the heart before reaching the pituitary. Option B is incorrect because hormones do not enter the hepatic portal system to reach the pituitary. Option C is incorrect as hormones do not travel to the pituitary through arteries.

Correct Answer: C

Rationale: The correct answer is C: nervous. The endocrine system and the nervous system work together to regulate various bodily functions and maintain homeostasis. The endocrine system releases hormones that control metabolism, growth, and reproduction, while the nervous system sends electrical signals to coordinate body movements and responses. Both systems communicate and coordinate with each other to ensure the body functions properly. Choices A, B, and D are incorrect because although they are important systems in the body, they do not play as direct a role in maintaining homeostasis as the nervous system does.