Answer: receptors.
- A receptors
- B nuclei
- C cell walls
- D cilia
Correct answer: A. receptors
Explanation: A hormone acts only on cells that carry receptors for it.
The hypothalamus releases hormones that direct the pituitary, which releases its own trophic hormone to a target gland; that gland's hormone output then feeds back (dashed lines) to suppress both the pituitary and hypothalamus once levels are sufficient - a self-correcting loop running the entire endocrine system.
Concept context
The endocrine glands, hormone classification and mechanism of action, and negative feedback regulation.