In the hypothalamus-pituitary-adrenal (HPA) axis, a patient's adrenal glands are surgically removed. What happens to circulating CRH and ACTH levels?
ABoth CRH and ACTH decrease, because there is no target gland to stimulate
BCRH decreases but ACTH increases, since the pituitary tries to compensate
CBoth CRH and ACTH increase markedly, because negative feedback from cortisol is lost
DACTH increases but CRH decreases, since pituitary output inhibits the hypothalamus
Without adrenal glands, cortisol production ceases. Cortisol normally feeds back to inhibit both CRH secretion at the hypothalamus and ACTH secretion at the anterior pituitary. When this negative feedback disappears, both CRH and ACTH rise dramatically — a pattern seen in Addison's disease. Tracing feedback loops by asking 'what is removed?' and 'what inhibition is therefore lost?' is the key reasoning pattern for this axis.
Question 2 True / False
The posterior pituitary synthesizes ADH and oxytocin and releases them in response to appropriate stimuli.
TTrue
FFalse
Answer: False
ADH (vasopressin) and oxytocin are synthesized in hypothalamic nuclei (the supraoptic and paraventricular nuclei). These hormones are transported down axons into the posterior pituitary, where they are stored and released into the bloodstream when needed. The posterior pituitary does not synthesize anything — it is essentially a storage and release site for hypothalamic products. Confusing synthesis with release is an extremely common error.
Question 3 Short Answer
Why does the hypothalamus communicate with the anterior pituitary via a specialized portal blood vessel system rather than via direct neural connections or the general circulation?
Think about your answer, then reveal below.
Model answer: The hypophyseal portal system delivers hypothalamic releasing and inhibiting hormones directly to the anterior pituitary at high local concentrations before those hormones dilute into the general circulation. This enables precise, rapid control of anterior pituitary hormone secretion that would be impossible if the hypothalamic signals had to travel through the full systemic circulation.
If hypothalamic hormones entered the general circulation first, they would be diluted to extremely low concentrations by the time they reached the anterior pituitary, making control imprecise. The portal system is a short-circuit: blood flows from hypothalamic capillaries directly to anterior pituitary capillaries, keeping concentrations high and response times fast. This architecture reflects the design principle of compartmentalized vascular delivery seen elsewhere in the body (e.g., the hepatic portal system).