Infusion of angiotensin II into the cerebral ventricles produces intense drinking behavior and antidiuretic hormone release despite an intact blood-brain barrier elsewhere. Which anatomical feature explains this central access?

• A The area postrema lacks a functional blood-brain barrier
• B The choroid plexus actively secretes angiotensin II into cerebrospinal fluid
• C Angiotensin II crosses the blood-brain barrier by carrier-mediated transport
• D The subfornical organ is a circumventricular organ with fenestrated capillaries ✓

Explanation

The subfornical organ is one of the circumventricular organs lying outside the blood-brain barrier, with fenestrated capillaries that permit circulating angiotensin II to reach neuronal AT1 receptors. Activation here stimulates thirst and vasopressin secretion, contributing to long-term blood pressure defense. The area postrema is also a circumventricular organ but mediates vomiting and chemoreception, not dipsogenesis. Peptides such as angiotensin II do not freely cross an intact blood-brain barrier, ruling out transport-based options.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.