The area postrema is a circumventricular brain region positioned on the dorsal surface of the medulla on the floor of the fourth ventricle.
The blood–brain barrier and the cerebrospinal fluid–brain barrier are absent in this region and consequently many substances that do not pass across capillaries in other regions of the brain can do so in the area postrema.
The chemoreceptor trigger zone (CTZ), located in the lateral area postrema is sensitive to blood-borne emetogens. Nerves from the CTZ connect with the vomiting centre.