The vasomotor center is a portion of the medulla oblongata

Medulla oblongata

The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...

 that regulates blood pressure

Blood pressure

Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

 and other homeostatic processes. Upon blood increase in carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 it stimulates the sympathetic system to constrict vessels. This is opposite to carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 in tissues causing vasoconstriction, especially in the brain. Cranial nerves IX (Glossopharyngeal nerve

Glossopharyngeal nerve

The glossopharyngeal nerve is the ninth of twelve pairs of cranial nerves . It exits the brainstem out from the sides of the upper medulla, just rostral to the vagus nerve...

) and X Vagus nerve

Vagus nerve

The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...

both feed into the vasomotor centre and are themselves involved in the regulation of blood pressure.