Medullary ischemic reflex
The medullary ischemic reflex is a big response to a drop in blood pressure in the brain particularly in the medulla, where the lack of oxygen due to decreased perfusion triggers an autonomic response from the cardiac and vasomotor centers. The cardiac and vasomotor centers respond to the decrease in blood pressure with sympathetic outflow to the heart and blood vessels.
Source: Wikipedia — Medullary ischemic reflex (CC BY-SA 4.0)