What substance is the most important vasodilator of cerebral blood vessels?

Answer 1

Carbon dioxide (CO2). The arteries in your brain are designed to dilate (get bigger) when they sense more carbon dioxide and constrict (get smaller) when they sense less carbon dioxide. This is why you will pass out of you hyperventilate: when you consciously increase your breathing rate, you are blowing off more and more of your CO2. Less CO2 => blood vessels in your brain constrict => less blood flow to your brain => you feel lightheaded and will pass out if you continue breathing too fast. This is also why the remedy is to breathe into a paper bag - your CO2 will build up in the bag, you will maintain CO2 levels in your blood, and you will not pass out. Fun fact: for people with serious head trauma, you are very worried about brain swelling, since this can be lethal. One thing doctors do in this situation to buy them some time is to purposefully hyperventilate the patient. This decreases the patient's blood CO2 levels, which cause the blood vessels in the brain to constrict, and this can relieve some of the swelling. Unfortunately, this trick only works for 4 hours, but should be enough time to get other treatment started and/or get the patient to neurosurgery.