Why fainting happens and how to nip it in the bud, from the Harvard Heart Letter
Faint, black out, swoon, pass out. They're all names for the same thing—a temporary loss of consciousness followed by a fairly rapid and complete recovery. Fainting occurs when something interrupts blood flow to the brain. Although usually harmless, fainting can cause injuries and sometimes signals a problem with the heart or circulatory system, reports the August 2009 issue of the Harvard Heart Letter.
The most common reason for fainting stems from crossed wires between the brain and the vagus nerve, the part of the nervous system that regulates blood pressure and heart rate. In response to some trigger, the blood vessels in your legs relax and the heart rate slows, making it difficult for blood to return to the heart. Blood pressure drops, and so do you. Triggers can range from fear and pain to standing for too long or even sneezing or going to the bathroom. Some people faint when they get out of bed or stand up from a chair.
Sometimes fainting is caused by trouble in the heart. This is more worrisome than fainting due to noncardiac causes. Heart rhythm problems are the most common culprit here. Anything that obstructs blood flow through the heart can also lead to fainting, such as a blocked or narrowed heart valve or a blood clot.