Why fainting happens and how to nip it in the bud, from 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.
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