Over one million Americans faint every year, and countless more do worldwide. Fear, pain, the sight of blood or prolonged standing – think the long lines of summer travel – can trigger fainting. The Conversation

These triggers set off a reflex in our nervous system that cause our hearts to paradoxically slow instead of speed up, and our blood vessels to dilate instead of constrict. Blood pressure and brain blood flow plummet, and we faint.

Excessive heat plays a role, too, as it makes it more difficult for the body to maintain blood pressure. With the weather warming across the country, now is a good time to know signs and symptoms that lead to fainting. As a physiologist, I can offer some pointers.