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.

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.

Physiologists, neurologists and cardiologists have long studied how people regulate blood pressure and maintain consciousness while standing. Upon standing, there is an immediate decline in blood pressure, and the body must respond to this decline. The normal response includes constriction of blood vessels, increased heart rate and the release of various hormones into the bloodstream.

Together these responses help to maintain blood pressure. By protecting blood pressure, our oxygen-thirsty brain ensures that it will continue to be adequately nourished. Usually the response is flawless – we can stand without any trouble.

But various triggers sometimes mess up this finely tuned response, and the summer heat makes it more challenging to regulate blood pressure.

Ultimately, if blood pressure is not maintained, brain blood flow will decline. The fancy term for this is cerebral hypoperfusion – the defining characteristic of fainting (i.e., syncope), whatever the cause or trigger.

The symptoms that precede syncope vary but may include sweating, nausea, paleness and abdominal discomfort.

Soldiers standing guard for long periods in the heat without moving can have excessive gravity-induced pooling of blood in their legs, which can trigger a faint. But simply shifting weight from one leg to another will contract the leg muscles and propel blood toward the heart and head. This will usually prevent a severe decline in blood pressure.

Astronauts returning to Earth after a sojourn in space are especially sensitive to gravity. When they first return to Earth, many astronauts cannot stand for more then a few minutes without experiencing syncope. Fortunately, this resolves once they get reaccustomed to Earth’s gravity over the ensuing days.

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