A bout of lightheadedness can come from all sorts of things. To help prevent yourself from passing out, Dr. Steven Lamm suggests tensing up your leg muscles. Photo by darkday.
Speaking with The Wall Street Journal, Dr. Lamm points out that oftentimes getting queasy is a physiological response to fear where the blood vessels dilate, blood pressure drops, and blood drains to your legs. So, you need to get it moving again:
Dr. Lamm offers an easy, on-the-spot antidote to squeamishness: tensing up leg muscles, then releasing them, over and over again. "We have a fixed amount of blood in our bodies, so if you can contract it far from your heart, you will have a better chance of getting that blood into your brain" and keeping yourself from swooning, he says.
Lamm's speaking specifically about dealing with being squeamish at the sight of blood, but it should work for most moments where you're feeling a bit lightheaded.
How to Get Over Being Squeamish at the Sight of Blood [The Wall Street Journal]