Have you ever turned woozy at the sight of blood? You may have more in common with an opossum than you think.

The marsupial faints whenever it feels threatened in an attempt to ward off predators. Some researchers think that people who faint when they see blood or are surprised lose consciousness for a similar reason: to ward off enemies.

This reaction is called neurocardiogenic syncope (SING-kə-pee). Although all fainting is because of a quick drop in blood and oxygen in the brain, this specific reaction is spurred by a startling event or viewing blood or an injury. Think of the stereotype of a burly guy fainting at having his blood drawn.

This type of fainting, which occurs in about 3 to 4 percent of people, happens specifically because of a trip in the automatic nervous system. The trip is an overreaction of the vagus (VAY-gəs) nerve, which controls urination and digestion in the body. When the vagus nerve overreacts, a person can experience a sudden loss in blood pressure and heart rate, and ultimately, consciousness.

Other researchers who court an evolutionary reason for fainting suggest the drop in blood pressure may have been useful to stanch blood flow if our ancestors suffered an injury. Because this kind of fainting tends to run in the family, researchers think the fainting is an inherited response.

Research in the 1980s found that if people tensed their muscles when exposed to blood, they were less likely to faint. Participants in these studies tensed their muscles for 10 to 15 seconds while looking at pictures of blood. Slowly, the researchers increased the participants’ exposure to blood until they were able to watch a live surgery.

If you suffer the same kind of phobia, this tactic may work for you. But just remember to clench your muscles, breathe deeply and try to remain calm — and conscious.