Horror movies have been thrilling fans since the late 1800s, and research has shown that people who watched a horror film had a spike in white blood cells—a basic and important part of the immune system—in response to the on-screen stressors they observed. But while watching a scary movie may help keep you healthy, the physiology portrayed in spine-chilling scenes isn’t always correct. Filmmakers have a history of making a few common mistakes in horror movie physiology:

• Unrealistic Blood: On the screen, blood is a bright red liquid that squirts out of wounds when a character is killed. Blood gets its red color from red blood cells that are filled with oxygen. But without oxygen—such as when someone dies—it changes to a dark reddish-purple hue. Also, when exposed to air, blood will also start to thicken into a gel-like substance, unlike the watery blood seen in movies.
• Sweat-free “Victims”: “Don’t open the door!” How many times have you yelled that opening that door is a bad idea? You may feel frightened just like the people on the screen, making your fight or flight response kick in. Watching horror movies can cause the same responses as fight or flight, which is why some people love them. Signals in your nervous system cause hormones such as adrenaline to release, which can make you feel out of breath, dizzy, sweaty or chilled. However, many of the characters in horror films—even after being chased by a person with a chainsaw—look like they haven’t broken a sweat.
• Injuries: Characters in horror flicks seem to have a very quick recovery time, no matter how badly they are hurt. In real life, however, several physiological processes take place after an injury. For example, muscle damage healing can take anywhere from 48 hours to six weeks. The best form of recovery for muscle injuries is to ice the area and rest, which most horror movies don’t show. Wound healing has a three-step healing process (blood clotting, inflammation and repair) that you don’t always see on the screen. Most horror movie characters never seek medical attention, but manage to avoid getting infected wounds while running through mud, climbing rusty old ladders and getting covered in alien slime.

Scientists and medical professionals may cringe when they see incorrect portrayals of physiology in horror movies. But maybe these liberties are what keeps them enjoyable.

Happy Halloween!

Liz Cambron is a PhD candidate in cellular and molecular biology at North Dakota State University. The Greenlee Lab focuses on insect physiology, including how environmental stress impacts pollinators.