Why do some people pass out on roller coasters?

You may have seen the videos of riders on a roller coaster. They will be having fun and screaming, and then suddenly they pass out for a few seconds before regaining consciousness.

PEOPLE PASSING OUT ON ROLLER COASTERS COMPILATION

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ROLLERCOASTER CHUG 🎢 #rollercoaster #chug #challenge

Insider's Guide to Kings Island Amusement Park near Cincinnati

Skyscraper Branson pass out watch till end!!

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People Passing Out #2 Funny Slingshot Ride Compilation

Passing Out On Roller Coasters Compilation

Guy Passes Out On Slingshot Ride - #funny #fail