How the Body Responds to Blood Loss - Mayo Clinic

It's a classic scenario especially at weddings. A member of the wedding party starts to sway then, bam, they're passed out on the floor. Lots of factors contribute to this, the main one being when you stand rigidly still, blood pools in your legs away from your heart and brain so you faint. The same sort of thing happens when accident victims or wounded soldiers lose blood from internal injuries. They're fine for a while then they crash. Researchers at Mayo Clinic have teamed up with the Department of Defense to study this issue in hopes of developing monitoring devices that can help save lives.