By Mayo Clinic News Network

It can be a classic occurrence, especially at weddings. A member of the wedding party starts to wobble and sway, then before you know it they’ve fainted and fallen to the floor.

A number of factors can contribute to this, the main one being that when you stand rigidlystill, 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 moment then they crash. Researchers at Mayo Clinic are teaming up with the U.S. Department of Defense to study the issue in hopes of developing monitoring devices that can eventually save lives.

Dr. Michael Joyner and his research team at Mayo Clinic are studying how the body responds to blood loss.

“The purpose of this experiment is to help the army develop better monitors to determine who needs a transfusion and who doesn’t need a transfusion in battlefield trauma situations.”

Triaging the wounded can be difficult, because when someone loses blood, vital signs such as blood pressure and heart rate don’t change much until the victim suddenly collapses.

 “What we’re trying to do is predict when the vital signs become unstable,” Joyner said.

The study, funded by the Department of Defense, includes two phases. First the research team slowly and in a very controlled manner, removes blood from a test subject.

 Then they put the blood back in and start phase two of the experiment. This involves putting the test subject into a negative pressure box. The negative pressure makes it difficult for blood to flow back to the heart so it pools in the legs.

 The amount of blood that reaches the brain and heart decreases, mimicking blood loss. The researchers say if the body responds to the negative pressure the same way it responds to actual blood loss, they can do more experiments without having to take blood from patients’ bodies. The information they gather will be used to develop monitoring devices that will hopefully help determine who needs immediate medical attention on and off of the battlefield