Thus, blood, and life-giving oxygen, are easier for the heart to circulate to the brain.
People can be exposed to gravitational force, or g-force, in different ways. It can be localized, affecting only a portion of the body, as in getting slapped on the back. It can also be momentary, such as hard forces endured in a car crash. A third type of g-force is sustained, or lasting for at least several seconds. ( ① ) Sustained, bodywide g-forces are the most dangerous to people. ( ② ) The body usually withstands localized or momentary g-force better than sustained g-force, which can be deadly because blood is forced into the legs, depriving the rest of the body of oxygen. ( ③ ) Sustained g-force applied while the body is horizontal, or lying down, instead of sitting or standing tends to be more tolerable to people, because blood pools in the back and not the legs. ( ④ ) Some people, such as astronauts and fighter jet pilots, undergo special training exercises to increase their bodies’ resistance to g-force.