Microlight » Human Factors » G-Forces
The human body is designed to cope with a continuous acceleration on the form of earth gravity. Other accelerations occur when the body is subjected to changes in rate, or direction of movement. These accelerations are felt as linear or angular momentum - forcing the body sideways or downwards or rearwards and so on. Since the blood has mass, the changes in momentum can affect the circulation of the blood in the body.
With the body in an upright posture, gravitational force will result in the blood pressure in the lower extremities being greater than that in the head; however the body is adapted to this situation. If the g force is increased - as when pulling out of a dive - the blood pressure in the head may fall to a point where the blood supply to the eyes and the brain ceases. Vision loss and loss of consciousness can result when this force reaches about 3.5 times the normal g force (+3.5G).
The first to suffer are the eyes, starting with a loss of colour vision (grey-out), and then maybe tunnel vision and with further increases in g force will lead to black-out. With blackout, the optic nerve can no longer send images to the brain and the pilot is temporarily blind, but still conscious.