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"Grey-out" occurs in case of..
  • A
    -2 Gz.
  • B
    +2 Gz.
  • C
    -4 Gx.
  • D
    +4 Gx.

Refer to figure.


Under increased +Gz acceleration, fluid and tissues are displaced downwards. This is most apparent in the face, where the skin can be seen to sag. Since the weight of the body may be increased many times while the power of the muscles remains unaltered, movements become progressively more difficult. If the head is lowered, it may not be possible to raise it again.

A pressure gradient develops in the blood between the heart and the brain, resulting in reduced blood pressure at head level. This can reduce the supply of oxygen to the eye and brain, ultimately leading to G-Induced Loss of Consciousness (G-LOC). The blood pressure in the blood vessels of the lower parts of the body rises with increasing G, and flow of blood back to the heart is reduced. The high blood pressure in parts of the body below heart level can be sufficient to rupture small capillaries in the feet and forearms, leaving a fine rash (‘G measles’).

  • As the level of acceleration is increased, inadequate blood pressure to supply oxygen to the retina causes partial loss of vision ('grey-out'). It also induces a tunnelling of vision as the eye cells at the edge of the retina, being furthest from the blood supply, suffer first. Followed by total loss of vision ('black-out'). Ultimately, it can lead to unconsciousness. Recovery from unconsciousness is frequently associated with jerky and uncontrolled movements of the head and limbs.

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