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will drain the blood, with the
obvious consequences, including
loss of vision, called grey out, at
+3.5G This could end up as black out
and unconsciousness at +6G. Both
are affected by hyperventilation,
hypoxia, heat, hypoglycaemia,
smoking and alcohol, all of which
are discussed more fully below.
The valsalva manoeuvre can be used to
help cope with high G (close your
throat and hold your nose), or you
could use a pressure suit.
Impact-wise, the body can tolerate
25G vertically and 45G horizontally
- if you don't wear shoulder straps,
tolerance to forward deceleration
reduces to below 25G, and you will
jackknife over your lapstrap with
your head hitting whatever is in
front of it at 12 times the speed it is
coming the other way.
Body Mass Index (BMI)
This is calculated by your weight in
kg divided by your height squared, in
metres squared.
If it is over 30, you are obese. This
could lead to diabetes and heart
disease, and reduce your ability to
cope with hypoxia, decompression
sickness and G tolerance.
The Central Nervous System
This consists of the brain and spinal
cord, though, for exam purposes, it
also includes the visual and aural
systems (eyes and ears),
proprioceptive system and
chemosensory system (smell).
The autonomic nervous system
regulates vital functions over which
you have no conscious control, like
heartbeat and breathing (unless
you're a high-grade Tibetan monk,
of course).
Although the brain is only 2% of the
body mass, it takes up to 20% of the
volume of each heartbeat – its blood
supply needs to be continuous, as
the brain cannot store oxygen.
Eyes
Vision is your primary (and most
dependable) source of information.
It gets harder with age to distinguish
moving objects; between the ages of
40 and 65, this ability diminishes by
up to 50%. This is only one of the
limitations of sight, and we need to
examine the eye to see how you
overcome them.
The eye is nearly round, and its
rotation in its socket is controlled by
external muscles:
Human Factors 247
It has three coatings; the sclerotic,
which is transparent at the front; the
choroid, which lines the sclerotic and
contains tiny blood vessels, and the
retina, which is the light sensitive bit
that detects electromagnetic waves
of the frequency of light, and
converts them to electrical signals
that are interpreted by the brain, and
which is sensitive to hypoxia.
This means you see with the brain as
well, giving a difference between
seeing and perceiving, discussed below.
This is because the eye's optical
quality is actually very poor (in fact,
you would get better results from a
pinhole camera), hence the need for
the brain, which can actually modify
what you see, based on experience,
and so is reliant on expectations. If
the brain fills in the gaps wrongly,
you get visual illusions.
The eye can, however react quickly
to changes in light conditions,
although it is slower to adapt from
light to dark because a chemical
(visual purple) needs to be created.
The optic nerve carries signals from
the eye to the brain, the lens focuses
light on to the retina, the iris controls
the diameter of the pupil (the black
bit in the middle where light gets
through), and the cornea refracts light
onto the lens. The area of sharp
vision is actually very small (at 4 feet
the size of a small coin), because of
the relatively small size of the fovea.
5°away from the foveal axis, it
reduces by a quarter, and onetwentieth
at 20° away. You should
be able to see another aircraft
directly at 7 miles, or 2.5 miles if it
was 45° off – at 60° it's down to half
a mile! The reason why you are
taught to scan is because the eye
needs to latch on to something,
difficult to do with a clear blue sky.
With an empty field of vision, your
eyes will focus at short distances,
about 1-2 metres ahead, and miss
objects further away.
Peripheral vision also has a different
neurological route to the brain. To
prove this, stand up, cover one eye
and make a fist with the other, to
cover up the good eye's central
vision. Now stand on one foot. Now
do it again, with your forefinger and
thumb making a very small circle
where the fist was – in other words,
you are using your central vision to
look through the circle. Standing on
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