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Review the effects of being in space on the
human body:
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Effects of acceleration/deceleration
Air pressure (or lack thereof!)
Temperature and humidity
Weightlessness (immediate and extended)
Ionising radiation
“Space Physiology and Medicine” by Nicogossian,
Huntoon and Pool. Published by Lea & Febiger.
◦ Library classmark – RC1150
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Accelerations (and conversely ‘decelerations’)
are given units of ‘g’ – multiples of Earth
standard gravitational acceleration (9.81ms-2)
Large, sustained accelerations are not
natural. Sustained here means tens →
hundreds of seconds.
Need a velocity of ~8 km s-1 to reach Earth
orbit. Launch lasts a few hundred seconds.
Area under the graph is the ‘g secs’ total. To achieve Earth
orbit approximately 815 g secs are experienced by an
astronaut
Mercury acceleration
profile (Friendship 7)
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Both launch and re-entry result in a series of
sharp jolts to the astronauts against a modest
g (>1) continuum.
How the body responds is the main medical
issue.
◦ The change in blood pressure, especially blood flow
to the brain and eyes.
◦ Damage to lung tissue
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Blood pressure: pressure exerted by the blood on the
walls of blood vessels.
During each heartbeat blood pressure varies from a
maximum (‘systlolic’) to a minimum (‘diastolic’ )
pressure.
Normally measured at a person’s upper left arm (BP
varies throughout the body, so need a ‘standard’
reference point).
Units of blood pressure are ‘millimetres of mercury’
(mmHg). [1 mmHg ≡ 133 Pa, ρHg = 13534 kg m-3]
Normally quoted as two numbers systolic over
diastolic pressure. i.e., “110/70”
Average values vary widely but ‘normal’ is considered
to be in the range 110/65 – 140/90
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Under normal conditions there is approximately a
20mmHg pressure difference between the pressure
leaving the heart (cardial), and the pressure of blood
entering the brain (cerebral). This difference is merely
due to the heart having to pump against gravity.
Under vertical acceleration, an additional gravitational
force (>1g) acts against the heart causing a drop in
cerebral pressure.
For accelerations >4.5g this causes the cerebral
blood pressure to drop to zero (hypoxia). As the
brain has no long term store of oxygen, this causes
unconsciousness within 6-7 seconds. Extended
periods cause brain damage through necrosis (tissue
death) and death.
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Just as there is a drop in pressure in the brain,
there is an increase in pressure in the lower
parts of the body (legs and abdomen) as the
blood accumulates.
This in turn restricts the amount of blood
flowing back to the heart, the heart then
restricts its output, further reducing blood
pressure (the dizzy feeling that some people
get when they stand up suddenly).
This ‘negative’ feedback process causes
cerebral pressure to drop even further.
The eye is also very sensitive to
blood pressure changes.
It has an internal vitreous humour
pressure of ~20 mmHg.
Blood flowing to the retina must have
a pressure greater than this pressure.
As blood pressure drops, peripheral
vision is lost, then finally central
vision (non-permanent damage, if
blood supply is restored within
minutes).
Occurs (just) before unconsciousness.
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How does the body react to
these effects?
◦ The carotid artery in the neck
(right) has receptors which
detect BP changes.
◦ As BP lowers smaller arteries
constrict to increase BP.
◦ Heart beat also increases to
keep blood volume constant
◦ The pooling effect of blood in
the lower body is a one-off
event. It takes the body ~10
seconds to restore normal
blood volume (some people
get dizzy if they stand up
suddenly, particularly if you
are borderline hypotensive).
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Acceleration also ‘squashes’ soft tissue.
Lungs are soft bags which are squeezed during
acceleration.
This causes various effects:
◦ Blood is squeezed out of the lungs, and affects oxygen
intake.
◦ The oxygen/CO2 exchange mechanism is compromised
leading to a build-up of CO2 in the blood.
◦ Very high accelerations (>15g) can lead to tearing of
lung tissue, lung cavity distortion and oxygen starvation.
◦ Soyuz 18 crew experienced re-entry acceleration ~20g
and one of the crew never flew again due to internal
damage.
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Technological solutions:
◦ Breathing high pressure gas. Counteracts the gforces acting on the lungs
◦ Wearing ‘g-suits’ (as used by combat pilots). These
squeeze the legs and abdomen reducing the
amount of ‘pooling’ that occurs and keeping BP
high.
◦ Change the astronaut’s position so that highest-g
is experienced in the least sensitive direction.
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Astronauts need to breath. Two ways of
supplying oxygen:
◦ Have astronauts remain in pressure (space) suits,
and supply oxygen from tanks or umbilicals. Ok for
short flights (Mercury, Vostok). Impractical for long
duration flights.
◦ Have a sealed and pressurised cabin to allow crew
to move and work freely. More complicated.
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Minimum pressure humans can operate in is
approximately 6% of standard atmospheric
pressure (using pure oxygen).
◦ Limited by oxygen consumption within the body
◦ Lower pressures can cause the evaporation of
bodily fluids (water, mucous membranes etc.)
◦ Haemorrhaging of surface capillaries.
◦ Would probably be very uncomfortable
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High pressures also problematic as loss of
cabin pressure would amplify the effects of
decompression (due to the larger pressure
differential between the cabin and vacuum).
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4 main concerns
◦ Barotrauma. Caused when gas is trapped within the body
due to external pressure changes. Mainly lowering of
pressure, but increasing pressure can cause severe ear
pain resulting in rupturing of the tympanic membrane).
Large pressure differentials can cause severe pain.
◦ Examples: middle ear, sinuses, teeth fillings and the gut
(long haul aircraft lower pressure in the passenger
cabin). Astronauts have had their fillings replaced during
training to stop this effect in teeth.
◦ In the Spaceshuttle, normal pressure is 1 atm (~105 Pa)
but can be controlled. Prior to an EVA pressure is
reduced to 0.69 atm and the rate of change is limited to
0.007 atm/sec – 0.07 atm/sec (in an emergency).
◦ Explosive decompression (everyone’s favourite).
◦ A pressure change so great and rapid that any gas
trapped within the body cannot flow out to equalise the
pressure. The result is a transient overpressure. A
transient overpressure ~0.1 atm can rupture the lungs.
Capillaries in the alveoli of the lungs rupture leading to
bleeding and ‘frothing’ of blood and air in the lungs.
◦ Leads to embolisms (air bubbles in the blood) which get
trapped and occlude blood vessels. Almost certainly
fatal.
◦ Swelling and rupturing of capillaries on the skin and
cornea (unpleasant, but probably not fatal).
◦ A pressure differential of 1 atm does not cause people to
explode and people (or aliens) do not get sucked out of
small holes.
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The results of explosive decompression
depend on:
1) Starting pressure
2) Rate of change of pressure
3) Absolute change in pressure
4) Ratio final/starting pressure
5) Ratio of lung volume at time of
decompression to maximum lung capacity
6) Ratio of venting area in cabin to airway
orifice of lung
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Example: If a cabin leak occurs from a small hole, the
size of which is comparable to the opening from the
mouth to the lungs (glottis). Flow rates will be similar
and lung pressure will drop with cabin pressure.
If this is not the case, lung volume will try to expand
to permit equalization of pressures. If this expansion
is limited (i.e. reach capacity of lungs) before
equalisation of pressure, the difference determines if
rupture occurs (i.e. greater than 0.1 atm ?).
In the Space-shuttle, the risk of decompression limits
the ejection altitude. Above certain altitudes is can
only occur is the crew wear oxygen masks.
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Non-explosive decompression
◦ Slow decompression itself is a problem.
Decompression sickness (the ‘bends’) occurs when
the pressure of gases dissolved in a bodily fluid
(normally the blood) exceeds the ambient pressure.
◦ The gas evolves from the blood (i.e., it forms
bubbles – think of opening a bottle of fizzy water).
◦ Bubbles in the blood are generally bad news. Can
form embolisms (blockages) and cause severe pain
in joints and lungs.
◦ Problems occur when dissolved gas pressure /
ambient pressure is >1.3.
◦ Bubbles are removed via the lungs and the
dissolved gas pressure equalises eventually.
◦ Different gases equalise at different rates: oxygen
equalises within seconds → minutes. Nitrogen takes
minutes →hours. This is one reason why pure
oxygen is used.
◦ Soyuz 11 (1971) crew killed when a valve opened at
too high an altitude during re-entry and vented the
atmosphere. No spacesuits meant the crew
suffocated.
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Pre space-shuttle NASA flew missions at 33% atm pressure (pure
oxygen). EVA suits had pressures of 25% atm so moving from
one to another didn’t cause decompression problems.
Spaceshuttle flies with an atmosphere of 1 atm. EVAs have to be
carried out at 25% atm due to pressure constraints in a ‘soft’
suit.
The astronaut therefore has to go through a decompression
procedure to allow for EVAs
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1 hour pure oxygen at 1 atm
70% atm at 26.5% oxygen for 24 hours
40 minutes in spacesuit (prior to EVA) at 70% atm and 100% oxygen
Reduction to 25% atm and pure oxygen in airlock prior to EVA.
Why so complicated? Various procedures tried (on the ground) to
check whole body washout times of dissolved gases. This
procedure reduces the chances of decompression sickness (~1%).
Altitude sickness
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This occurs due to poor oxygen pickup at low pressures. At sea level the
partial pressure of oxygen in the air is 3.06 psi. In the STS it is 3.2 psi.
In the alveoli (small sacs in lung tissue where gas exchange occurs) the
partial pressure is 2.01 psi. Problems of oxygen uptake occur if this is
not maintained.
If there is less oxygen (this falls with altitude, hence the name), the
height at which an individual can still take enough oxygen is approx.
1830m, where alveoli oxygen pressure is 1.50 psi. Above this altitude
problems appear progressively.
When the problem occurs the symptoms are; vision impairment, loss of
breath, exhaustion, reduced mental capacity (e.g. confusion, poor
learning ability), motor impairment. Finally at 0.67 psi unconsciousness
occurs.
Equally, high alveoli oxygen levels ( > 2.01 psi) are bad. This results in
oxygen toxicity. Note however, that oxygen concentrations themselves
are not the problem, the pressure is also important.
(Mauna Kea observatory in Hawaii is at 4000 m – working here is like
having a constant stinking hangover).
CO2
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This is a product of respiration. On Earth it comprises 0.04% of the
atmosphere by concentration. But in an enclosed space with crew this
builds up. Extra CO2 causes increases in heart rate, respiration rate, and
changes acid base of the body. On STS, CO2 is limited to 0.15 psi, above
this there is a mission contingency, i.e. crew don breathing masks if it
reaches 0.30 psi (2% CO2).
The CO2 content in a cabin is controlled by scrubbers. These are usually
chemical absorption devices, i.e. cans of chemicals (e.g. lithium
hydroxide) which react with CO2, trapping it in their volume. These cans
have finite capacity, so on space stations they need regular replacement
(cf. the weekly house keeping on Salyut/Mir). Cans can be recycled.
When heated they release their CO2. If this is done in a sealed port and
vented externally, they can then be reused. On the STS the cans are
disposable.
2LiOH (s) + CO2 (g) → Li2CO3(s) + H2O(g)
1% ≡ 0.01 atm
≡ 0.147 PSI
Water vapour.
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Percentage humidity is water vapour pressure/max water vapour
pressure in the atmosphere at the given temperature.
High relative humidity is associated with condensation (on any
local relatively cool surfaces), and is conducive to microbial and
fungal growth (and electrical short-circuits). It is thus to be
avoided.
Absolute humidity is also important (i.e. actual partial pressure
of water vapour). Low humidity causes dry eyes and skin, as well
as dry mucus membranes, chipping of lips etc. Breathing tract
suffers damage and this in time encourages respiratory illness.
High percentage humidity is uncomfortable.
Humidity also affects heat loss and heat balance. 0.19 psi is
optimal for habitability (relative humidity ~30-60%). STS
maintains 0.12 to 0.27 psi.
Temperature.
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Bodies perform best within an optimum temperature range.
Outside this range shivering/sweating try to correct imbalance in
body temp.
This causes discomfort/impairs performance. Oddly no single
optimum temp exists, it varies from person to person by 5oC.
STS maintains temp between 18 and 27oC. Crew wear/discard
light clothing to suit themselves.
The operation of apparatus inside the STS means that in normal
flight the problem is one of reducing cabin temperature. This is
done via heat pumps which redistribute heat to radiators
mounted on the interior of the cargo bay doors. It is thus
essential that these are opened as early as possible into a
mission.
Failure to open the doors can require an abort of the flight.
Temperature
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Hyperthermia (‘heat stroke’) occurs when the body
temperature rises above 40.6oC. Symptoms include
red and hot skin, sweating, nausea, confusion.
Hypothermia occurs when the body temperature
drops below 35oC. Symptoms include shivering,
numbness in extremities (blood vessels constrict to
reduce heat loss through radiation).
Humans can only exist in a narrow range of
temperatures without thermal protection (it gets
uncomfortably cold below 10oC and hot above 35oC –
but this is humidity dependent).
Toxic gas build-up.
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Venting of fumes from rocket exhausts into the cabin is a design
flaw that occurred on several early missions What usually
happened was that during descent pressure equalisation of cabin
and exterior occurs. This can permit drawing of fumes into the
cabin.
Apollo-Soyuz mission: a valve opened prematurely and nitrogen
tetroxide gas (a fuel oxidiser) was sucked into the spacecraft. All
3 astronauts suffered gas poisoning and inflammation of the
lungs (pulmonary edema) and spent two weeks in hospital.
Today the STS has potential problems after landing, as fumes
can build up in nozzles etc. and then disperse as a cloud around
the landed shuttle. So crew disembark into a cabin with its own
air supply. After earlier STS missions the crew sat in the craft
until external monitoring indicated no hazard.
Toxic gas build-up
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Fumes can also build up in a cabin during a mission.
Outgassing of materials releases gases which can be
poisonous. On Earth these are simply vented away. In
flight they accumulate.
The result is that you need ‘scavengers’ to remove toxic
chemicals or gases, or you need to prevent the flight of
any material subject to outgassing.
NASA has strict ratings as to permitted outgassing of
materials. This also limits the amount of scientific or
engineering work that may be carried out in the crew area.
Clearly no process can occur which releases fumes into the
crew atmosphere (or which consumes oxygen !).
The atmosphere therefore has to be circulated (and
‘scrubbed’).
Fire
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A separate issue not strictly related to atmospheres
is the need to avoid fire. This consumes oxygen and
can release poisonous gases. The oxygen resupply
and toxic gas removal systems have limited
capacities, and finite response times.
If the oxygen content is high, or has been increased
in preparation for an EVA, flash fires present an
enormous hazard (Apollo 1 crew were killed (1967),
the Soyuz T-10-1 crew barely escaped (1983)).
Smoke from a fire can fill the cabin and make
movement difficult to carry out. Fire is thus a hazard
that requires major efforts to avoid, and serious
countermeasures.
Weightlessness (well, ‘microgravity’)
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During spaceflight the most dramatic effects on
the body are associated with microgravity
The body was has evolved for a 1 g constant load
in the vertical, with regular periods of rest
perpendicular to this.
Spaceflight violates this natural order of things.
The effects on the body’s systems are varied and
include:
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balance (neurovestibular system),
bones (calcium loss),
muscle (nitrogen loss)
cardio-pulmonary system
Balance and Space Motion Sickness (SMS)
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The most obvious external symptom is that
50% of astronauts suffer from space nausea.
Some experience discomfort, others the onset
of stomach heaving, and some are simply
sick.
Onset of symptoms is very rapid once orbit is
achieved. It can take 8 hours for the worst to
pass, and then almost all astronauts recover.
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Earlier astronauts had a much lower
incidence of illness (indeed there
were virtually no reports of illness
during the early sixties). It was
therefore supposed that some
change in mission organisation had
triggered the onset of nausea.
The most obvious change was that
early astronauts had been securely
strapped in, with no freedom of
movement. The surfaces in their
cabin were fixed relative to them,
and horizons were thus fixed. It was
supposed that later missions, where
crew had freedom of movement,
confused the crew with a mismatch
of horizon and expectations. Result
nausea.
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Travel sickness (or ‘earth motion sickness’ - EMS, sea
sickness) are very similar. Long periods watching a
rushing past landscape, whilst local objects are
stationary. Sends conflicting messages to the brain.
This has been tested by NASA. They constructed a
room, with a chair which slowly rotated with both
pitch and yaw. Candidates then had to carry out
tasks, alternatively focusing on their hands and the
background. A sequence of 1200 hand eye coordinated movements is required.
No candidate completed the sequence first time
without illness. Seeming confirmation of the theory.
Repeated tests increase the tolerance to this problem.
Drugs administered before the tests also increase
tolerance.
The neurovestibular (‘balance’) system is also
affected.
The three semi-circular canals
(superior, posterior and
horizontal) contain a viscous
fluid that brushes against hairs
on the internal surface of the
cochlea. In μg this fluid is no
longer flowing under gravity
and leads to conflicting nerve
impulses.
The left and right ears may also be
affected differently. Leads to
dizziness and nausea
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In μg blood also tends to migrate upwards –
probably due to the way blood flows from the
heart (initially upwards to the brain).
This causes increased pressure on blood vessels
and nerves, again leading to dizziness and
nausea.
Tests carried out on Earth indicate there is a
difference between EMS and SMS. Spontaneous
vomiting is very rare in EMS, but common in SMS.
SMS normally improves within a few hours, but
some astronauts have suffered for 2 – 4 days.