Aviation and High Altitude and Space Physiology pdf - Aviation and Space and Deep Sea Diving Physiology

lowing discussions.

very different when the person is breathing pure oxygen, as we see in the fol-

of acclimatized people can barely survive when breathing air. But the effect is

oxygen pressure in the alveoli. At the summit of Mount Everest, only the best

oxygen is continually being absorbed into the blood, leaving about 35 mm Hg

in the alveoli would be 40 mm Hg. However, some of this remaining alveolar

199 mm Hg would be oxygen and four fifths would be nitrogen; that is, the P

only 199 mm Hg. If there were no use of oxygen by the body, one fifth of this

person, 7 mm of the 206 mm Hg must be carbon dioxide, leaving

acclimatized

this must be water vapor, leaving only 206 mm Hg for all the other gases. In the

at the top of 29,028–foot Mount Everest. Forty-seven millimeters of mercury of

sea-level value of 760 mm Hg to 253 mm Hg, which is the usual measured value

oxygen. For instance, assume that the barometric pressure falls from the normal

falls to about 7 mm Hg because of increased respiration.

person, who increases his or her ventilation about fivefold, the P

acclimatized

falls from the sea-level value of 40 mm Hg to lower values. In the

In the case of carbon dioxide, during exposure to very high altitudes, the alve-

the body temperature is normal, regardless of altitude.

centration. Water vapor pressure in the alveoli remains 47 mm Hg as long as

These two gases dilute the oxygen in the alveoli, thus reducing the oxygen con-

alveoli. Also, water vaporizes into the inspired air from the respiratory surfaces.

Even at high altitudes,

Carbon Dioxide and Water Vapor Decrease the Alveolar Oxygen.

at Different Elevations

18 mm Hg.

at sea level about 159 mm Hg, but at 50,000 feet only

proportionately, remaining at all times slightly less than 21 per cent of the total

metric pressure decreases, the atmospheric oxygen partial pressure decreases

of all the hypoxia problems in high-altitude physiology because, as the baro-

50,000 feet, 87 mm Hg. This decrease in barometric pressure is the basic cause

barometric pressure is 760 mm Hg; at 10,000 feet, only 523 mm Hg; and at

at different altitudes, showing that at sea level, the

oxygen pressures

Table 43–1 gives the approximate

Effects of Low Oxygen Pressure on the Body

body. This chapter deals with these problems.

forces, weightlessness, and so forth) on the human

in aviation, mountain climbing, and space vehicles,

Aviation, High-Altitude,

537

and Space Physiology

As we have ascended to higher and higher altitudes

it has become progressively more important to
understand the effects of altitude and low gas pres-
sures (as well as several other factors—acceleratory

Barometric Pressures at Different Altitudes.

baro-

metric and

barometric pressure—Po

Alveolar P

carbon dioxide is continually excreted from the pulmonary blood into the

olar Pco

Now let us see how the pressures of these two gases affect the alveolar

unacclimatized person breathing air, beginning at an

Acute Effects of Hypoxia

operates perfectly.

47,000 feet, provided the oxygen-supplying equipment

23,000 feet and when breathing pure oxygen is about

feet when one is breathing air. In addition, because an

air. For instance, the arterial saturation at 47,000 feet

tion curves in Figure 43–1, one notes that an aviator

Unpressurized Airplane

and When Breathing Oxygen in an

The “Ceiling” When Breathing Air

ascends to about 39,000 feet; then it falls rapidly to

when one is breathing pure oxygen. Note that the sat-

The red curve of Figure 43–1 shows arterial blood

air (see Table 43–1).

139 mm Hg instead of the 18 mm Hg when breathing

as high as

nitrogen becomes occupied by oxygen. At 30,000 feet,

When a person breathes pure oxygen instead of air,

at Different Altitudes

Effect of Breathing Pure Oxygen on

and much less at still higher altitudes.

falls rapidly, as shown by the blue curve of the figure,

cent. Above 10,000 feet, the arterial oxygen saturation

10,000 feet, even when air is breathed, the arterial

while breathing oxygen. Up to an altitude of about

Figure 43–1 shows arterial blood oxygen saturation at

than in the unacclimatized person, as we discuss later.

person but only to 53 mm Hg in the acclimatized. The

tude, it falls to about 40 mm Hg in the unacclimatized

is 104 mm Hg; at 20,000 feet alti-

level, the alveolar P

person. At sea

acclimatized

unacclimatized

Table 43–1 shows the approximate P

The fifth column of

Aviation, Space, and Deep-Sea Diving Physiology

538

Unit VIII

Alveolar P

at Different Altitudes.

s in the alveoli

at different altitudes when one is breathing air for both
the

and the

difference between these two is that alveolar ventila-
tion increases much more in the acclimatized person

Saturation of Hemoglobin with Oxygen at Different Altitudes.

different altitudes while a person is breathing air and

oxygen saturation remains at least as high as 90 per

until it is slightly less than 70 per cent at 20,000 feet

Alveolar P

most of the space in the alveoli formerly occupied by

an aviator could have an alveolar Po

hemoglobin oxygen saturation at different altitudes

uration remains above 90 per cent until the aviator

about 50 per cent at about 47,000 feet.

Comparing the two arterial blood oxygen satura-

breathing pure oxygen in an unpressurized airplane
can ascend to far higher altitudes than one breathing

when one is breathing oxygen is about 50 per cent and
is equivalent to the arterial oxygen saturation at 23,000

unacclimatized person usually can remain conscious
until the arterial oxygen saturation falls to 50 per cent,
for short exposure times the ceiling for an aviator in
an unpressurized airplane when breathing air is about

Some of the important acute effects of hypoxia in the

Table 43–1

Effects of Acute Exposure to Low Atmospheric Pressures on Alveolar Gas Concentrations and Arterial Oxygen Saturation*

Alveoli

Saturation

Alveoli

Saturation

Pressure

Air

Oxygen

Barometric

Arterial

Breathing Air

Breathing Pure Oxygen

50,000

40,000

141

30,000

226

24 (7)

18 (30)

24 (38)

139

20,000

349

24 (10)

40 (53)

73 (85)

262

100

10,000

523

110

36 (23)

67 (77)

90 (92)

436

100

760

159

40 (40)

104 (104)

97 (97)

673

100

(mm Hg)

(%)

(mm Hg)

(%)

Altitude (ft)

(mm Hg)

* Numbers in parentheses are acclimatized values.

Altitude (thousands of feet)

Arterial oxygen saturation (per cent)

100

Breathing pure oxygen
Breathing air

ing air and when breathing pure oxygen.

Effect of high altitude on arterial oxygen saturation when breath-

Figure 43–1

). This occurs

in the nonpulmonary tissues, which is called

growth of

, so that the amount

over a period of weeks

The cardiac output

tion—Increased Tissue Capillarity.

Peripheral Circulatory System Changes During Acclimatiza-

conditions.

of the lungs, which are poorly perfused under usual

increase in pulmonary arterial blood pressure; this

interface still more. A final part results from an

expands the surface area of the alveolar-capillary

results from an increase in lung air volume, which

which oxygen can diffuse into the blood. Another part

monary capillary blood volume, which expands the

altitude.

increase as much as threefold during exercise. A

21 ml/mm Hg/min, and this diffusing capacity can

by 20 to 30 per cent, and this increase times the

In addition, the blood volume also increases, often

of 40 to 45 to an average of about 60, with an average

a time, the hematocrit rises slowly from a normal value

increase in red blood cell production. Ordinarily, when

Increase in Red Blood Cells and Hemoglobin Concentration

for the alkalosis.

tion. Thus, the respiratory centers are much more

bicarbonate excretion. This metabolic compensation

Chapter 30. The kidneys respond to decreased P

kidneys for the respiratory alkalosis, as discussed in

activity of the center.

center, thus increasing the respiratory stimulatory

This in turn decreases the pH in the fluids surround-

the cerebrospinal fluid as well as in the brain tissues.

The cause of this fading inhibition is believed to be

ceptor stimulus from hypoxia, and ventilation in-

inhibition fades away, allowing the respiratory center

. But during the ensuing 2 to 5 days, this

peripheral arterial chemoreceptors in the carotid and

to stimulate respiration by way of the

of low P

pH of the body fluids. These changes

carbon dioxide, reducing the P

The immediate increase in pulmonary ventilation on

chemoreceptors increase ventilation still more, up to

remains at very high altitude for several days, the

without the increased ventilation. Then, if the person

high altitude, and it alone allows the person to rise

Therefore, compensation occurs within seconds for the

arterial chemoreceptors, and this increases alveolar

Increased Pulmonary Ventilation—Role of Arterial Chemore-

vascularity of the peripheral tissues, and (5) increased

increased diffusing capacity of the lungs, (4) increased

tion, (2) increased numbers of red blood cells, (3)

The principal means by which acclimatization comes

still higher altitudes.

the body. And it becomes possible for the person to

, so that it causes fewer deleterious effects on

acclimatized

A person remaining at high altitudes for days, weeks,

Acclimatization to Low P

narily falls to about 50 per cent of normal, and after

stays at 15,000 feet for 1 hour, mental proficiency ordi-

movements. For instance, if an unacclimatized aviator

ment, memory, and performance of discrete motor

decreased mental proficiency, which decreases judg-

matized person, in coma, followed shortly thereafter

18,000 feet and end, above 23,000 feet in the unaccli-

sionally nausea, and sometimes euphoria.These effects

mental and muscle fatigue, sometimes headache, occa-

altitude of about 12,000 feet, are drowsiness, lassitude,

Aviation, High-Altitude, and Space Physiology

Chapter 43

539

progress to a stage of twitchings or seizures above

by death.

One of the most important effects of hypoxia is

18 hours at this level it falls to about 20 per cent of
normal.

or years becomes more and more

to the

low Po

work harder without hypoxic effects or to ascend to

about are (1) a great increase in pulmonary ventila-

ability of the tissue cells to use oxygen despite low Po

ceptors.

Immediate exposure to low Po

stimulates the

ventilation to a maximum of about 1.65 times normal.

several thousand feet higher than would be possible

about five times normal.

rising to a high altitude blows off large quantities of

and increasing the

inhibit the brain

stem respiratory center and thereby oppose the effect

aortic bodies

to respond with full force to the peripheral chemore-

creases to about five times normal.

mainly a reduction of bicarbonate ion concentration in

ing the chemosensitive neurons of the respiratory

An important mechanism for the gradual decrease

in bicarbonate concentration is compensation by the

by reducing hydrogen ion secretion and increasing

for the respiratory alkalosis gradually reduces plasma
and cerebrospinal fluid bicarbonate concentration and
pH toward normal and removes part of the inhibitory
effect on respiration of low hydrogen ion concentra-

responsive to the peripheral chemoreceptor stimulus
caused by the hypoxia after the kidneys compensate

During Acclimatization.

As discussed in Chapter 32,

hypoxia is the principal stimulus for causing an

a person remains exposed to low oxygen for weeks at

increase in whole blood hemoglobin concentration
from normal of 15 g/dl to about 20 g/dl.

increased blood hemoglobin concentration gives an
increase in total body hemoglobin of 50 or more per
cent.

Increased Diffusing Capacity After Acclimatization.

It will

be recalled that the normal diffusing capacity for
oxygen through the pulmonary membrane is about

similar increase in diffusing capacity occurs at high

Part of the increase results from increased pul-

capillaries and increases the surface area through

forces blood into greater numbers of alveolar capil-
laries than normally—especially in the upper parts

often increases as much as 30 per cent immediately
after a person ascends to high altitude but then
decreases back toward normal
as the blood hematocrit increases
of oxygen transported to the peripheral body tissues
remains about normal.

Another circulatory adaptation is

increased numbers of systemic circulatory capillaries

in-

creased tissue capillarity (or angiogenesis

in increasing work capacity, consider this: The work

To give an idea of the importance of acclimatization

uptake that the body can achieve.

In general, work capacity is reduced in direct pro-

not only skeletal muscles but also cardiac muscles.

muscles is greatly decreased in hypoxia. This includes

hypoxia, as discussed earlier, the work capacity of all

High Altitudes and Positive Effect

Reduced Work Capacity at

high-altitude natives.

, indicating that oxygen transport to the tissues is

for the lowlanders, despite the very low arterial

altitude natives is only 15 mm Hg less than the venous

altitude. Note also that the venous P

in the natives at high altitude is only 40 mm Hg,

who live at 15,000 feet. Note that the arterial oxygen

highly facilitated in these natives. For instance, Figure

than the hearts of lowlanders.

amounts of cardiac output, are considerably larger

their hearts, which from birth onward pump extra

ratio of ventilatory capacity to body mass. In addition,

the body size is somewhat decreased, giving a high

The chest size, especially, is greatly increased, whereas

years. Acclimatization of the natives begins in infancy.

acclimatized lowlanders, even though the lowlanders

tization, the natives are superior to even the best-

and live there all their lives. In all aspects of acclima-

feet. Many of these natives are born at these altitudes

group in the Peruvian Andes lives at an altitude of

Many native human beings in the Andes and in the

at High Altitudes

of Native Human Beings Living

Natural Acclimatization

than can their sea-level counterparts.

than in sea-level inhabitants. Therefore, it is presumed

13,000 to 17,000 feet, cell mitochondria and cellular

caused by pulmonary hypertension at high altitude.

instance, capillary density in right ventricular muscle

increase in capillarity is especially marked. For

In active tissues exposed to chronic hypoxia, the

to high altitude.

Aviation, Space, and Deep-Sea Diving Physiology

540

Unit VIII

especially in animals born and bred at high altitudes
but less so in animals that later in life become exposed

increases markedly because of the combined effects
of hypoxia and excess workload on the right ventricle

Cellular Acclimatization.

In animals native to altitudes of

oxidative enzyme systems are slightly more plentiful

that the tissue cells of high altitude–acclimatized
human beings also can use oxygen more effectively

Himalayas live at altitudes above 13,000 feet—one

17,500 feet and works a mine at an altitude of 19,000

might also have lived at high altitudes for 10 or more

Delivery of oxygen by the blood to the tissues is also

43–2 shows oxygen-hemoglobin dissociation curves for
natives who live at sea level and for their counterparts

but because of the greater quantity of hemoglobin,
the quantity of oxygen in their arterial blood is greater
than that in the blood of the natives at the lower

in the high-

exceedingly effective in the naturally acclimatized

of Acclimatization

In addition to the mental depression caused by

portion to the decrease in maximum rate of oxygen

capacities as per cent of normal for unacclimatized and
acclimatized people at an altitude of 17,000 feet are as
follows:

Acclimatized for 2 months

Work capacity

(per cent of normal)

Unacclimatized 50

Native living at 13,200 feet but working at

almost equal to that of a lowlander at sea level, but

Thus, naturally acclimatized native persons can

17,000 feet

achieve a daily work output even at high altitude

even well-acclimatized lowlanders can almost never
achieve this result.

Acute Mountain Sickness and
High-Altitude Pulmonary Edema

A small percentage of people who ascend rapidly to
high altitudes become acutely sick and can die if not

(Venous values)

100 120 140

Pressure of oxygen in blood (P

) (mm Hg)

Quantity of oxygen in blood (vol %)

Sea-level dwellers

Mountain dwellers

(15,000 ft)

(Arterial values)

level residents. PAHO Scientific Publication No. 140, Life at High

contents as recorded in their native surroundings. (Data from

showing the respective arterial and venous P

) and sea-level residents (

red curve

residents (

Figure 43–2

Oxygen-hemoglobin dissociation curves for blood of high-altitude

blue curve),

levels and oxygen

Oxygen-dissociation curves for bloods of high-altitude and sea-

Altitudes, 1966.)

the lower vessels. Because the heart cannot pump

pressure in the vessels of the lower body increases,

tion, the pressure becomes nearly 300 mm Hg. And, as

increased (to about 450 mm Hg). In the sitting posi-

5 G and

Thus, if the centrifugal acceleratory force is

centrifuged toward the lowermost part of the body.

positive G,

When an aviator is subjected to

translocated by centrifugal forces.

latory system, because blood is mobile and can be

The most impor-

Effects of Centrifugal Acceleratory Force on the Body—

1 G.

down by his belt is equal to the weight of his body, the

applied to his body; if the force with which he is held

negative G

5 G.

pull-out from a dive, the force acting on the seat is

of gravity. If the force with which he presses against

1 G because it is equal to the pull

gravity and is equal to his weight. The intensity of this

aviator is simply sitting in his seat, the force with which

When an

Measurement of Acceleratory Force—“G.”

. It is also obvious that the force

obvious that as the velocity increases, the

of curvature of the turn. From this formula, it is

is velocity of travel, and

is centrifugal acceleratory force,

When an airplane makes a turn, the force of cen-

Centrifugal Acceleratory Forces

vehicle turns, centrifugal acceleration.

at the end of flight, deceleration; and every time the

beginning of flight, simple linear acceleration occurs;

acceleratory forces affect the body during flight. At the

motion in airplanes or spacecraft, several types of

on the Body in Aviation and

Effects of Acceleratory Forces

a lower altitude.

further compounds the problem. Most of these people

blood flow where the blood is poorly oxygenated; this

vessels, thus causing an excess of pulmonary shunt

fails. Third, the alveolar arteriolar spasm diverts much

sure rises excessively, and the right side of the heart

oles become constricted, the pulmonary arterial pres-

alveoli are now in the low-oxygen state, all the arteri-

alveoli, as explained in Chapter 38. But because

of the lung hypoxia. This results from the hypoxic

oxygen delivery also begins to decrease. Second, the

the blood viscosity increases severalfold; this increased

threefold: First, the red cell mass becomes so great that

The causes of this sequence of events are probably

altitude.

fall, (5) congestive heart failure ensues, and (6) death

enlarged, (4) the peripheral arterial pressure begins to

tization, (3) the right side of the heart becomes greatly

hematocrit become exceptionally high, (2) the pul-

the following effects occur: (1) the red cell mass and

, in which

chronic mountain sickness

Occasionally, a person who remains at high altitude

within hours.

dysfunction that can be lethal. Allowing the person

occurs. Extension of the process to progressively

unconstricted pulmonary vessels. The postulated

parts, so that more and more of the pulmonary

to constrict potently, but the constriction is much

The severe hypoxia causes the pulmonary arterioles

unknown, but a suggested answer is the following:

. The cause of this is still

The cerebral edema can then lead to severe

causes fluid to leak into the cerebral tissues.

thus increasing capillary pressure, which in turn

arterioles increases blood flow into the capillaries,

vessels, caused by the hypoxia. Dilation of the

. This is believed to result

ascent. Two events frequently occur:

given oxygen or removed to a low altitude. The sick-

Aviation, High-Altitude, and Space Physiology

Chapter 43

541

ness begins from a few hours up to about 2 days after

1. Acute cerebral edema

from local vasodilation of the cerebral blood

disorientation and other effects related to cerebral
dysfunction.

2. Acute pulmonary edema

greater in some parts of the lungs than in other

blood flow is forced through fewer and fewer still

result is that the capillary pressure in these areas of
the lungs becomes especially high and local edema

more areas of the lungs leads to spreading
pulmonary edema and severe pulmonary

to breathe oxygen usually reverses the process

Chronic Mountain Sickness

too long develops

monary arterial pressure becomes elevated even more
than the normal elevation that occurs during acclima-

often follows unless the person is removed to a lower

viscosity tends to decrease tissue blood flow so that

pulmonary arterioles become vasoconstricted because

vascular constrictor effect that normally operates to
divert blood flow from low-oxygen to high-oxygen

all the

of the blood flow through nonalveolar pulmonary

recover within days or weeks when they are moved to

Space Physiology

Because of rapid changes in velocity and direction of

trifugal acceleration is determined by the following
relation:

in which f

m is the

mass of the object, v

r is radius

force of

centrifugal acceleration increases in proportion to the
square of the velocity
of acceleration is directly proportional to the sharpness
of the turn (the less the radius).

he is pressing against the seat results from the pull of

force is said to be

the seat becomes five times his normal weight during

If the airplane goes through an outside loop so that

the person is held down by his seat belt,

negative force is

(Positive G)

Effects on the Circulatory System.

tant effect of centrifugal acceleration is on the circu-

blood is

the person is in an immobilized standing position,
the pressure in the veins of the feet becomes greatly

these vessels passively dilate so that a major portion
of the blood from the upper body is translocated into

used by astronauts.

Therefore, we see the reason for the reclining seats

continue for as long as several minutes at a time.

, this amount of acceleration can be withstood

could not withstand this much acceleration, but in a

high as 8 G. In the standing position, the human body

eration as high as 9 G, and the second-stage booster as

Figure 43

negative.

, one positive and the other

tion can be tremendous; both of these are types

However, blast-off acceleration and landing decelera-

when the spacecraft goes into abnormal gyrations.

spacecraft cannot make rapid turns; therefore, cen-

Unlike an airplane, a

Acceleratory Forces in Space Travel.

Effects of Linear Acceleratory Forces

certainly would still be less than 10 G.

this procedure were used, the limit of safety almost

tions despite submersion in water. Therefore, even if

tissues, and diaphragm into seriously abnormal posi-

the lungs still allows displacement of the heart, lung

acting in the body. However, the presence of air in

retically, a pilot submerged in a tank or suit of water

ating compression bags as the G increases. Theo-

in the lower abdomen and legs. The simplest of these

by delaying the onset of blackout. Also, special

vessels of the abdomen can be prevented, there-

abdomen, some of the pooling of blood in the large

collapse that might occur during positive G. First, if

Protection of the Body Against Centrifugal Acceleratory

tive G. As a result, the eyes often become temporarily

brain to prevent intracerebral vascular rupture.

that blood is centrifuged toward the cranial vessels,

expected for the following reason: The cerebrospinal

cranium show less tendency for rupture than would be

brain to rupture. However, the vessels inside the

sure reaches 300 to 400 mm Hg, sometimes causing

20 G, for instance) and centrifugation of the blood

Occasionally, negative G forces can be so great

hyperemia of the head. Occasionally, psychotic distur-

nent harm, although causing intense momentary

manently than the effects of positive G. An aviator can

The effects of negative G on the body are

before vertebral fracture occurs is about 20 G.

vertebrae. The degree of positive acceleration that the

Effects on the Vertebrae.

acceleration is continued, the person will die.

ness shortly thereafter. If this great degree of

exes.

20 mm Hg within another 10 to 15 seconds. This sec-

sure of about 55 mm Hg and a diastolic pressure of

below 22 mm Hg for the

sitting person. Note that both these pressures fall

3.3 G is suddenly applied to a

diastolic arterial pressures (top and bottom curves,

Figure 43

in this way in the lower body, the less

unless blood returns to it, the greater the quantity of

Aviation, Space, and Deep-Sea Diving Physiology

542

Unit VIII

blood “pooled”
that is available for the cardiac output.

–3 shows the changes in systolic and

respectively) in the upper body when a centrifugal
acceleratory force of

first few seconds after the

acceleration begins but then return to a systolic pres-

ondary recovery is caused mainly by activation of the
baroreceptor refl

Acceleration greater than 4 to 6 G causes “black-

out” of vision within a few seconds and unconscious-

Extremely high acceleratory

forces for even a fraction of a second can fracture the

average person can withstand in the sitting position

Negative G.

less dramatic acutely but possibly more damaging per-

usually go through outside loops up to negative accel-
eratory forces of

-4 to -5 G without causing perma-

bances lasting for 15 to 20 minutes occur as a result of
brain edema.

(

into the head is so great that the cerebral blood pres-

small vessels on the surface of the head and in the

fluid is centrifuged toward the head at the same time

and the greatly increased pressure of the cerebrospinal
fluid acts as a cushioning buffer on the outside of the

Because the eyes are not protected by the cranium,

intense hyperemia occurs in them during strong nega-

blinded with “red-out.”

Forces.

Specific procedures and apparatus have been

developed to protect aviators against the circulatory

the aviator tightens his or her abdominal muscles to
an extreme degree and leans forward to compress the

“anti-

G” suits have been devised to prevent pooling of blood

applies positive pressure to the legs and abdomen by
infl

might experience little effect of G forces on the circu-
lation because the pressures developed in the water
pressing on the outside of the body during centrifugal
acceleration would almost exactly balance the forces

on the Body

trifugal acceleration is of little importance except

of linear acceleration

–4 shows an approximate profile of accel-

eration during blast-off in a three-stage spacecraft,
demonstrating that the first-stage booster causes accel-

semireclining position transverse to the axis of acceler-
ation
with ease despite the fact that the acceleratory forces

Time from start of G to symptoms

(sec)

Arterial pressure

(mm Hg)

100

tolerance to positive acceleration. J Aviation Med 22:382, 1951.)

from Martin EE, Henry JP: Effects of time and temperature upon

person to an acceleratory force from top to bottom of 3.3. G. (Data

arterial pressures after abrupt and continuing exposure of a sitting

Figure 43–3

Changes in systolic (top of curve) and diastolic (bottom of curve)

nearby heavenly body is still active. However, the

gravity to pull on the body, because gravity from any

inside its chambers. The cause of this is not failure of

bottom, sides, or top of the spacecraft but simply

. That is, the person is not drawn toward the

near-zero G force, which is sometimes called

, or a state of

Weightlessness in Space

system for recycling has yet to be achieved.

process of photosynthesis. A completely satisfactory

release oxygen. Others depend on biological methods,

physical procedures, such as electrolysis of water to

again. Some recycling processes depend on purely

proposed for use of the same oxygen over and over

supply. For this reason, recycling techniques have been

For space travel lasting more than several months,

plugs.

760 mm Hg. The presence of nitrogen in the mixture

equal to those in normal air are used, with four times

used, but in the modern space shuttle, gases about

ing pure oxygen at about 260 mm Hg pressure was

earlier space missions, a capsule atmosphere contain-

centration low enough to prevent suffocation. In some

spacecraft. Most important, the oxygen concentration

Because there is no atmosphere in outer space, an arti-

“Artificial Climate” in the

cushion the shock of landing.

brae, or leg. Consequently, the trained parachutist

of the body, resulting in fracture of his pelvis, verte-

the earth with extended legs, and this will result in

from a height of about 6 feet. Unless forewarned, the

is properly trained in landing. Actually, the force of

1/81 the impact force without a parachute. Even so,

second, and the force of impact against the earth is

other words, the speed of landing is about 20 feet per

chutist to about one ninth the terminal velocity. In

The usual-sized parachute slows the fall of the para-

pounds can occur on the parachute shrouds.

parachute, an

per hour (175 feet per second). If the parachutist has

that after falling for about 12 seconds, the person will

exactly balances the acceleratory force of gravity, so

Finally, the deceleratory force of the air resistance

the air resistance tending to slow the fall also increases.

per second; and so on. As the velocity of fall increases,

(if there is no air resistance); in 2 seconds it is 64 feet

However, because of the acceleratory force of gravity,

When the parachuting aviator leaves the airplane, his

Deceleratory Forces Associated with Parachute Jumps.

than is necessary at lower velocities.

for a short time. Therefore, deceleration must be

this, a human being can withstand far less deceleration

versus Mach 100 about 10,000-fold. But in addition to

of the velocity, which alone increases the

for safe deceleration. The principal reason for this dif-

0.12 mile, whereas a person traveling at a speed of

spacecraft re-enters the atmosphere. A person travel-

Aviation, High-Altitude, and Space Physiology

Chapter 43

543

Problems also occur during deceleration when the

ing at Mach 1 (the speed of sound and of fast air-
planes) can be safely decelerated in a distance of about

Mach 100 (a speed possible in interplanetary space
travel) would require a distance of about 10,000 miles

ference is that the total amount of energy that must
be dispelled during deceleration is proportional to
the square
required distance for decelerations between Mach 1

if the period of deceleration lasts for a long time than

accomplished much more slowly from high velocities

velocity of fall is at first exactly 0 feet per second.

within 1 second his velocity of fall is 32 feet per second

be falling at a “terminal velocity” of 109 to 119 miles

already reached terminal velocity before opening his

“opening shock load” of up to 1200

the force of impact is still great enough to cause con-
siderable damage to the body unless the parachutist

impact with the earth is about the same as that which
would be experienced by jumping without a parachute

parachutist will be tricked by his senses into striking

tremendous deceleratory forces along the skeletal axis

strikes the earth with knees bent but muscles taut to

Sealed Spacecraft

ficial atmosphere and climate must be produced in a

must remain high enough and the carbon dioxide con-

as much nitrogen as oxygen and a total pressure of

greatly diminishes the likelihood of fire and explosion.
It also protects against development of local patches
of lung atelectasis that often occur when breathing
pure oxygen because oxygen is absorbed rapidly when
small bronchi are temporarily blocked by mucous

it is impractical to carry along an adequate oxygen

such as use of algae with their large store of chloro-
phyll to release oxygen from carbon dioxide by the

A person in an orbiting satellite or a nonpropelled
spacecraft experiences weightlessness

micro-

gravity

floats

gravity acts on both the spacecraft and the person at

Minutes

Acceleration (G)

First

booster

Second
booster

Space

ship

Acceleratory forces during takeoff of a spacecraft.

Figure 43–4

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Zhang LF: Vascular adaptation to microgravity: what have

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Hochachka PW, Beatty CL, Burelle Y, et al: The lactate

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ight and ground-based models. J Appl

unweighting: space

Adams GR, Caiozzo VJ, Baldwin KM: Skeletal muscle

to 2 to 3 G.

periods (e.g., 1 hour each day) of centrifugal accelera-

more effectively attenuate these changes. One such

measures, in addition to exercise, that can prevent or

emergency landing. Therefore, considerable research

nauts after they land, especially in the event of an

of other planets, such as Mars, the effects of prolonged

tness. As space

bone, and muscle

ight cardiovascular,

ing to the full gravity of Earth. Astronauts returning

ability to stand

exes, and reduced orthostatic tolerance. These

capacity, reduced blood volume, impaired barorecep-

, which includes decreased work

each month even though they continue to exercise.

ight. Studies of astronauts

vascular system, skeletal muscles, and bone despite rig-

ights and prolonged exposure to microgravity,

Prolonged Exposure to Weightlessness.

Cardiovascular, Muscle, and Bone “Deconditioning” During

mechanisms.

tendency to faint (and still do, to some extent) when

rst few days after returning to earth. They also had a

the exercise program had been less vigorous, the astro-

prolonged space missions.

for an extended period of time. For this reason, exer-

from the bones, as well as loss of bone mass. Most of

cardiac output, and (5) loss of calcium and phosphate

strength and work capacity, (4) decrease in maximum

decrease in red blood cell mass, (3) decrease in muscle

are the following: (1) decrease in blood volume, (2)

The observed effects of prolonged stay in space

the same time lack of gravitational signals.

arriving in the equilibrium centers of the brain, and at

rst 2 to 5 days of space travel. This probably

sickness, with nausea and sometimes vomiting, during

to oppose the force of gravity.

static pressures, and (3) diminished physical activity

of travel, (2) translocation of

lessness: (1) motion sickness during the

of weightlessness is not too long. Most of the problems

cance, as long as the period

. The

Physiologic Problems of Weightlessness (Microgravity)

For this reason, the person simply is not attracted

the same time, so that both are pulled with exactly the

Aviation, Space, and Deep-Sea Diving Physiology

544

Unit VIII

same acceleratory forces and in the same direction.

toward any specific wall of the spacecraft.

physiologic problems of weightlessness have not
proved to be of much signifi

that do occur are related to three effects of the weight-

first few days

fluids within the body

because of failure of gravity to cause normal hydro-

because no strength of muscle contraction is required

Almost 50 per cent of astronauts experience motion

the fi
results from an unfamiliar pattern of motion signals

these same effects also occur in people who lie in bed

cise programs are carried out by astronauts during

In previous space laboratory expeditions in which

nauts had severely decreased work capacities for the
fi

they stood up during the first day or so after return to
gravity because of diminished blood volume and
diminished responses of the arterial pressure control

During very long

space fl
gradual “deconditioning” effects occur on the cardio-

orous exercise during the fl
on space flights lasting several months have shown that
they may lose as much 1.0 percent of their bone mass

Substantial atrophy of cardiac and skeletal muscles
also occurs during prolonged exposure to a micro-
gravity environment.

One of the most serious effects is cardiovascular

“deconditioning”

tor refl
changes greatly limit the astronauts’

upright or perform normal daily activities after return-

from space flights lasting 4 to 6 months are also sus-
ceptible to bone fractures and may require several
weeks before they return to pre-fl

flights become

longer in preparation for possible human exploration

microgravity could pose a very serious threat to astro-

effort has been directed toward developing counter-

countermeasure that is being tested is the application
of intermittent “artificial gravity” caused by short

tion of the astronauts while they sit in specially
designed short-arm centrifuges that create forces of up

References

red blood cell mass in spacefl
1996.

to spacefl

paradox in human high-altitude physiological perform-

flooding at high altitude:

space fl

—History of High-Altitude Physiology