J Appl Physiol
89: 379–384, 2000.
highlighted topics
Physiology of a Microgravity Environment
Historical Perspectives: Physiology in microgravity
JOHN B. WEST
Department of Medicine, University of California San Diego, La Jolla, California 92093-0623
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West, John B. Historical Perspectives: Physiology in microgravity. J
Appl Physiol 89: 379–384, 2000.—Studies of physiology in microgravity
are remarkably recent, with almost all the data being obtained in the
past 40 years. The first human spaceflight did not take place until 1961.
Physiological measurements in connection with the early flights were
crude, but, in the past 10 years, an enormous amount of new information
has been obtained from experiments on Spacelab. The United States and
Soviet/Russian programs have pursued different routes. The US has
mainly concentrated on relatively short flights but with highly sophisticated equipment such as is available in Spacelab. In contrast, the
Soviet/Russian program concentrated on first the Salyut and then the
Mir space stations. These had the advantage of providing information
about long-term exposure to microgravity, but the degree of sophistication of the measurements in space was less. It is hoped that the International Space Station will combine the best of both approaches. The
most important physiological changes caused by microgravity include
bone demineralization, skeletal muscle atrophy, vestibular problems
causing space motion sickness, cardiovascular problems resulting in
postflight orthostatic intolerance, and reductions in plasma volume and
red cell mass. Pulmonary function is greatly altered but apparently not
seriously impaired. Space exploration is a new frontier with long-term
missions to the moon and Mars not far away. Understanding the physiological changes caused by long-duration microgravity remains a daunting challenge.
Spacelab; Mir space station; bone demineralization; muscle atrophy;
space motion sickness; orthostatic intolerance
IN MANY AREAS OF PHYSIOLOGY,
articles on historical perspectives might begin with a reference to work done
some 2,000 years ago. For example, a historical perspective on pulmonary gas exchange could reasonably
refer to the writings of Galen in the 2nd century CE but
might also briefly mention the theories of Empedocles
in the 5th century BCE. In contrast, physiological
studies in microgravity are remarkably young, with
almost all the data being obtained in the past 40 years.
The first living creature to be launched into space was
the Soviet dog Laika in November 1957, and the first
human spaceflight was that of Yuri Gagarin on April
12, 1961 (8). Of course, the physiological measureAddress for reprint requests and other correspondence: J. B. West,
UCSD Dept. of Medicine 0623A, 9500 Gilman Drive, La Jolla, CA
92093–0623 (E-mail:[email protected]).
http://www.jap.org
ments in connection with these early flights were relatively crude, but, over the past 40 years, the degree of
sophistication has greatly increased, culminating in
the extraordinarily productive studies in the Spacelabs
of the 1990s.
Although physiological measurements during spaceflight go back only 40 years or so, there were remarkable
visionaries who considered the possibilities of human
spaceflight and its likely problems over 100 years ago.
One of the most important was Konstantin Eduardovich
Tsiolkovsky (1857–1935), who, as a young deaf Russian
mathematics teacher, sketched a spacecraft design in
1883 and published his first article on space travel in
1895. There is a dramatic memorial to him in Moscow. He
recognized the importance of escape velocity, that is, the
velocity required for a spacecraft to escape the gravitational attraction of the Earth, and he realized that this
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HISTORICAL PERSPECTIVES
SOME MILESTONES IN SPACEFLIGHT AND THEIR
IMPACT ON THE PHYSIOLOGY OF MICROGRAVITY
Soviet Vostok program. As indicated earlier, the first
human spaceflight was by Yuri Alekseyevich Gagarin
(1934–1968) aboard Vostok-1 on April 12, 1961. The
flight lasted 1 h and 48 min, during which one Earth
orbit was completed. Physiological variables that were
monitored included electrocardiogram (ECG) and chest
movements by pneumography; a television camera also
recorded the cosmonaut’s activities. Gagarin remained
well and ejected from the spacecraft at an altitude of
7,000 m, landing by parachute.
Approximately 4 mo later, Gherman Titov flew on
Vostok-2 for over 1 day, during which he made 17
Earth orbits. He developed what later became known
as space motion sickness (or space adaptation syndrome), and he also became the first person to sleep in
space. Later flights in the Vostok series incorporated
more sophisticated physiological monitoring, including
electrooculogram, electroencephalogram, galvanic skin
resistance, and neuropsychological evaluations. Postflight orthostatic intolerance, that is, difficulty sustaining the standing posture on return to Earth, was seen
in some cosmonauts. The first woman in space, Valen-
tina Vladimirovna Tereshkova (1937–), flew in Vostok-6 in June 1963.
US Project Mercury. In the first United States human space program, Alan Bartlett Shephard, Jr.
(1923–1998) performed a 15-min suborbital flight on
May 5, 1961; and John Herschel Glenn (1921–) accomplished the first orbital flight on February 20, 1962. In
all, two suborbital and four orbital missions were accomplished during Project Mercury, and all six astronauts returned to Earth safely. The principal findings
were weight loss, mainly from dehydration, and postflight orthostatic intolerance.
Soviet Voskhod program. Voskhod-1, which was
launched in October 1964, was the first spacecraft with
more than one crew member and, in fact, was made up of
three, including the first physician in space, Dr. Boris
Borisovich Yegorov (1937–1994). He conducted several
medical experiments, including tests of pulmonary and
vestibular function, and he also took the first blood samples in space. On the Voskhod-2 mission, Aleksey
Arkhipovich Leonov (1934–) became the first person to
perform extravehicular activity (EVA) (space walking).
US Gemini program. The first flight of this twoperson spacecraft took place during March 1965. All in
all, the 10 Gemini missions accomplished a number of
objectives necessary for a lunar landing. These included proving the feasibility of a spaceflight lasting
almost 14 days and demonstrating the ability to rendezvous and dock with another spacecraft. The first
EVA in the US program was carried out during Gemini-4, although there were problems with overheating of the space suit. Physiological findings in the
Gemini program included loss of bone density in the os
calcis, loss of bone calcium and muscle nitrogen, postflight reduction in exercise capacity, loss of red cell
mass, unexpectedly high metabolic cost of EVA, and
postflight orthostatic intolerance in all crew members.
Soviet Soyuz program. This program was apparently
originally planned in preparation for a human moon
landing. The spacecraft carried three crew members
and had sophisticated orbital maneuvering, rendezvous, and docking capabilities. In addition to tests of
maneuvering and welding in space, physiological investigations continued on cardiovascular and musculoskeletal responses. There were several tests of the
value of physical exercise in space to reduce cardiovascular deconditioning. Unfortunately, four cosmonauts
lost their lives, one when a parachute failed to deploy
successfully during reentry and three others when a
valve opened accidentally and the crew members died
as a result of sudden depressurization.
US Project Apollo. As every schoolboy knows, this
ambitious program to land a man on the moon and
return him safely to Earth was an outstanding success.
However, there was tragedy at the beginning when
three astronauts died in a fire on the launch pad during
a test. As a result, the atmosphere of the spacecraft
during launch was changed from pure oxygen to 60%
oxygen-40% nitrogen, and the nitrogen was gradually
replaced with oxygen after launch, as cabin pressure
was reduced. During the whole of the Mercury, Gemini,
and Apollo series, the crew members breathed pure
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would require liquid propellants because of their greater
efficiency vs. solid propellants. In 1926, an American,
Robert Hutchings Goddard (1882–1945), launched the
first liquid propellant rocket; in one of his papers, he
discussed the possibility of a rocket reaching the moon.
Hermann Julius Oberth (1894–1989) in Germany formulated some of the technical problems of spaceflight, but
many of his ideas were dismissed as fantasies. The technical breakthrough came during World War II in Peenemunde, Germany, where Wernher von Braun (1912–
1977) and his co-workers developed the A4 rocket and its
sophisticated guidance system. These workers immediately recognized the potential importance of what they
had done for future space travel. However, at the time,
the rocket was renamed the V2 and many were launched
against London with terrifying consequences.
After World War II, von Braun and other German
rocketeers were brought to the United States and the
V2 became the Redstone launcher. Subsequently, enormous developments in launchers followed, culminating
in the Saturn V, which allowed Neil Alden Armstrong
(1930–) and Edwin Eugene Aldrin, Jr. (1930–) to walk
on the moon in July 1969, a spectacular milestone in
the history of human exploration.
Short periods of weightlessness or microgravity can
be produced in other ways. As early as World War I, a
few seconds of microgravity could be produced in diving
aircraft (2). The higher performance of aircraft during
World War II enabled longer periods of microgravity to
be obtained. In 1950, the flight maneuver known as a
Keplerian arc was proposed to produce periods of microgravity as long as 45 s (3, 9). Later, rocket-launched
capsules, which could contain small animals, allowed
periods of microgravity for many minutes during the
free fall after the rocket had burnt out (4).
HISTORICAL PERSPECTIVES
lower part of the body on return to the normal 1-G
environment. Extensive use was made of “countermeasures” to attempt to reduce the deterioration that occurs
in spaceflight. Regular exercise was performed using
both a treadmill and a stationary bicycle, and the cosmonauts wore so-called “penguin suits” during working
hours. These are made of elastic material and are designed to stress the skeletal musculature to counteract
the effects of weightlessness.
US Skylab program. This was the only US space
station and allowed exposure of three astronauts for as
long as 84 days. It was ingeniously built, using the
spent third stage of the Saturn V booster rocket, and,
with its volume of ⬃294 m3, it provided laboratory
space and living quarters that were orders of magnitude larger than those on the Mercury, Gemini, and
Apollo spacecraft. The crew members of Skylab 4 were
launched in November 1973, and the duration of 84
days for its three crew members far eclipsed any previous duration record. This combination of a long period in orbit, the large amount of space available, the
relatively sophisticated physiological equipment, and
the freedom from operational objectives gave unprecedented opportunities for physiological measurements
in microgravity. Among the projects that were carried
out were studies of bone mineral loss and mineral
balance. Bone loss was seen in the os calcis at a rate
that was similar to that encountered in bed rest studies. Importantly, the loss of bone calcium appeared to
be relentless despite the vigorous use of countermeasures such as exercise. In addition to calcium, significant nitrogen and phosphorus were also lost, presumably as a result of muscle atrophy. There was also a
reduction in leg volume, part of which could be attributed to muscle atrophy and part to fluid loss. Extensive
blood studies were done to determine the cause of the
loss of red cell mass. Cardiac deconditioning was also
monitored using LBNP.
Skylab demonstrated the feasibility of relatively sophisticated laboratory measurements in space. However, it was disappointing that few studies were reported in peer-reviewed journals. Instead, the main
body of the results were written up in a National
Aeronautics and Space Administration (NASA) publication (5). This unfortunate decision helped to fuel the
suspicion by the science community at large that
NASA-funded studies of human physiology in space
lack the scientific rigor of some other studies. This
perception exists in some quarters to the present day.
Soviet Mir program. With the launch of the Mir
space station in February 1986, the world’s first permanently inhabited space station became a reality. A
feature of the Mir design was its multiple docking
ports, which allowed an evolutionary approach
whereby the station could be incrementally expanded.
The original hardware, which was derived from the
early Salyut stations, had a compartment at the front
with five docking ports and another docking port at the
rear. Various modules have been added, and these
have allowed the installation of sophisticated scientific
equipment for physiological studies, including a large
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oxygen at a pressure of 5 psi, giving a cabin oxygen
pressure of ⬃260 Torr. This was in contrast to the
Soviet program in which, from the outset, the cosmonauts breathed 21% oxygen at 760 Torr.
Physiological changes noted during the Apollo flights
confirmed those of the earlier Mercury and Gemini
programs and included reduced postflight exercise tolerance, decreased red cell mass and plasma volume,
dehydration and weight loss, vestibular disturbances,
and postflight orthostatic intolerance. It is interesting
that symptoms of space motion sickness were not reported by US astronauts before the Apollo flights,
whereas they had been noted as early as the second
Soviet manned flight, Vostok-2. However, in Apollo 8
and 9, five of six crew members suffered from space
motion sickness and in one instance the flight plan had
to be modified. Another feature of the Apollo flights
was that one of the astronauts developed cardiac arrhythmias (on Apollo 15).
Soviet Salyut program. Salyut I, which was launched
in April 1971, became the world’s first space station. It
was launched on a Proton rocket, and a crew of three
cosmonauts was delivered to the station using Soyuz
10. This was the beginning of the Soviet (and later,
Russian) emphasis on long-duration human space exposure. In fact, the US and Soviet paths of human
space research diverged at this point, the only exception being Skylab, described below. The Soviet program concentrated on long-term space missions with
relatively small amounts of experimentation done during the mission itself, partly because of the limited
space and resources of Salyut and the ensuing space
station, Mir. By contrast, the US program developed
the space shuttle, which allowed highly sophisticated
experiments on human physiology to be carried out in
the Spacelab program, but the duration of exposure
was limited to 2–3 wk. In a sense, the two programs
were therefore complementary. Note that, although
the Soviets were limited in the amount of data that
could be obtained onboard their space station, they
made extensive measurements before and after the
long exposure to microgravity.
Some remarkably long-duration flights were made in
the various Salyut missions. For example, cosmonaut
V. V. Ryumin spent periods of 175 and 185 days in
space, separated by just 6 mo; therefore, he logged
almost a year in orbit. Subsequently, on the Mir space
station, V. G. Titov and M. Kh. Manarov completed
flights lasting 1 yr. Dr. Valeriy Polyakov, a physician,
conducted many medical studies during his 8-mo flight
in 1988 and 1989. Polyakov subsequently spent 438
days continuously in space on the space station Mir.
With the extended duration of the Salyut missions,
increased emphasis was placed on monitoring of crew
health. Extensive medical examinations were performed
weekly, and these included cardiovascular evaluations by
ECG, respiration by pneumography, arterial pressure by
oscillometry, and central and peripheral hemodynamics
by impedance studies. The measurements were carried
out at rest, during exercise on an ergometer, and using
lower body negative pressure (LBNP), which simulates to
some extent the gravitational pooling of blood in the
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Table 1. Spacelabs on which major physiological research studies were conducted
Spacelab
Major Experiments
Spacelab 1 (SL-1)
November 1983
Spacelab 3 (SL-3)
German Spacelab 1 (D-1)
April 1985
October 1985
Spacelab Life Sciences-1
(SLS-1)
June 1991
International Microgravity
Laboratory-1 (IML-1)
Japanese Spacelab (SL-J)
January 1992
German Spacelab 2 (D-2)
April 1993
Spacelab Life Sciences-2
(SLS-2)
International Microgravity
Laboratory-2 (IML-2)
Life and Microgravity
Sciences (LMS)
Neurolab
October 1993
Vestibular function, central venous pressure, red cell kinetics, immune responses,
circadian rhythm, endocrine studies
Space motion sickness, animal urine monitoring system, animal holding facility
Vestibular function, radiation biology, developmental biology, central venous
pressure
Cardiovascular and pulmonary function, bone metabolism, muscle morphology
and function, vestibular adaptation, hematology, immunology, renal/endocrine
function
Vestibular and central nervous system studies, proprioception, gravisensitivity in
plants, developmental biology
Developmental biology, biological rhythms, radiation biology, orthostatic
intolerance
Gravitational biology, cardiovascular and pulmonary function, endocrine studies,
hematology
Continuation of SLS-1 studies with modifications based on the previous results
September 1992
July 1994
June 1996
April 1998
Vestibular function in fish, bone resorption, radiation biology, orthostatic
intolerance
Muscle physiology and morphology, bone metabolism, cardiovascular and
pulmonary function, circadian rhythm
Autonomic function, sensory physiology, vestibular function, sleep studies
including control of ventilation, nervous system development
For more information, see http://lsda.jsc.nasa.gov/shuttle.cfm.
refrigerator/freezer, echocardiograph, automated capillary blood analyzer, and a large treadmill.
A feature of the Mir program has been the large
number of foreign crew members that have spent some
time on the space station. Also, the US space shuttle
has docked with Mir on a number of occasions. Unfortunately, an accident occurred in June 1997 during the
attempted docking of a supply vehicle, and one of the
modules was damaged and became depressurized. At
the time of this writing, the future of Mir is uncertain.
US Spacelab program. This ambitious program has
enabled high-caliber research to be carried out in a
highly sophisticated laboratory environment. For
physiological purposes, Spacelab refers to a pressurized laboratory module of about 7 m in length and 4 m
in diameter, the atmosphere of which is 21% oxygen
(remainder nitrogen) at sea level pressure of 760 Torr.
Both sides of the module are covered by standard
instrument racks that are supplied with electrical
power, cooling, and vacuum. Thus Spacelab provides a
“shirtsleeves” research environment similar to that of a
ground-based laboratory. The first Spacelab was
launched in November 1983 and was partly a proving
mission, but a number of experiments were carried out
successfully (Table 1). It should be emphasized that
Spacelab was also designed to support research in
other disciplines, such as astronomy, solar physics,
space plasma physics, atmospheric physics, Earth observations, and materials science. Many of these experiments use a variety of pallets or laboratory test
benches that are located in the shuttle bay behind the
laboratory module.
The Spacelab program was interrupted by the explosion of the space shuttle Challenger shortly after liftoff
in January 1986, which took the lives of all seven crew
members on board. This set the Spacelab program back
about 4 yr. However, in June 1991, the first Spacelab
devoted to life sciences, Spacelab Life Sciences-1 (SLS-
1), flew and the mission was extremely productive from
a research standpoint. In all, 11 Spacelab missions
with major or minor emphases on physiology were
flown over a period of 15 yr, and a large number of
areas were covered (Table 1). The last Spacelab, known
as Neurolab because of its emphasis on neuroscience,
flew in April 1998, and it was one of the most productive from a scientific point of view.
A feature of the Spacelab scientific program has been
the very close involvement of the university academic
community. The result has been a large number of
peer-reviewed publications in some of the top scientific
journals. This is in contrast to the studies carried out
in Skylab, which, as pointed out earlier, were mainly
published in a NASA report.
It is sad to relate that the Spacelab program has
come to an end, and many shuttle flights are now being
used for the construction of the International Space
Station. Parenthetically, it seems difficult to justify
abandoning the Spacelab program at this stage, when
it has been so enormously productive and so many
important space life science questions remain to be
answered. Admittedly, the Spacelab program is expensive but so is all space science research, particularly
that involving humans. Scientists interested in human
space science problems now see a hiatus of several
years before even the most elementary measurements
can be made aboard the International Space Station.
Furthermore, it is hard to imagine that the station will
ever have the sophisticated facilities of Spacelab, at
least in our lifetime. Although it is argued that the big
advantage of the International Space Station is that
experiments can be conducted over a long duration of
exposure to microgravity, it is not clear why the two
programs cannot be run together, at least until the
International Space Station is up and running. Certainly, the abandonment of the highly productive
Spacelab program has led many people to question the
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Launch Date
HISTORICAL PERSPECTIVES
PHYSIOLOGICAL CONSEQUENCES OF MICROGRAVITY
The various mini-reviews on Physiology of a Microgravity Environment, for which this historical perspective is an introduction, will discuss the state of the art
in a number of areas, including bone and mineral
physiology and skeletal muscle, cardiovascular, and
pulmonary physiology. Therefore, this section is only a
brief introduction to some of the most important physiological sequelae of microgravity. (For additional reviews, see Refs. 1, 6, and 7.)
Bone and mineral metabolism. Much of the evidence to
date suggests that calcium loss from bone progresses
inexorably during periods of microgravity, and some
studies suggest that full recovery is extremely slow, or
indeed does not occur, on return to the 1-G environment.
Therefore, mineral changes may be the most important
limiting factor for long-term human spaceflight.
Some of the earliest studies of bone loss were made
on Skylab, when photon absorptiometry studies
showed losses in the os calcis during the 84-day mission. These measurements suggested that it was
mainly trabecular bone in the center of the os calcis
that was affected. However, subsequent measurements
showed that compact bone also undergoes changes.
The measurements made by photo absorptiometry
were correlated with calcium balance studies, which
indicated a substantial calcium loss. Indeed, additional
studies, particularly in the Soviet Salyut program,
showed loss from vertebral bone, and it became clear
that the degree of negative calcium balance indicated
generalized bone loss. Some studies suggest a loss of an
average of 1–2% of bone mineral density per month in
some sites. Bone demineralization is also seen during
prolonged bed rest, although not to the same extent as
in microgravity.
The studies that have been carried out on crew members in both the US and Soviet programs have been
complemented by numerous animal experiments, which
have shown marked skeletal changes after as few as 7
days of microgravity. Some experiments suggest that
microgravity not only causes mineral loss but also inhibits bone formation. A hazard of the increased urinary
calcium concentration is renal calculi, and a crew member on Salyut 7 reported renal colic, although this did not
abort the mission. Countermeasures to reduce bone mineralization include weight-loading exercises, wearing a
suit that provides continuous compression of the skeleton, and nutritional supplements. It is still not clear how
useful these countermeasures are, although some Soviet
studies suggest they are of some value.
Muscle physiology. All skeletal muscles respond to
the presence or absence of motor activity; therefore, it
is not surprising that muscle atrophy is a feature of
microgravity. In the early Gemini flights, weight loss of
the astronauts occurred as a result of both a reduced
body fluid volume and a reduced muscle mass. In one
80-day shuttle flight, decreases in muscle volume were
measured by magnetic resonance imaging and showed
decreases in leg muscles of 4–6%. Similar changes
have been seen in experimental animals, for example,
in rats flown for as few as 5 days. Muscle biopsies in
astronauts have shown that after only 5 days of microgravity, mean cross-sectional areas of muscle fibers
were 11 and 24% smaller in type I and type II fibers,
respectively.
The reduction in muscle mass is accompanied by
reduced muscle force, as measured on Skylab and,
particularly, the longer Soviet Salyut missions. There
is also evidence of increased muscle fatigue, which has
been confirmed by electromyographic studies. Motor
coordination also appears to be impaired by long-term
spaceflight. These physiological changes are accompanied by metabolic alterations, such as changes in protein metabolism. Countermeasures for muscle atrophy
have included vigorous exercise programs and, in Soviet studies, wearing a penguin suit. However, there
has been considerable variation between subjects on
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extent to which NASA-funded research is driven by
interests related to science vs. those of the engineering
community.
International Space Station. This is an ambitious
project with components from the United States, Russia, Europe, Japan, Canada, and other countries. From
the vantage point of physiology in microgravity, it will
offer unique opportunities for the study of long-term
effects of weightlessness. However, the program is extremely expensive, and it has been plagued by long
delays. At the time of this writing, only two modules
are in orbit. In particular, the Russian service module,
which is a critical component for further construction
and crew habitation, is long overdue. As indicated
earlier, it will probably be many years before the International Space Station will allow the degree of sophistication of research that was carried out on
Spacelab. However, it is clearly essential to have longterm studies of physiology in microgravity if further
space exploration is to take place. For example, a Mars
mission will take of the order of 1,000 days, which is
about three times longer than anybody has been in
space so far.
Spaceflights involving only animals. A substantial
number of experimental animals have been flown in
connection with experiments on Spacelabs. For example, on Neurolab, rats were flown in special holding
facilities, and some dissections were carried out in
microgravity. No animals have been sent up to the Mir
space station. However, the Soviet/Russian program
included a number of flights in the unmanned Cosmos
program, and US investigators collaborated in at least
eight of these (see http://lsda.jsc.nasa.gov/cosmos.cfm
for more details). A variety of animals were exposed to
microgravity in this way, including a number of primates. Of course, because there were no humans on
board, the animals were not tended and food and water
were provided automatically. In general, relatively few
measurements were made during the actual exposure
to microgravity, but, of course, extensive studies were
made on the animals before and after the spaceflight.
Particularly valuable information was found on bone
demineralization and muscle atrophy. The US space
program also had a small series of animal flights in the
early biosatellite program that included primates.
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HISTORICAL PERSPECTIVES
side the great veins is reduced because of changes in
the effects of gravity on the mediastinum, with the
result that the pressure measured by the catheter is
not an accurate reflection of the transmural pressure of
the veins.
Pulmonary function. The lung is exquisitely sensitive to gravity, which normally causes regional differences in ventilation, blood flow, gas exchange, alveolar
size, intrapleural pressure, and parenchymal stress.
Therefore, it is not surprising that a variety of changes
in pulmonary function have now been described. The
distributions of ventilation and blood flow become more
uniform, although not entirely so, and there are reductions in some lung volumes, including functional residual capacity and residual volume. However, overall,
gas exchange is little affected. One of the reasons for
monitoring pulmonary function during spaceflight is
because of the vulnerability of the lung to changes in
the atmosphere of the spacecraft. A dramatic example
was a fire that occurred on Mir, which, fortunately, was
rapidly brought under control. Aerosol contamination
of the spacecraft is another potential hazard.
Hematology. Spaceflight is accompanied by reductions in both plasma volume and red blood cell mass.
For example, on SLS-1, plasma volume was reduced by
23% on mission day 2 and 14% on mission day 8.
Recovery was complete within 6 days of return. The
reduction of red blood cell mass is seen within about 4
days of launch and apparently reaches a maximum
some 50 days later. It appears that red blood cells are
produced in the bone marrow but not released into the
blood. The mechanisms for this is not yet clear.
The above section represents only a very brief survey
of the wide range of physiological abnormalities encountered during spaceflight. It serves as an introduction to the more extensive invited reviews that follow
in this series.
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2. Ferry G. L’Aptitude à L’Aviation; Le Vol en Hauteur et le Mal des
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6. Nicogossian AE, Huntoon CL, and Pool SL. (Eds.) Space
Physiology and Medicine. Philadelphia, PA: Lea & Febiger, 1994.
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AI. (Eds.) Space Biology and Medicine (3 vol.). Washington, DC:
American Institute of Aeronautics and Astronautics, 1993.
8. Rauschenbach BV, Sokolskiy VN, and Gurjian AA. Historical aspects of space exploration. In: Space Biology and Medicine,
edited by Nicogossian AE, Mohler SR, Gazenko OG, and Grigoryev AI. Washington, DC: American Institute of Aeronautics and
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the efficacy of these measures, and one astronaut who
exercised extensively on a treadmill during a flight still
developed extensive muscle atrophy. Other possible
countermeasures, including the use of growth hormone
and insulin-like growth factor-I, have been studied in
experimental animals with variable results.
Vestibular/central nervous system. Space motion
sickness has been a serious problem, particularly in
the flights of the space shuttle, where functional impairment in the first couple of days of a mission represents a substantial portion of the useful time. One
hypothesis is that the condition results from conflicting
afferent information from visual, vestibular, skin,
joint, and muscle receptors. Although the information
from these receptors is compatible and consistent with
prior experience under 1-G conditions, a conflict develops in microgravity. At first sight, space motion
sickness appears to be similar to terrestrial motion
sickness. However, there are differences in symptomatology, and there is not a good correlation between the
incidence of the two conditions. For example, some
astronauts may rarely develop ground-based motion
sickness but be prone to the condition in microgravity.
The clinical features include malaise, anorexia, lack
of drive, nausea, and vomiting. However, vomiting may
suddenly occur without preceding nausea. There is a
high incidence of about 50% in cosmonauts and nearly
70% in astronauts flying in the space shuttle. It is
interesting that the condition was not seen in the
Mercury and Gemini programs, possibly because the
tight confinement of the astronauts prevented them
from moving their heads. This is known to be an
aggravating factor. Another possibility is that the
symptoms were underreported.
Cardiovascular system. A great deal of attention has
been given to postflight orthostatic intolerance, partly
because this represents a potential hazard during reentry when increased G levels are experienced. Part of
the explanation for the condition may be the fluid shift
that occurs from the lower extremities to the upper
body as a result of the removal of normal gravity.
Typically, astronauts show periorbital puffiness and
facial edema, whereas there are decreases in the girth
of the upper leg and calf. It is postulated that the
increased central fluid volume causes a diuresis, which
results in a negative fluid balance and a reduced circulating blood volume during exposure to microgravity.
There is also evidence of changes in the baroreceptor
system.
The tendency for orthostatic intolerance can be
tested during spaceflight using LBNP, which simulates
the normal gravitational forces to some extent. LBNP
is also used as a countermeasure. Another useful countermeasure is fluid loading before reentry when, typically, an astronaut ingests 1 liter of water with salt
tablets.
Cardiac output increases initially on exposure to
microgravity and then declines. It is interesting that
the increase in cardiac output occurs despite a reduction of central venous pressure measured with the
catheter tip near the left atrium. The explanation of
this apparent paradox may be that the pressure out-