Microgravity – Effects on the Human Body During and After Long-Duration Space Flight

Space flight experiences of the astronauts and cosmonauts have proven that humans can remain in space for long durations of several months. Mere survival is not sufficient for space travel. Human beings must be able to work efficiently and live comfortably and then to return to Earth without any after-effects (Humans in space, NSBRI).

Mission profile- The Short Stay Mission

This mission allows a stay of 30-90 days on Mars and 400-600 days in the round trip. This mission class requires a large amount of energy to be expended in transit, even after taking advantage of either a Venus swingby or a deep space propulsive maneuver in order to limit Mars and Earth entry speeds. This mission profile has three distinguishing features. First, the time spent on the Martian surface is relatively short. In fact, over 90 percent of the total mission time is spent in the zero-g In-Space Environment, allowing little time for the astronauts to recover in the 0.38g Martian gravity. This brings us to the second point; the Venus swingby leg of the mission can last up to 360 days. Depending on the orbital positions of Earth and Mars at the time of launch, the Venus swingby may occur on the inbound or the outbound leg of the mission. Should the longer Venus swingby leg occur on the way to Mars, questions are raised about the ability of astronauts to endure physically demanding entry maneuvers and their ability to perform effectively on the surface. Then, after 90 percent of the 400 to 650 day mission is spent in the zero-g In-Space Environment with no appreciable recovery time in 0.38g, concerns for the safety of the crew arise from the cumulative exposure to the zero-g environment. Third, the Venus swingby brings the crewed spacecraft inside of the orbit of Venus (~0.7AU). Here the crew would be subjected to more intense solar particle exposure and their spacecraft would therefore require heavy shielding. In the event of a major solar particle event, the crew would need to take refuge in an even more heavily shielded portion of the spacecraft. The disadvantage here is the increased mass of the spacecraft which, on a mission with high energy requirements already, would increase energy requirements further. Due to the fact that the propellant is the largest contributor to the overall mass of any spacecraft, every attempt should be made to minimize the energy requirements, and therefore the mass, of any Mars-bound spacecraft.

Support

I have chosen the Short-Stay mission profile due to a large amount of time spent in a microgravity environment. I feel this will best support the research I will be conducting on recovery time related to long duration space flight. The paper will not contain much on the possible positive/negative effects of time spent on the Mars surface at.38g. While this mission profile does have its benefits it will also have its consequences, Due to the limited size of the spacecraft there will not be much room for exercise equipment. The astronauts will be limited to only a few activities with which to perform to deal with the negative effects of microgravity. This may take a larger toll on their bodies and will have to be accounted for in the final analysis and recommendations. There will be 5 astronauts on the voyage, they should be a variation of individuals as to showcase possible differences in the effects on their bodies. 3 male and 2 female preferably of differing ages between 26-60.

The Atmosphere to Human beings

Air or the mixture of gases and the pressure at which it exists are just right for human beings to breathe. The air contains 78% of nitrogen (N2), 21% oxygen (O2), 0.5% water vapor (H2O) and a mixture of carbon dioxide, argon, helium, neon, krypton, hydrogen, methane and xenon. This mixture may show some variations depending on the humidity, temperature and altitude. The breathing cycle or respiration consists of two main processes of inspiration and expiration (The Atmosphere, NASA).

. The inspiration is the inhalation of air or oxygen into the lungs through the trachea and the bronchi into the lungs. The exchange of gases takes place at the pulmonary alveoli in the lungs and the oxygen transferred into the blood will be “pumped” to all parts of the body through the arteries. Just like the distribution there is a collection of carbon dioxide from all the organs. This deoxygenated blood travels in the veins back to the lungs through the heart (The Atmosphere, NASA). The atmospheric pressure influences the process. When the oxygenation is occurring, the lungs expand to contain a large volume and in expiration a smaller volume is let out. Respiration is a natural process for human beings on Earth. The atmospheric pressure pushes the air into the body when it is more than the pulmonary in inspiration and the pulmonary pressure which is higher than the atmospheric pressure pushes it out in expiration. It happens easily because atmospheric pressure is an unquestionable feature of Earth. The density and kinetic energy of the air molecules influence the atmospheric pressure (The Atmosphere, NASA). The faster moving molecules influence the atmospheric pressure. The situation becomes dramatically different when there is no atmospheric pressure because there are no moving molecules on the planets of the Moon or Venus. The movements of the molecules also produce temperature differences. The atmospheric pressure is just right in composition and pressure for facilitating the body function of respiration which is essential to human survival.

Space has three main differences when compared to Earth. The atmosphere, the atmospheric filter and the presence of gravity on Earth make it just right for human life. The absence of these in Space has to be overcome or remedial measures are taken so that Man can live there (The Atmosphere, NASA).

Harmful radiations

Another important function of the atmosphere is the prevention of the effects of ultra-violet radiation of the sun. The ionizing radiation is filtered so that it does not harm the skin or cause cancers. These ionizing radiations are electro-magnetic radiations or energy due to nuclear particles which cannot harm the skin by producing cancers as they are prevented from reaching our bodies. They can rupture chromosomes and cause mutations which can lead to cancers. Space is rich in ionizing radiations (The Atmosphere, NASA). Galactic cosmic rays are harmless electro-magnetic radiations found in space but these are not found in our Solar system. Sporadic radiation from the sun that is called solar particulate radiation is another kind. It arrives over a few days and is called solar particle events or flares. The ionizing radiation hazards in space include solar flare, solar wind and galactic cosmic rays.

The survival in space becomes a challenge as there is nothing like atmosphere there (The Atmosphere, NASA). The few gas molecules present do not build up into sufficient atmospheric pressure. This atmosphere is a vacuum so that human space travellers moving into that atmosphere need to carry their own atmosphere pressurized appropriately with the right mixture of gases in a leakproof cabin. Should they need to venture out on spacewalks, they need to wear suits pressurized with oxygen.

Body temperature

Space is very cold. The temperature in the cabin and the astronaut suits must be adjusted and well-controlled as temperature variation is extreme. As there is nothing in space the solar energy just passes freely. This energy changes into heat when some obstacle comes in its ways like the Earth, gas molecules, other planets or a spaceship. This aspect is to be remembered when astronauts perform spacewalks. If the Extravehicular Activity or EVA is in the path of the sun, the spaceship or the astronaut absorbs the unshielded energy and becomes heated up. The spacecraft and the astronaut need to be protected by insulating material. Human beings are comfortable only when the body temperature is 98.4oF or around it. The spacecraft and the suits are thereby actively regulated for the temperature. Heating and cooling are involved to allow humans to live and work comfortably in space.

Gravity

The presence of gravity on earth helps all living beings to remain attracted to the surface of the earth. Space has practically no force of gravity. The human body would not be seen functioning as on Earth. Microgravity causes the body to act differently.

The constant tug of gravity keeps our feet planted on the ground on the earth. This is not possible in Space.

The Stress of Transition

The astronaut feels differently in space and his body functions become slightly altered in the new environmental conditions. In fact the astronaut has to make many alterations in his style right from takeoff and at the different stages of the flight. The high acceleration rate of the spacecraft creates an adjustment problem and stress at the launch. The cramped quarters and the lack of privacy on the shuttle are other causes of stress. As the shuttle moves away from the earth, the effects of the exposure to greater radiation become evident. The lack of gravity in space or microgravity causes a shift of body fluids to the head. The stress of reentry and landing is another problem. The loss of muscle mass in the flight due to lack of proper exercise and the effects of weightlessness of objects are additional problems. All of the problems are to be solved to reduce the discomfort of space travel. After launch, the spacecraft is carried through the atmosphere into space. At this moment begins the feeling of weightlessness (Lujan and White, Human Physiology in Space). Then begins the astronauts’ acrobatics whereby he walks all walls and ceiling of the cabin, turns somersaults and balances heavy objects on the fingertips. This is the time when space motion sickness begins.

Space Adaptation syndrome or Space motion sickness

The interpretations of the brain to visual, auditory and other signals like touch become confused at the beginning of the weightlessness and may provoke nauseous sensations. The sense of position is confused due to the altered interpretations of the visual, auditory and vestibular system signals. Headaches, loss of appetite and a queasy feeling in the stomach affects the sick astronauts and interfere with their efficiency (Lujan and White, Human Physiology in Space). Some vomit, others become disoriented or dizzy or have visual motion illusions. These symptoms are experienced by 50% of astronauts during the first 2-3 days of flight (Nooij, 2007). It is believed that a transition from 3g. to 1 g. produces similar symptoms which are termed as Sickness Induced by Centrifugation (SIC). Both illnesses are caused by movements of the head. The vestibular system involvement is proved by observation of the linear motion perceptions and eye movements (Nooij, 2007). The conflict theory on motion sickness has some drawbacks. The “subjective vertical mismatch theory” used gravity to explain the SAS (Bles, 1998). This explanation incorporates self motion, attitude perception and eye movements and the role of gravity. Further experiments found that astronauts who were affected by the SAS were the same people who were affected by the SIC. If one did not have SAS, he did not have SIC too.

The overall incidence of SAS is the same as SIC at 45%. Why some astronauts are affected while others are not cannot yet be explained. A suggestion that astronauts can be trained to overcome the phenomenon has also arisen (Nooij, 2007). Anti-space sickness drugs are also being tried.

The Cardiovascular System in microgravity

On earth the distribution of blood through the body to the peripheral parts is influenced by gravity. In space the blood makes a headward shift. Fluids are distributed more towards the head and upper part of the body away from the extremities so that the face looks filled out and the legs are thinner in circumference and called bird-legs. The over-abundance of fluids in the upper part of the body is sensed by the body which responds by the kidneys eliminating the excess fluid. The astronauts reduce their drinking in addition. The excess fluid is eliminated and the total fluid would be much less than that on Earth. The heart function is reduced as there is less fluid to pump and the activities in the cabin are less than those of normal walking and running. The space –normal condition has been reached.

The blood pressure and distribution of blood volume of Man when exposed to gravity of 9.81./sec2 are different when in Space or microgravity (Antonutto, 2007). The pressure in the lower limbs is higher than in the brain. The heart pumps blood in such a manner as to carry it to the brain too. Baroreflexes and chemoreflexes are the evolutionary sensitive mechanisms which control blood flow and oxygen flow. These sensors are found at the bifurcation of the common carotid on the way to the brain. In spaceflight, suddenly a larger central venous pressure and central blood volume occur. The extracellular fluid shift also occurs in the same direction upwards. Body fluid displacement is an immediate loss of fluid reducing the central blood volume and plasma volume (Nicogossian et al, 1993). Pooling of blood in the extremities occurs because of the increase in venous compliance (Herault et al, 2000). The neural control of the cardiovascular system also has been noted. The arterial baroreflex control becomes impaired and influences the heart rate at rest (Fritsch-Yelle, 1994). The metaboreflexes of the muscles during exercise show an enhancement (Iellamo et al, 2006). Long-term bed rest studies like in Lower Body Negative Pressure and stand tests indicated defective lower limb vasoconstriction (Herault et a, 2000). Though much reorganization occurs, certain functions are not changed.

Hand grip and cold pressor stimuli do not show any change in the cardiovascular and sympathetic neural responses (Fu et al, 2002). Steady-state exercise maintains the relationship between cardiac output and oxygen consumption in microgravity (Shykoff et al, 1997). Levine et al have noted that the maximum cardiac output and oxygen consumption is unchanged (1996). Once the astronauts adapt to microgravity, their performance and efficiency of work improve even though their muscle mass is reduced (Antonutto, 2007).

Artificial gravity

When the astronauts return to earth and the conditions of gravity, blood travels downward to the limbs with compliant veins. Central blood volume, venous return and the stroke volume of the heart become reduced. The heart rate does not increase to compensate. “The relationship between cardiac output and oxygen consumption in steady state exercise is displaced down ward” (Antonutto, 2007, p.140). So there is a reduction in the maximal cardiac output and maximal oxygen consumption (Capelli et al, 2006). Cardiovascular deconditioning has occurred and attempts must be made to reverse it.

The physicians who see the returning astronauts, attempt to prevent the decay of the cardiovascular system and the musculoskeletal system by artificial gravity or hypergravity (Antonutto, 2007). Studies have been done in the range of 4-9g but more has been done in the gravity range of 3g and below. Future systems of artificial gravity are more likely to use this range as a measure against the decay of cardiovascular deconditioning (Clément and Pavy-Le Traon 2004). Pulmonary blood flow is reduced and ventilation is very much increased at 3 g.

The ventilation-perfusion ratio is increased due to the increase in the respiratory drive, a reduction in the homogeneity of the alveolar ventilation perfusion and reduction of pulmonary blood flow. The systemic circulation is reduced due to reduced cardiac diastolic filling and because arterial pressures above a certain level were reduced with lesser tissue perfusion pressure (Rowell, 1993). The skull protects the brain and the perfusion in the eye may be affected by hypergravity. Dynamic leg exercise however produces a better pumping action. Studies showed that the hypergravity exercises could not be extended for even 13 minutes as some participants could not tolerate them. Most however could finish the exercises and did not complain of the dimness of vision or impending loss of consciousness (Rosenhamer, 1968). The oxygen consumption of exercising muscles increases with gravity in the head-to-foot direction. The maximal working capacity will be decreased in planets with greater gravity than Earth and reduced on the Moon, Mars and microgravity. The efficiency of the lungs is reduced at high gravity but improves with exercise.

Components of blood in space

The haematocrit value of blood remains the same before flight and after reaching space.

The fluid loss causes a decrease in the volume of plasma (Lujan and White, NSBRI). The RBCs also decrease in number leading to the lack of change in the concentration of red cells in the blood or haemtocrit value. This is termed space anaemia. In the haemoconcentration theory, the body understands the need for lesser RBCs as a response to the increased fluid in the upper part of the body. Fluid is lost through the kidney and by way of drinking less. The kidney reduces the production of erythropoietin which suppresses the RBC formation. Another theory states that with the reduction of muscle mass in space and thereby lesser oxygen would be required. The body responds by reducing the number of RBCs. Another theory speaks about the increase in destruction of RBCs which regulates the number of RBCs to keep the haematocrit value steady. However if this theory is correct, the number of reticulocytes would increase. Another theory says that the loss of Calcium could alter the bone structure and bone marrow which then reduces the red cell formation (Lujan and White, NSBRI).

Human lymphocytes have shown that the mitogenic T-cell activation is completely depressed in microgravity (Cogoli, 2006). This experiment was done in the Spacelab in 1983.

Another interesting finding was “was the alteration of the cytoskeleton 30 sec after exposure to microgravity observed in Jurkat cells, a cell line derived from T lymphocytes as seen in samples treated with fluorescent antibodies against vimentin, a constituent of the cytoskeleton” (Cogoli, 2006). In later experiments, different activating factors causing inhibition of T cell activation in vectorless gravity were found. This indicated that gravitational unloading only affected very specific steps of the signal transduction pathways (Vadrucci, in print).

Maintenance of fluids

The fluid balance of the intracellular fluid and extracellular fluid like the interstitial fluid and blood plasma is maintained by the methods of intake and output. The ways to change intake and output would strike a balance through the functions of the kidney and endocrine hormones in the body. The total volume of water in the body would be the same. The electrolytes perform certain functions in the body by changing into active ions for various metabolic reactions. Of these sodium ion is the one which is connected to the fluid balance and the amount of urine excretion.

The changes in the body fluids start at the launchpad itself. The Launch and Entry Suit is provided for the launching but is not protective of fluid loss occurring in the body (Lujan and White, NSBRI). Urine excreted during launch comes to about 1 liter. The astronauts are fitted with napkins in their suits as the urge to urinate occurs during the launching. The head-down and legs-up launch position create an upward shift of the fluid and the body responds by passing urine so that the fluid maintenance is managed. In space, when more fluid is lost, thirst is not felt as the hormones which regulate the excretion of salts and minerals are secreted (Lujan and White, NSBRI). Later in the mission the regulation would have reached a space-normal level.

Muscles

The character of muscles can change depending on the stress they are exposed to (Lujan and White, NSRBI). Muscles can be built up and broken as necessary within 7-14 days. As muscles adapt to changing situations, the protein in the muscles is broken, thrown away and replaced. Changes in the environment can change the muscles in size and pattern. Muscles that are not used go into hibernation mode. If disuse has been of long duration, wasting or atrophy occurs. Astronauts do not need the use of anti-gravity muscles in space. These would undergo natural atrophy (Lujan and White, NSRBI).

Skeletal muscle responds to stress and is highly adaptable. Microgravity affects the structure, function and neuromuscular control of the skeletal muscle Countermeasures include “aerobic training, resistive training, electrical muscle stimulation, low body negative pressure training, and various means of achieving artificial gravity” (Narici, 2007). Detraining and inactivity can produce varying degrees of atrophy based on muscle fiber type. Postural muscles having Type I slow fibres like the soleus, vastus intermedius and adductor longus are more prone than non postural muscles like tibialis anterior and extensor digitorum longus having Type II fast fibers. A decrease of 37% in muscle mass was found in rats in the missions US Shuttle and Russian Cosmos (Narici, 2007). The picture in humans is slightly different in short-term space flight. Prolonged bed rest of 12 weeks however has shown decrease of 35% in one muscle and and 42 % in the other in Type I fibers and lesser decrease in Type II fibers of 20% and 25% (Rudnick et al, 2004). After 84 days the decrease was seen to be 15% in Type I and 8% in Type II. Type I fibers appear to be more prone to disuse-atrophy (Narici, 2007). Contractile properties of single fibers show variation due to a reduction in the myofibrillar protein density and a decrease in the number of cross-bridges. It has been suggested that the shortening velocity was modulated by muscle activity. The single fiber peak power decreased in conditions of actual and simulated microgravity. Human and animal muscles show obvious atrophy because of a decrease in the size of fiber, the number of fibers being the same (Thomason et al, 1990). Postural muscles show more decrease and even in the postural, the degree of decrease is different. The greatest decrease in volume is seen in the plantar flexors followed by the dorsiflexors, knee extensors, knee flexors

and intrinsic lower back muscles (Le Blanc et al, 1997). Bed rest studies indicate that the degree of atrophy depends on the time factor (Adams et al, 1994). After 120 days of microgravity or simulated microgravity, 30% decrease was seen. In another study after 90 days of bed rest, 30% of calf muscle mass was found reduced (Narici, 2007). Animal and human muscle atrophy occur due to the lack of balance between the rates of protein synthesis and degradation. Synthesis decreases and degradation increases in the first 2 weeks. The equilibrium between the two is achieved in the next 2 weeks and muscle protein content becomes stable (Thomason et al, 1989). After three months of staring the space flight aboard the MIR, a decrease of 45% was seen in protein synthesis when compared to pre-flight values (Stein et al, 1999). Protein breakdown also decreased so the reduction in muscle mass was actually due to decrease in protein synthesis rather than increase of breakdown (Ferrando et al, 2002). Muscle architecture also shows a variation in space flight. Disuse atrophy shows a decrease in fascicle length and pennation angle (Reeves et al, 2002). A 10% decrease in pennation angle and a 13% decrease in fascicle length was seen in young, male healthy adults who had 90 days bed rest (Reeves et al, 2002). Those who had intensive exercise every three days showed a slight change in that the level of atrophy seen was less when compared to the control group. The reduction of muscle sarcomeres was in parallel and in series thereby causing a decrease in the power of the muscle.

The power of the lower limb muscles was reduced by 30% after 42 days of bed rest. The ratio of tetanic force to cross-sectional area decreased after 17 days of bed rest. After Skylab, the force of the muscles showed a decrease of 6.5% to 20%. The large fall of maximal explosive power is seen only after space flight. After 42 days of bed rest, the maximal explosive power was found to decrease by 76% of pre-flight levels. It has been suggested that the microgravity situation causes a rearrangement of the motor control system which has led to the decrease in the maximal explosive power (Narici, 2007). The mitochondrial enzymes show a decreased activity

while the glycolytic enzymes do not show a difference. Muscles of different fibers show a similar qualitative pattern. An increase of muscle fatigability is seen during the space flight and persisting into recovery due to an increase of fast twitch fibers, reduced ability to oxidize fatty acids and increased utilization of carbohydrates (Baldwin et al, 1993) and reduced blood flow in the exercise. Muscle damage is increased in space flight. The whole muscle swells due to microcirculatory changes and interstitial oedema. Few studies have investigated this. The decrease in muscle power for bilateral muscle functions was more than for unilateral functions in weightlessness. This bilateral deficit is increased in disuse atrophy. Astronauts tried to keep away the disuse atrophy through inflight exercise to no avail (Narici, 2007).

Types of exercises good in space flight

Daily supine cycle ergometer exercise is good for cardiovascular functions (Chase et al, 1966). Aerobic exercises may not be good for inflight as the mechanical load is too small for preventing muscle atrophy or producing hypertrophy. Cycle exercise for 60 minutes at 40% maximal aerobic power did not protect against atrophy after 20 days bed rest (Suzuki et al, 1994). Resistive exercises managed to protect 80% of the muscle loss and are best for preventing the negative effects on skeletal muscle.

Strength in space

Rigid support is provided by the skeleton. Demineralization causes the loss of Calcium from the bones. The skeletal system becomes weak and unable to withstand stress in space. However the recovery process starts only on return to the Earth. Bone marrow changes accompany the demineralization process. The welfare and performance of the astronauts depend on the maintenance of muscle and integrity. The most effective counter-measure of the balance is by healthy nutrition, medicines and exercise. Volume of selected muscles, lean body mass, and spinal bone marrow composition can be measured by Magnetic Resonance Imaging and Bone Mineral Loss and Recovery.

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