Space physiology is the study of physiological responses of the body in space and spacecrafts. Major differences between the environments of earth and space are atmosphere, radiation and gravity. These three factors challenge the human survival in space. Atmospheric factors include atmospheric pressure, temperature, humidity and gas composition. Spacecraft or spacelab is provided with stable and sophisticated environmental control system, which maintains all the atmospheric factors close to earth’s environment. Astronauts also wear launch and entry suit (LES). LES is a pressurized suit that protects the body from space environment. Another factor which affects the body in the space is weightlessness. Weightlessness is because of absence of gravity (microgravity).


While traveling by spacecraft, the astronauts experiencesome intense symptoms only during blast off, due to acceleration and during landing because of deceleration. Otherwise, the accelerative forces are least while traveling in a spacecraft, since the spacecraft cannot make rapid changes in speed or direction like an aircraft. Most of the physiological changes occur due to weightlessness in space travel. These changes are responsible for the adaptation of astronaut’s body to space environment. Further, problems develop only when they return to earth. They require a longtime to readapt to earth environment. Effects of weightlessness in spacecraft are:

1. Effects on Cardiovascular Systems and Kidneys

Cardiovascular changes are due to the fluid shift. Due to absence of gravity, blood moves from lower part to upper part of the body (upper trunk and head). It causes enlargement of heart to cope up with increased blood flow. In addition, there is an accumulation of other body fluids in upper part. Now, the compensatory mechanism in the body interprets the increase in blood and other fluids as a serious threat and starts correcting it by excreting large amount of fluid through kidneys. It causes decrease in blood volume and the heart need not pump the blood against gravity in space. So, initially enlarged heart starts shrinking slowly and becomes small. Thus during the initial fluid shift, astronauts experience dizziness or feeling of fainting. Along with water, kidneys excrete electrolytes also. Because of this, osmolarity of body fluids is not altered. So the thirst center is not stimulated and the astronauts

do not feel thirsty during space travel.

2. Effects on Blood

Plasma volume decreases due to excretion of fluid through urine. RBC count also decreases and it is called space anemia.

3. Effects on Musculoskeletal System

Because of microgravity in space, the muscles need not support the body against gravity. Astronauts move by floating instead of using their legs. This leads to decrease in muscle mass and muscle strength. Endurance of the muscles also decreases. Bones become weak. Osteoclastic activity increases during space travel. Calcium removed from bone is excreted through urine.

4. Effects on Immune System

Space travel causes suppression of immune system in the body.

5. Space Motion Sickness

After obtaining weightlessness, some astronauts develop space motion sickness. It is characterized by

nausea, vomiting, headache and malaise (generalized feeling of discomfort or lack of wellbeing

or illness that is associated with sensation of exhaustion). It persists for two or three days and then disappears. It is thought that the motion sickness occurs due to abnormal stimulation of vestibular apparatus and fluid shift.

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