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Back pain is one of the most common medical problems encountered in space - over 65% of astronauts and cosmonauts experience it sometime during space flight.

Here on Earth, gravity acts on the spine, compressing it so that it shortens. At night when we sleep, the spine elongates because gravity is not acting on it.

The same process happens in space. Taking the force of gravity away lets the spine expand. The vertebrae are no longer pulled down on top of each other and they spread apart slightly, making the cosmonaut appear taller.

Researchers believe that the back pain may be caused by this stretching of the spinal column. When the spinal column extends, it creates tension in the nerves in, and extending from, the spinal cord. This could lead to problems such as muscle spasms and changes in sensation.

Body fluids – reduced by one-fifth

Mark's body should no be functioning in a 'space normal' condition, with only 80% of his 'earth- normal' fluids. His kidneys should have adapted and he should feel well-adjusted.

He will not be very thirsty and his urination levels should be normal.

Only 80% of red and white blood cells remain

Although Mark's body has reduced the number of red and white blood cells by as much as 20%, he will not notice the difference.

Immune system

The immune systems of the crewmembers will be weaker, and they will have to be cautious about hygiene to avoid infections.

The heart a third smaller

Mark's heart will already be almost a third smaller than before lift off and his heart beat will have returned to normal. These changes will not cause problems, as the heart has merely adjusted to 'space normal' conditions.

In weightlessness, Mark will not have to use his muscles as much to get around, and so his heart does not have to work as hard as on earth. Cosmonauts exercise as much as possible to prevent the reconditioning of the heart muscle.

Losing bone mass

The weakening of the skeleton, that began on the first day, continues. Losing bone mass is one of the most serious effects of space travel.

A cosmonauts can lose up to one percent of bone mass per month in space.

Mark will exercise for two and a half hours per day to prevent bone and muscle loss. He will be strapped to his training apparatus to prevent him from flying all over the shuttle.

Losing muscle

Cosmonauts can lose 10-20 percent of the muscle mass on a shirt mission. The muscles most affected by space travel are the leg and spine muscles.

Emotional influences

Read more about the psychological pressures of space in Health24's Body in Space feature.

Internal clock

Cosmonauts sleep roughly six hours per day. The spacecraft orbits the earth every 90 minutes, so the crew is exposed to 45 minutes of light followed by 45 minutes of darkness. This causes sleep disruptions, which is why almost half the medication used on board the space shuttle is used to help cosmonauts sleep.

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