Humans have adapted to the pull of Earth's gravitational field and the human body relies on its influence to develop properly. Living for long periods of time in space or on planets, such as Mars, which have a gravitational force significantly less than Earth's can prove harmful to health.

When you struggle to carry that heavy load of groceries up a flight of stairs you quickly realise how gravity makes your body work. In fact, it makes your body hurt, however, the lack of gravity can cause irreversible damage to the body.

It is gravity that determines how strong our muscles or bones need to be. The body responds to gravity and reacts in a certain way. The body builds muscle mass and strong bones in response to gravity, but in zero gravity there is no signal for the body to respond to so muscles and bones waste away. Muscles that usually work against gravity, such as the calves and muscles of the spine, can lose up to 20% of their mass in zero gravity. For bones the loss is more extreme with between 40% and 60% loss of mass.

On Earth, gravity causes a blood pressure gradient to form in our body. When standing, gravity pushes the blood down towards the feet and increases blood pressure within the feet to around 200 mmHg while blood pressure in the brain is much less at around 80 mmHg. In space there is no pressure gradient and the blood pressure is uniform throughout the body at 100 mmHg.

Brain sensors pick up an unusually high pressure to the brain and immediately assume there is too much blood. The body responds by removing fluids from the body in the form of urine. Within 3 days of zero gravity astronauts can lose up to 20% of blood volume. With less blood to pump around the body the heart works less and as a result heart muscle can deteriorate. Within 3 days of returning to Earth blood volume returns to normal. Astronauts naturally feel thirsty when they return as the body needs to increase blood volume.

To maintain a blood pressure gradient astronauts use a "Lower Body Negative Pressure Chamber" as pictured on the right. This device causes blood to pool in the lower legs causing the heart to work harder and therefore reduce the deterioration of heart muscle.

Maintaining good condition in zero gravity is difficult which makes the 6 month journey to Mars all that much more problematic. When astronauts land on Mars with 40% of Earth's gravity they will have to manage on their own without any help.

To maintain astronaut's fitness a number of devices have been developed that offer resistance training as pictured on the right.

What happens to muscles and bones in low gravity environments?

How does the heart react to zero gravity?

How does a Lower Body Negative Pressure Chamber combat the influence of zero gravity on the heart?

Explain why astronauts that live in zero gravity have a lower blood volume than when they live on Earth?