While flying in space, astronauts have no weight. It is a condition of weightlessness.

How does this affect their heart and blood vessels？ What happens to their hearts and blood vessels when they go into space？

On earth gravity (引力) makes blood flow down towards the feet, so the heart must work to pump blood up. In space blood flows up towards the heart. The effect is similar to what happens when you lie with your feet up and your head down. The flowing blood is felt by receptors in the walls of arteries (动脉) in the upper part of the body and in the heart. These receptors send signals to the brain, but the brain reads them incorrectly. To the brain, the signals means the amount of blood in the body has increased, so the brain tells other parts of the body to react to the increase in the blood. For example, the body produces more urine (尿液), resulting in loss of body fluid and salt. At the same time the astronaut has less desire to drink and so does not replace the lost fluid.

As a spacecraft begins to fall back to earth atmosphere, the crew begins to feel gravity. Blood again flows towards the feet and collects there. The heart beats faster in an effort to pump enough blood to the brain. Some crew members may not get enough blood to the brain and the heart beats too fast. They become sick and even lose consciousness as their spacecraft reenters the earth atmosphere. To prevent this, special clothing is invited for astronauts to prevent blood collecting in their legs. They put it on just before the craft reenters the earth’s atmosphere. They also drink about a liter (升) of water and eat some salt to replace the fluid they have lost while in space.

Weightlessness affects the heart and blood vessels in another way. The heart does not work as hard in a weightless environment as it does in an environment of gravity. When the heart works less hard, the heart muscles become weaker. It is replaced by fibrous tissue and fat

41.If one lies with his feet up and his head down on earth, his()

A.blood will flow up towards the heart.

B.blood will flow down towards the feet.

C.heart will pump the blood up to the head.

D.heart will have to work harder.

42.What can we infer from the second paragraph of the passage()

A.The brain reads the signals incorrectly because it tends to go wrong in outer space

B.In space an astronaut should drink some water even when he does not feel thirsty.

C.In space the amount of water in the body of an astronaut increase.

D.The receptors in the walls of arteries often send wrong signals to the brain.

43.When the spacecraft reenters the earth’s atmosphere some astronauts become sick or even lose consciousness because()

A.their brains cannot get enough blood

B.their hearts are not strong enough

C.their blood collects in their feet

D.they begin to feel the gravity again

44.When the spacecraft enter the earth’s atmosphere, the astronauts should do the following except()

A.trying to regain their consciousness

B.eating some salt

C.drinking some water.

D.putting on special clothing

45.After a long time in space, an astronaut’s heart muscles become weaker because()

A.they are short of exercises when the heart works less hard.

B.the heart works too hard under an environment of weightlessness.

C.the astronaut’s hard work puts too great a burden on the heart.

D.they are completely replaced by fibrous tissue and fat.