Here on Earth where most of us live,we spend about a third of everyday lying down sleeping and two-thirds standing or sitting in an upright position.
That's not really how it goes in space when people are weightless and the zero gravity environment causes more liquid to shift to the head.Faces get swollen,legs lose volume and appear to be smaller.Many astronauts have complained of eye and back problems after coming back down to Earth and its gravity.
And now scientists say they have discovered some new risks with long-term space flight.A study published recently in JAMA Network Open,a medical journal,examined 11 healthy astronauts who had been on the International Space Station for six months.Eight of them had unusual characteristics observed in their blood.For instance,six of the astronauts had reverse（反向的）blood flow from their heads.
The lead author of the study says he doesn't know if that's actually harmful.The blood is still leaving the head from other pathways,so flowing backwards through a jugular vein（颈静脉）may not be dangerous.But he says it does show a change in how blood moves through the body while in space.
Another issue the study found was blood clots（凝块）.One astronaut had one.Another showed signs of a partial blood clot.That is potentially harmful as the clots can block the flow of blood to the lungs.The astronaut who had one was treated for the rest of the spaceflight and made it home safely.
What does all this mean？Well,one researcher says these issues have probably been occurring since humans first went into space and that they would likely resolve themselves when astronauts came back down to Earth.Knowing about them now gives doctors something else to monitor when people leave our atmosphere.



What causes the physical problems for astronauts？ ______

A. Sleeping for a long time.
B. Staying in the spaceship.
C. The shift in their bodies.
D. The zero gravity environment.

How many issues did the study newly find？ ______

A. One.
B. Two.
C. Three.
D. Four.

What can we know about the blood clot？ ______

A. It is a common problem.
B. It was first found by astronauts.
C. It is a new kind of disease.
D. It is a threat to people's health.

What may be the best title for the text？ ______

A. Doctors have a new task for astronauts
B. Zero gravity stops astronauts flying in space
C. New risks are found in long-term space flight
D. It's dangerous for astronauts to fly into space