Travel is exciting and summer is a busy period for holiday travel. Many people will travel great distances in airplanes, cars or other vehicles. Experts say people should know about other health problems that can strike when traveling by air.
One of these is a condition called hypoxia (缺氧症). It results from a lack of oxygen to the brain. Experts say the body begins losing oxygen minutes after an airplane leaves the ground. The air pressure in a plane during flight is lower than at sea level. This makes it more difficult for the body to effectively use the same amount of oxygen as it would on the ground. Fewer oxygen molecules （分子） cross the tissues (组织) in the lungs and reach the bloodstream.
The result is a five to twenty percent drop in the amount of oxygen in the blood. This reduces the amount of oxygen that reaches the organs of the body.
One effect of this lack of oxygen to the brain is a headache. When this happens, the heart attempts to fix the situation by beating harder and faster. This can make the traveler feel tired.
These signs of hypoxia are not dangerous in a healthy person. But a drop in oxygen level can cause a health emergency in people with heart or lung problems. They might lose consciousness or even suffer a heart attack.
Experts say that smoking cigarettes and drinking alcoholic liquids also reduce the body’s ability to use oxygen. So they suggest that people not drink alcohol or smoke cigarettes either before or during a flight. They also say persons with heart or lung problems should seek advice from their doctors before flying.
小题1:The author writes the passage mainly to ___.
A．warn people in airplanes of possible dangers
B．offer some advice on avoiding health problems
C．explain the causes and effects of hypoxia
D．advise people not to smoke or drink alcohol小题2:According to the passage, what leads to hypoxia?
A．The brain lacks enough oxygen.
B．The air pressure in a plane is too high
C．Our body needs less oxygen during takeoff.
D．The bloodstream contains no oxygen molecules.小题3:The underlined part “fix the situation” means “___”.
A．use more oxygen it needs
B．increase the air pressure
C．reduce the amount of blood
D．bump more blood to the brain小题4:Why should passengers not smoke or drink before flying?
A．They weaken the body’s ability to use oxygen.
B．Smoking may pollute the air inside the airplane.
C．Drinking alcohol may raise one’s blood pressure.
D．Drinking is forbidden at the airport.小题5:Do you think which the following title is the best title for this passage?
A．Tralling
B．how to travel
C．some suggestions about on the plane while travelling
D．the causes and effects of hypoxia
可知，机器做功越多，其功率就越大
可知，只要知道时间t内机器所做的功，就可以求得这段时间内任一时刻机器做功的功率