People have smoked cigarettes for a long time now. The tobacco which is used to make cigarettes was first grown in what is now part of the United States. Christopher Columbus, who discovered America, saw the Indians smoking. Soon the dried leaves were transported to Europe. In the late 1800s, the Turks made cigarettes popular.
Cigarette smoke contains at least two harmful substances, tar and nicotine. Tar, which forms as the tobacco bums, damages the lungs and therefore affects breathing. Nicotine, which is found in the leaves, causes the heart to beat faster and increases the breathing rate. Nicotine in large can kill a person by stopping a person′s breathing muscles. Smokers usually take in small amounts that the body can quickly break down.
Nicotine can make new smokers feel dizzy (头晕) or sick to their stomachs. The heart rate for young smokers increases 2 to 3 beats per minute. Nicotine also lowers skin temperature and reduces blood flow in the legs and feet. It plays an important role in increasing smokers′ risk of heart disease and stroke.
Smoking cigarettes is dangerous. Cigarette smoking was the cause of lung cancer and several other deadly diseases.
___________ doesn′ t make one′ s heart beat as usual.
A.Tar
B.Nicotine
C.Lung disease
D.Leave