Tham khảo tài liệu 'cracking the toefl ibt 2009 edition part 31', ngoại ngữ, toefl - ielts - toeic phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 302 i Cracking the TOEFL iBT Female Professor Okay I know from having taught this class before that the topic I m about to cover has the potential to get people s blood boiling so I d like to start off by making something clear I m going to discuss the physical effects of tobacco on the body. This is not the place for a debate over social issues. If you all want to get into it over lunch after class is over you re welcome to. Just don t expect me to join in. Anyway what we want to talk about is what happens to your body when you smoke or chew tobacco. The primary active ingredient in tobacco is one you re probably all familiar with nicotine. Nicotine is a class of chemical called an alkaloid meaning that it s a close relative of the caffeine in coffee and the natural drugs found in some plants. It generally has a stimulant effect it gets your body systems going makes them more active. It actually makes up a very small percent of the tobacco plant about 5 percent by mass not much right Also when you smoke tobacco most of the nicotine burns away before you can even inhale it. That means that most of the nicotine in your cigarettes never gets to your lungs. It s not ingested. So between those two things the low proportion of nicotine in the tobacco plant and the high proportion of the nicotine that s destroyed before you ingest it you re not really getting a whole lot with each puff. Even so the nicotine in cigarettes has a powerful effect on the body for a few reasons. For one thing while some drugs pass through the body largely without being absorbed nicotine inhaled into the lungs is absorbed almost completely. This is because the lungs have a lot of surface area that s usually used to exchange oxygen and carbon dioxide when you breathe. All that surface area provides lots and lots of places for nicotine to pass into the bloodstream. Then there s the fact that nicotine crosses the blood-brain barrier quickly. The blood-brain barrier is a sort of gatekeeping system
