tailieunhanh - Cracking the Toefl IBT 2009 edition part 33

Tham khảo tài liệu 'cracking the toefl ibt 2009 edition part 33', ngoại ngữ, toefl - ielts - toeic phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | bloodstream. Then there s the fact that nicotine crosses the blood-brain barrier quickly. The blood-brain barrier is a sort of gate-keeping system in the bloodstream. It severely limits the ability of substances and organisms to pass from the blood into the brain. This helps keep the chemical environment of the brain stable which it needs to function properly and it also keeps the brain know nasty infections can t get in. even if the rest of the body is sick fighting them off. Because nicotine is one of those chemicals that crosses the blood-brain barrier quickly it affects the nervous system right from its command center. And it has two big effects. One is that it stimulates the production of the hormone adrenaline. More adrenaline means an increase in your heartbeat your breathing and possibly even cognition that is. the thinking and perceiving nerves in your brain. That would help .right .to explain why people report feeling energized and sharper more aware after smoking a cigarette. Their breathing and heart rates are jazzed up and their thinking nerves are loo. You might be thinking at this point. Well that s how people get addicted to smoking it makes them feel more energetic and stuff and you re partially right but there s another physiological reason. Scientists have evidence that nicotine stimulates the production of not only adrenaline but also another chemical dopamine. Levels of dopamine rise when you re doing something enjoyable like eating your favorite food. So what happens researchers refer to it as a reinforcement or reward mechanism. Dopamine signals well-being so when nicotine stimulates dopamine production it s kind of like exciting your pleasure center. That s probably the source of the rush people associate with smoking and why they keep reaching for more cigarettes to get that same feeling again after it passes. Now those are the most obvious direct effects but nicotine alters body functions in other ways that are also .