首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
If it weren’t for nicotine, people wouldn’t smoke tobacco. Why? Because of the more than 4,000 chemicals in tobacco smoke, nicot
If it weren’t for nicotine, people wouldn’t smoke tobacco. Why? Because of the more than 4,000 chemicals in tobacco smoke, nicot
admin
2009-04-23
112
问题
If it weren’t for nicotine, people wouldn’t smoke tobacco. Why? Because of the more than 4,000 chemicals in tobacco smoke, nicotine is the primary one that acts on the brain, altering people’s moods, appetites and alertness in ways they find pleasant and beneficial. Unfortunately, as it is widely known, nicotine has a dark side: it is highly addictive. Once smokers become hooked on it, they must get their fix of it regularly, sometimes several dozen times a day. Cigarette smoke contains 43 known carcinogens(致癌物质), which means that long-term smoking can amount to a death sentence. In the US alone, 420,000 Americans die every year from tobacco-related illness.
Breaking nicotine addiction is not easy. Each year, nearly 35 million people make a concerted effort to quit smoking. Sadly, less than 7 percent succeed in abstaining for more than a year; most start smoking again within days. So what is nicotine and how does it insinuate(慢慢滋长) itself into the smoker’s brain?
The nicotine found in tobacco is a potent drag. Smokers, and even some scientists, say it offers certain benefits. One is enhanced performance. One study found that non-smokers given doses of nicotine typed about 5 percent faster than they did without it. To greater or lesser degree, users also say nicotine helps them to maintain concentration, reduce anxiety, relieve pain, and even dampen their appetites(thus helping in weight control). Unfortunately, nicotine can also produce deleterious effects beyond addiction. At high doses, as are achieved from tobacco products, it can cause high blood pressure, distress in the respiratory and gastrointestinal systems and an increase in susceptibility to seizures and hypothermia(体温降低).
First isolated as a compound in 1828, in its pure form nicotine is a clear liquid that turns brown when burned and smells like tobacco when exposed to air. R is found in several species of plants, including tobacco and, perhaps surprisingly, in tomatoes, potatoes, and eggplant(though in extremely low quantities that are pharmacologically insignificant for humans).
As simple as it looks, the cigarette is highly engineered nicotine delivery device. For instance, when tobacco researchers found that much of the nicotine in a cigarette wasn’t released when burned but rather remained chemically bound within the tobacco leaf, they began adding substances such as ammonia(氨水) to cigarette tobacco to release more nicotine. Ammonia helps to keep nicotine in its basic form, which is more readily vaporized by the intense heat of the burning cigarette than the acidic form. Most cigarettes for sale in the US today contain 10 milligrams or more of nicotine. By inhaling smoke from a lighted cigarette, the average smoker takes 1 or 2 milligrams of vaporized nicotine per cigarette. Today we know that only a miniscule amount of nicotine is needed to fuel addiction. Research shows that manufacturers would have to cut nicotine levels in a typical cigarette by 95% to forestall its power to addict. When a smoker puffs on a lighted cigarette, smoke, including vaporized nicotine, is drawn into the mouth. The skin and lining of the mouth immediately absorb some nicotine, but the remainder flows straight down into the lungs, where it easily diffuses into the blood vessels lining the lung walls. The blood vessels carry the nicotine to the heart, which then pumps it directly to the brain. While most of the effects a smoker seeks occur in the brain, the heart takes a hit as well. Studies have shown that a smoker’s first cigarette of the day can increase his or her heart rate by 10 to 20 beats a minute. Scientists have found that a smoked substance reaches the brain more quickly than on swallowed, snorted(such as cocaine powder) or even injected. Indeed, a nicotine molecule inhaled in smoke will reach the brain within 10 seconds. The nicotine travels through blood vessels, which branch out into capillaries(毛细血管) with the brain. Capillaries normally carry nutrients but they readily accommodate nicotine molecules as well. Once inside the brain, nicotine, like most addictive drags, triggers the release of chemicals associated with euphoria and pleasure.
Just as it moves rapidly from the lungs into the bloodstream, nicotine also easily diffuses through capillary walls. It then migrates to the spaces surrounding neurons—ganglion cells that transmit nerve impulses throughout the nervous system. These impulses are the basis for our thoughts, feelings, and moods. To transmit nerve impulses to its neighbor, a neurone releases chemical messengers known as neurotransmitters. Like nicotine molecules, the neurotransmitters drift into the so-called synaptic space between neurons, ready to latch onto the receiving neurone and thus deliver a chemical "message" that triggers an electrical impulse.
The neurotransmitters bind onto receptors on the surface of the recipient neurone. This opens channels in the cell surface through which enter ions, or charged atoms, of sodium. This generates a current across the membrane of the receiving cell, which completes delivery of the "message". An accomplished mimic, nicotine competes with the neurotransmitters to bind to the receptors. It wins and, like the vanquished chemical, opens ion channels that let sodium ions into the cell. But there’s a lot more nicotine around than the membrane. This bigger current causes increased electrical impulses to travel along certain neurons. With repeated smoking, the neurons adapt to this increased electrical activity, and the smoker becomes dependent on the nicotine.
选项
A、Y
B、N
C、NG
答案
B
解析
转载请注明原文地址:https://kaotiyun.com/show/3IC7777K
0
大学英语四级
相关试题推荐
期货市场的风险承担者,即()。
某套利者在黄金期货市场上以961美元Z盎司卖出一份7月份的黄金期货合约,同时他在该市场上以950美元/盎司买入一份11月份的黄金期货合约。若经过一段时间之后,7月份价格变为964美元/盎司,同时11月份价格变为957美元/盎司,该套利者同时将两合约对冲平仓
看涨期权购买者的收益一定为期权到期日市场价格和执行价格的差额。()
买入套期保值付出的代价是()。
A、Traintravelcanhelphimknowaboutpeople.B、Traintravelisanuncomfortableexperienceforhim.C、Traintravelisbettert
A、Theyarethemainfactorsinfluencingteensuicide.B、Theyarebeingunfairlycriticized.C、Kidsshouldbeforbiddentouseth
Youshouldstartyouressaywithabriefdescriptionofthepictureandthendiscusswhethergoodacademicperformanceequalsto
Forthispart,youareallowed30minutestowriteanessayentitledManandArtificialIntelligencebycommentingonthesaying
A、Declarativememoryandprocedurememory.B、Declarativememoryandshort-termmemory.C、Short-termmemoryandprocedurememory.
A、Hekeepsforgettingtheimportantthingshehastodo.B、HehasgreatdifficultyrememberingKoreanwords.C、Hecan’tfindthe
随机试题
Ifit______tomorrow,we______putoffthevisittotheYangpuBridge.
急性胆囊炎引起的腹痛特点是
关于肠梗阻错误的是
加有少量钴盐(氯化钴)的硅胶(二氧化硅)颗粒干燥剂
证券服务机构为证券的发行、上市、交易等证券业务活动制作、出具( )时,应当勤勉尽责,对所依据的文件资料内容的真实性、准确性、完整性进行核查和验证。
在建设新农村号召的鼓励下,某地农村成立了一家农资公司,主要经营农产品和农副产品。该农资公司近11个月的玉米销售量为:根据以上材料,回答下列问题:运用算数平均数法进行预测,则第12个月的销售量预测值为()吨。
下列支出中,属于商业银行营业外支出的是()。
Butthesuccessofscience,bothitsintellectualexcitementanditspracticalapplication,dependsupontheself-correctingc
在VisualFoxPro中,如果在表之间的联系中设置了参照完整性规则,并在删除规则中选择了“限制”,当删除父表记录时,系统的反应是()。
有如下类声明:classHow{intk;public:How():k(99){}};则类How所拥有的构造函数的数量是
最新回复
(
0
)