Edexcel IGCSE Biology 复习笔记 2.7.6 Smoking & the Human Gas Exchange System

• Smoking causes chronic obstructive lung disease (COPD), coronary heart disease and increased risks of several different types of cancer, including lung cancer
• Chemicals in cigarettes include:
  • Tar - a carcinogen (a substance that causes cancer)
  • Nicotine - an addictive substance which also narrows blood vessels
  • Carbon monoxide - reduces the oxygen-carrying capacity of the blood

   

Nicotine

• Nicotine narrows blood vessels leading to an increased blood pressure
• It also increases heart rate
• Both of these effects can cause blood clots to form in the arteries leading to heart attack or stroke

Carbon monoxide

• Carbon monoxide binds irreversibly to haemoglobin, reducing the capacity of blood to carry oxygen
• This puts more strain on the breathing system as breathing frequency and depth need to increase in order to get the same amount of oxygen into the blood
• It also puts more strain on the circulatory system to pump the blood faster around the body and increases the risk of coronary heart disease and strokes

Tar

• Tar is a carcinogen and is linked to increased chances of cancerous cells developing in the lungs
• It also contributes to COPD, which occurs when chronic bronchitis and emphysema (two different diseases which are frequently linked to smoking) occur together
• Chronic bronchitis is caused by tar which stimulates goblet cells and mucus glands to enlarge, producing more mucus
  • It destroys cilia and mucus (containing dirt, bacteria and viruses) builds up blocking the smallest bronchioles and leading to infections
  • A smoker's cough is the attempt to move the mucus

   

• Emphysema develops as a result of frequent infection, phagocytes that enter the lungs release elastase, an enzyme that breaks down the elastic fibres in the alveoli
  • This means the alveoli become less elastic and cannot stretch so many burst
  • The breakdown of alveoli reduces the surface area for gas exchange
  • As it progresses, patients become breathless and wheezy - they may need a constant supply of oxygen to stay alive

   

The breakdown of alveoli in emphysema reduces the surface area for gas exchange

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