Respiration

Cigarette smoke contains several chemicals that damage the gas exchange system and the rest of the body.

Three of the key chemicals

Three smoking-related lung diseases

Chronic bronchitis

• Tar inflames the lining of the airways. In response, the goblet cells and mucus glands grow larger and pour out far more mucus than they would in a healthy lung.
• The mucus builds up in the smallest bronchioles, blocking them and creating an ideal place for bacteria to grow. Frequent chest infections follow.
• Tar also damages the cilia on the airway lining, so the natural mechanism that sweeps mucus up out of the lungs no longer works.
• The smoker has to cough to clear the mucus, which is the familiar "smoker's cough".

Emphysema

• Repeated infections attract phagocytes (a type of white blood cell) into the lungs. The phagocytes release an enzyme called elastase, which destroys the elastic fibres that normally hold the alveolar walls together.
• Without elastic fibres, the alveoli cannot recoil properly. Many alveoli burst, merging into larger air spaces.
• The total surface area for gas exchange falls, sometimes to a tenth of the normal value.
• The patient becomes breathless and wheezy with even gentle activity, and in late stages may need a constant oxygen supply.

Lung cancer

• Many of the chemicals in tar are carcinogens that can damage the DNA of cells lining the airways.
• A damaged cell can become a cancer cell that divides uncontrollably, forming a tumour.
• Smoking is the single biggest cause of lung cancer; the risk falls steeply if the smoker quits.

Coronary heart disease

Smoking is also a major risk factor for coronary heart disease. Nicotine raises blood pressure, carbon monoxide reduces oxygen delivery, and both put strain on the coronary arteries that supply the heart muscle. Fatty deposits called atheromas are more likely to build up inside these arteries, eventually leading to a heart attack if a coronary artery becomes blocked.