• Tar is a chemical in cigarettes and is a carcinogen, which means it causes cancer
• Cancer occurs if mutations affect the regulation of mitosis in cells. Tar forms a sticky layer inside the lungs and increases the rate of mutations in the DNA of bronchial epithelial cells, potentially leading to a tumour
• Tumours develop if mutations occur in oncogenes or tumour-suppressor genes of the bronchial epithelial cells. This causes uncontrolled mitosis which develops into a mass of cells in the lumen of the airways
• The tumour becomes larger because it has no method of programmed cell death and survives because it develops its own blood supply (vascularisation)
• The tumour then starts to interfere with the normal working of the lungs, such as by squeezing against blood vessels or cancer cells entering into the lymphatic system, where they may develop another tumour
• Symptoms of lung cancer include coughing up blood, a persistent cough, coughing an increased amount of mucus, back or shoulder pain, wheezing and breathing difficulties and sudden weight loss

 COPD

• Tar can also cause Chronic obstructive pulmonary disease (COPD)
• COPD includes a range of lung-based diseases, such as asthma, chronic bronchitis and emphysema
• Tar stimulates goblet cells in the ciliated epithelium and causes them to become enlarged. In turn, the goblet cells produce more mucus
• This destroys the cilia in the trachea and prevents them from sweeping mucus away from the lungs
• The mucus contains bacteria, dust and other microorganisms and can then block narrow bronchioles, causing coughing, scar tissue and infection
• The infection attracts phagocytes to the lungs and the phagocytes release elastase, an enzyme which damages the elasticity of the alveolar walls
• Without enough elastin, the alveoli break down and may burst. This creates large air spaces in the alveoli and patients become wheezy and breathless. Once the disease progresses, people often need a constant supply of oxygen to stay alive

Worked example

Describe the signs and symptoms of Chronic obstructive pulmonary disease (COPD).

Symptoms include shortness of breath, a chronic or persistent cough, chest tightness, wheezing and difficulty breathing when exercising or during any physical activity.