Cambridge International A Level Biology Revision Guide

Chapter 9: Gas exchange and smoking
Lung cancer
Tar in tobacco smoke contains several substances that
are carcinogens (Chapter 5). These react, directly or
via breakdown products, with DNA in epithelial cells
to produce mutations, which are the first in a series of
changes that lead to the development of a mass of
cells, known as a tumour (Figure 5.13 on page 104 and
Figure 9.9).
As the cancer develops, it spreads through the bronchial
epithelium and enters the lymphatic tissues (page 164) in
the lung. Cells may break away and spread to other organs
(metastasis, page 105), so that secondary tumours become
established. A tumour like this is known as a malignant
tumour.
Lung cancer takes 20–30 years to develop. Most of the
growth of a tumour occurs before there are any symptoms.
The most common symptom of lung cancer is coughing
up blood, as a result of tissue damage. People with lung
cancer also have chest pain and find it difficult to breathe.
It is rare for a cancer to be diagnosed before it reaches 1 cm
in diameter.
Figure 9.9 A scanning electron micrograph of a bronchial
carcinoma – a cancer in a bronchus. Cancers often develop
at the base of the trachea where it divides into the bronchi as
this is where most of the tar is deposited. The disorganised
malignant tumour cells at the bottom right are invading the
normal tissue of the ciliated epithelium (× 1000).
Tumours in the lungs, such as that shown in Figure 9.9,
are located by one of three methods:
■■
■■
bronchoscopy, using an endoscope to allow a direct
view of the lining of the bronchi (Figure 9.1)
chest X-ray
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CT scan (similar to that shown in Figure 9.8).
By the time most lung cancers are discovered, they are
well advanced; treatment involves surgery, radiotherapy
and chemotherapy and is dependent on the type of lung
cancer, how far it has developed and whether it has
spread into other areas of the body (whether there is
metastasis). If the cancer is small and in one lung, then
either a part or all of the lung is removed. However,
metastasis has usually happened by the time of the
diagnosis so, if there are secondary tumours, surgery will
not cure the disease. This is why smoking is linked to so
many different cancers. Chemotherapy with anti-cancer
drugs or radiotherapy with X-rays (or another form of
radiation) is used.
Short-term effects on the
cardiovascular system
The two components of tobacco smoke that cause shortterm
effects on the cardiovascular system are nicotine and
carbon monoxide.
Nicotine
Nicotine is the drug in tobacco. It is absorbed very readily
by the blood and travels to the brain within a few seconds.
It stimulates the nervous system to reduce the diameter
of the arterioles and to release the hormone adrenaline
from the adrenal glands. As a result, heart rate and blood
pressure increase and there is a decrease in blood supply
to the extremities of the body, such as hands and feet,
reducing their supply of oxygen. Nicotine also increases
the risk of blood clotting.
Nicotine is a highly addictive drug that influences
reward centres in the brain. It stimulates nerve endings in
the brain to release the transmitter substance dopamine,
which is associated with reinforcing pleasurable
experiences. This makes it very hard to give up smoking.
Carbon monoxide
Carbon monoxide diffuses across the walls of the alveoli
and into the blood in the lungs. It diffuses into red
blood cells where it combines with haemoglobin
to form the stable compound carboxyhaemoglobin
(page 171). This means that haemoglobin does not become
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