Gas Exchange in Animals and Plants

Lessons

Senior 5 · Biology

Gas Exchange in Animals and Plants

5 lessons

Lesson 4 of 5

Smoking, lung disease and heart disease

25 min

One habit, many organs

According to the World Health Organization's fact sheet on tobacco (26 June 2026), tobacco kills more than 7 million people every year, including over 1.6 million non-smokers exposed to second-hand smoke, and about 80% of the world's 1.2 billion tobacco users live in low- and middle-income countries. In Rwanda, Law N° 08 of 2013 relating to the control of tobacco bans smoking in public places and the sale of tobacco to anyone under 18. This lesson explains, using what you now know about the lungs and the blood, exactly how smoke does its damage.

Definition

Carcinogen: a substance that can cause cancer, by damaging DNA.

Chronic obstructive pulmonary disease (COPD): a long-term lung disease in which the flow of air out of the lungs is obstructed; it includes chronic bronchitis (long-term inflammation of the airways, with excess mucus) and emphysema (breakdown of the alveolar walls).

Atherosclerosis: the build-up of fatty deposits, called atheroma or plaques, in the walls of arteries, which narrows and hardens them.

Coronary heart disease (CHD): disease caused by atherosclerosis in the coronary arteries, which supply the heart muscle.

What is in tobacco smoke?

Component	What it is	Main harm
Tar	A sticky mixture of many chemicals that settles in the airways	Damages cilia, irritates airways, contains carcinogens such as benzopyrene
Nicotine	A drug that acts on the nervous system	Addictive; raises heart rate and blood pressure; makes platelets stickier
Carbon monoxide (CO)	A poisonous gas from incomplete burning	Binds to haemoglobin and cuts oxygen transport; damages artery linings

Damage to the gas-exchange system

Cilia stop working. Tar paralyses and destroys the cilia of the ciliated epithelium, so mucus is no longer swept up out of the airways.
More mucus. Goblet cells and mucous glands enlarge and make more mucus. It collects in the airways and is coughed up: "smoker's cough". Bacteria trapped in it cause repeated chest infections.
Chronic bronchitis. The airway walls are inflamed and thickened, and mucus narrows the bronchioles, so air flow is obstructed.
Emphysema. Phagocytes drawn into the inflamed lungs release an enzyme, elastase, that breaks down elastin. The walls between alveoli break down, leaving fewer, larger air spaces (Figure 7). The surface area for gas exchange falls, and the lungs lose their elastic recoil, so air is trapped and hard to breathe out. Sufferers become breathless, at first on exertion and later even at rest.

Worked example 1: diagnosing COPD with a spirometer

A doctor measures how much air a person can blow out in the first second of a forced breath ( $\text{FEV}_1$ ) and in the whole breath (the forced vital capacity, FVC). A ratio $\frac{\text{FEV}_1}{\text{FVC}}$ below 0.70 shows obstruction of the airways (the widely used GOLD criterion). Interpret these results.

Person	FEV₁ / dm³	FVC / dm³
A: non-smoker, 45 years	3.2	4.0
B: has smoked for 30 years, 50 years old	1.5	3.0

A: $\frac{3.2}{4.0} = 0.80$ , above 0.70: no obstruction.
B: $\frac{1.5}{3.0} = 0.50$ , well below 0.70: obstruction.
Explanation: in COPD the narrowed bronchioles and the lost elastic recoil slow the flow of air out, so much less can be blown out in the first second, even though a fair volume can be blown out in total.

Conclusion: B's results fit COPD; a doctor would confirm it with other tests.

Lung cancer

Carcinogens in tar reach the cells lining the bronchi. Over years, they cause mutations in the genes that control cell division (you will meet mutations in Unit 12). A cell with enough mutations divides without control, forming a tumour. A malignant tumour can invade nearby tissue and spread to other organs through the blood or lymph, which is called metastasis. Symptoms include a cough that will not go away, coughing up blood, breathlessness, chest pain and loss of weight. Because the damage builds up slowly, lung cancer is usually found in people who have smoked for many years, and often late, when it is hard to treat.

Nicotine and carbon monoxide in the cardiovascular system

Nicotine stimulates the release of adrenaline, which raises the heart rate and constricts arterioles, so blood pressure rises. It makes platelets stickier, so blood clots form more easily. It is strongly addictive, which is why stopping is hard.
Carbon monoxide combines with haemoglobin to form carboxyhaemoglobin. Haemoglobin binds CO far more strongly than oxygen (roughly 200 times), and the bond is hard to reverse, so less haemoglobin is free to carry oxygen. The heart must pump more blood to deliver the same oxygen. CO also damages the endothelium lining the arteries.

From smoking to a heart attack

High blood pressure, CO and other chemicals damage the endothelium of an artery.
White blood cells move into the damaged wall, with cholesterol carried by low-density lipoproteins (LDL). A fatty deposit, the atheroma, builds up and is covered by fibrous tissue; calcium may harden it.
The plaque narrows the lumen, so less blood can flow, and the artery loses elasticity, which raises blood pressure further.
If the plaque's surface ruptures, platelets stick to it and a blood clot (thrombus) forms; nicotine's sticky platelets make this more likely (Figure 8).

In the coronary arteries, narrowing starves the heart muscle of oxygen during exertion, causing chest pain called angina. A clot that blocks a coronary artery kills the heart muscle beyond it: a myocardial infarction, or heart attack. A clot in an artery to the brain causes a stroke.

Worked example 2: the cost of carbon monoxide

Fully saturated blood carries about $200\,\mathrm{cm^3}$ of oxygen per $\mathrm{dm^3}$ . Suppose 8% of a heavy smoker's haemoglobin is bound to CO (an illustrative value). (a) How much oxygen can each $\mathrm{dm^3}$ of blood now carry? (b) By what percentage must the volume of blood pumped increase to deliver the same oxygen?

(a) Only 92% of the haemoglobin is available: $200 \times 0.92 = 184\,\mathrm{cm^3\,dm^{-3}}$ .
(b) Blood needed $= \frac{1}{0.92} = 1.087$ times as much, an increase of about 8.7%.

Meaning: the heart works harder all day, while its own blood supply carries less oxygen: a dangerous combination when the coronary arteries are already narrowed.

It is never too late to stop

The WHO (questions and answers on the health benefits of smoking cessation, 25 February 2020) lists what happens after a person stops smoking:

Time after stopping	Change
20 minutes	Heart rate and blood pressure drop
12 hours	Carbon monoxide in the blood drops to normal
2 to 12 weeks	Circulation improves and lung function increases
1 to 9 months	Coughing and shortness of breath decrease
1 year	Risk of coronary heart disease is about half that of a smoker
10 years	Risk of lung cancer falls to about half that of a smoker
15 years	Risk of coronary heart disease is that of a non-smoker

The best choice of all is never to start: nicotine is addictive, and most smokers begin as teenagers.

Warning

Common mistakes

"Nicotine causes lung cancer." The carcinogens in tar cause lung cancer; nicotine causes addiction and harms the heart and blood vessels.
"Carbon monoxide blocks the airways." CO acts in the blood, by binding to haemoglobin.
"Emphysema makes alveoli bigger, so the surface area increases." The air spaces are bigger but far fewer, so the total surface area falls.
"Light" cigarettes, shisha and smokeless tobacco are safe. WHO states that all forms of tobacco use are harmful and there is no safe level of exposure.

British Heart Foundation: "Coronary heart disease, clogged arteries and atherosclerosis". Watch how fatty material builds up in the artery wall and how a clot forms on a plaque, then match each stage to Figure 8.

Tip

Exam technique: keep the three main components apart. Tar → Tissues of the airways (cilia, mucus, cancer). Nicotine → Nerves, heart rate and sticky platelets. Carbon monoxide → Carrying of oxygen by haemoglobin. In a long answer, link each effect to a disease.

✦

Practice: smoking and disease

Name the three main harmful components of tobacco smoke and give one effect of each.
Explain why smokers often have a persistent cough.
Describe how emphysema develops and why it makes a person breathless.
A person has an FEV₁ of 2.1 dm³ and an FVC of 3.5 dm³. Calculate the ratio and state whether it shows obstruction.
Explain how carbon monoxide reduces the oxygen delivered to tissues.
Outline the stages by which atherosclerosis can lead to a heart attack.
If 5% of haemoglobin is bound to CO, how much oxygen can 1 dm³ of blood carry, if the normal value is 200 cm³?
Using the WHO table, evaluate the claim "There is no point stopping smoking after 20 years, because the damage is done."

Show answersHide answers

Tar: damages cilia, causes mucus build-up, carcinogens cause cancer. Nicotine: addiction, higher heart rate and blood pressure, sticky platelets. CO: binds haemoglobin, less oxygen carried; damages artery linings.
Tar paralyses the cilia, so mucus is not swept away, and goblet cells make extra mucus; the mucus collects and must be coughed up.
Inflammation draws phagocytes into the alveoli; they release elastase, which breaks down elastin in the alveolar walls; the walls break down into fewer, larger spaces with less surface area and less elastic recoil, so less oxygen is absorbed and air is hard to breathe out.
$\frac{2.1}{3.5} = 0.60$ ; below 0.70, so it shows obstruction.
CO binds to haemoglobin to form carboxyhaemoglobin, very strongly and almost irreversibly, so less haemoglobin is free to carry oxygen.
Endothelium damaged → LDL cholesterol and white cells enter the wall → atheroma builds up → lumen narrows → plaque ruptures → clot forms → coronary artery blocked → heart muscle beyond dies (myocardial infarction).
$200 \times 0.95 = 190\,\mathrm{cm^3}$ .
The claim is false. The WHO table shows benefits at any stage: CO falls to normal within 12 hours, heart disease risk halves within a year, and lung cancer risk halves within 10 years. WHO also notes gains in life expectancy even for people who stop at 60.

Key points

Tar damages cilia and airways and carries carcinogens; nicotine is addictive and strains the heart; CO cuts the oxygen carried by haemoglobin.
COPD = chronic bronchitis + emphysema; it obstructs air flow (FEV₁/FVC below 0.70).
Lung cancer: carcinogens cause mutations that lead to uncontrolled cell division.
Atherosclerosis narrows arteries; a clot on a plaque causes a heart attack or stroke.
Stopping smoking brings benefits within hours and at any age (WHO, 2020).

Sources and credits

Written by Ganzaa.
Facts on deaths and users from Tobacco, fact sheet, World Health Organization, 26 June 2026; the benefits of stopping from Tobacco: Health benefits of smoking cessation, World Health Organization, 25 February 2020. Facts cited, not copied.
Rwanda's law: Law N° 08 of 2013 relating to the control of tobacco, Official Gazette of the Republic of Rwanda, via RwandaLII.
Photo: Emphysema, centrilobular, Yale Rosen, CC BY-SA 2.0, via Wikimedia Commons.
Video: Coronary heart disease, clogged arteries and atherosclerosis, British Heart Foundation, embedded from YouTube.

Figures 7 and 8, the tables, the worked examples and the practice questions are by Ganzaa. The carboxyhaemoglobin value in Worked example 2 and the spirometry results in Worked example 1 are illustrative.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1One habit, many organs

2What is in tobacco smoke?

3Damage to the gas-exchange system

4Lung cancer

5Nicotine and carbon monoxide in the cardiovascular system

6From smoking to a heart attack

7It is never too late to stop