Human Gas Exchange & Smoking

IGCSE Edexcel Biology
2.46–2.49 Breathing system, alveoli, gas exchange and effects of smoking
Key Concepts: Gas exchange occurs in the alveoli of the lungs. Oxygen diffuses into the blood and carbon dioxide diffuses out. The alveoli are adapted to maximise efficient gas exchange. Smoking damages the gas exchange system.

Section A — The Breathing System

1. Describe the route taken by air from the mouth/nose to the alveoli. Name each structure in order. [4]
2. Describe what happens to the ribcage and diaphragm during inhalation (breathing in). [3]
3. Complete the table showing the composition of inhaled and exhaled air. [4]
Gas Inhaled air (%) Exhaled air (%)
Oxygen
Carbon dioxide
Nitrogen
Water vapour

Section B — Alveoli and Gas Exchange

4. State four features of alveoli that make them efficient for gas exchange. [4]
5. Describe the process of gas exchange in the alveoli, including the direction of diffusion of each gas. [4]
6. Explain why maintaining a high concentration gradient is important for efficient gas exchange. [2]

Section C — Effects of Smoking

7. State the harmful effects of each substance found in cigarette smoke. [6]
SubstanceHarmful effect(s)
Nicotine
Tar
Carbon monoxide
8. Explain how smoking leads to emphysema and how this affects gas exchange. [4]
9. Explain why carbon monoxide in cigarette smoke reduces the oxygen-carrying capacity of the blood. [2]

Total marks: 33

Mark Scheme

1. Mouth/nose → trachea → bronchi → bronchioles → alveoli [4 — 1 per structure in correct order]
2. Intercostal muscles contract, ribcage moves up and out [1]; diaphragm contracts and flattens [1]; volume of thorax increases, pressure decreases, air moves in [1] [3]
3. O₂: ~21% inhaled / ~16% exhaled; CO₂: ~0.04% inhaled / ~4% exhaled; N₂: ~79% both; Water vapour: low inhaled / high (saturated) exhaled [4 — 1 per gas, accept approx. values]
4. Any four: large total surface area (millions of alveoli); thin walls (one cell thick) = short diffusion distance; moist surface for gas to dissolve; rich blood supply (capillaries) maintains concentration gradient [4]
5. Oxygen diffuses from the alveolus (high [O₂]) into the blood capillary (low [O₂]) [2]; CO₂ diffuses from the blood capillary (high [CO₂]) into the alveolus (low [CO₂]) [2] [4]
6. A steep concentration gradient means diffusion occurs faster [1]; blood flow and breathing continuously remove products and replenish reactants, maintaining the gradient [1] [2]
7. Nicotine: addictive; increases heart rate/blood pressure; narrows arteries; Tar: contains carcinogens → lung cancer; coats cilia/airways → reduces ciliary action → mucus accumulates → infections; Carbon monoxide: binds irreversibly to haemoglobin; reduces O₂-carrying capacity of blood [6 — 2 per row]
8. Tar irritates the airways; cilia are damaged/destroyed so mucus and bacteria accumulate [1]; chronic infection causes inflammation [1]; alveoli walls break down and merge, reducing surface area [1]; less surface area means less gas exchange, so the person becomes short of breath [1] [4]
9. Carbon monoxide binds to haemoglobin (forming carboxyhaemoglobin) with a greater affinity than oxygen [1]; haemoglobin cannot carry as much oxygen, so less oxygen is delivered to cells [1] [2]