What you'll learn
Gas exchange in humans is a fundamental topic in CIE IGCSE Biology that examines how oxygen enters the bloodstream and carbon dioxide is removed. This topic appears consistently across Paper 2 (core and extended) and Paper 4 (alternative to practical), with questions testing your knowledge of respiratory system structure, the mechanism of breathing, and adaptations for efficient gas exchange. Understanding the relationship between structure and function at the alveolar level is particularly important for scoring well in extended response questions.
Key terms and definitions
Gas exchange — the process by which oxygen diffuses from the air into the blood and carbon dioxide diffuses from the blood into the air
Alveoli — tiny air sacs in the lungs where gas exchange occurs; singular: alveolus
Ventilation — the movement of air into and out of the lungs, commonly called breathing
Trachea — the windpipe; a tube reinforced with C-shaped cartilage rings that carries air from the throat to the bronchi
Bronchi — two tubes (singular: bronchus) that branch from the trachea and lead to each lung
Bronchioles — smaller tubes that branch from the bronchi and end in alveoli
Diffusion gradient — the difference in concentration of a substance between two areas, which drives the movement of molecules from high to low concentration
Surface area to volume ratio — the relationship between the area available for exchange and the volume of tissue requiring supply; humans need specialized gas exchange surfaces because their surface area to volume ratio is too low for simple diffusion across body surfaces
Core concepts
Structure of the human respiratory system
The human respiratory system consists of a series of tubes that deliver air to the gas exchange surface. Understanding the sequence and function of each structure is essential for CIE IGCSE Biology examinations.
Air pathway sequence:
- Air enters through the nose or mouth
- Passes through the pharynx (throat) and larynx (voice box)
- Travels down the trachea (windpipe)
- Divides into left and right bronchi (one for each lung)
- Further subdivides into smaller bronchioles
- Reaches the alveoli where gas exchange occurs
The trachea contains C-shaped rings of cartilage that prevent collapse during breathing but allow flexibility for swallowing. The bronchi also contain cartilage, but the bronchioles do not—they are held open by the surrounding lung tissue.
The lungs are located in the thorax (chest cavity) and are protected by the ribcage. They are separated from the abdomen by the diaphragm, a sheet of muscle crucial for breathing. Each lung is surrounded by two pleural membranes that contain a thin layer of fluid, reducing friction during breathing movements.
The mechanism of breathing (ventilation)
Breathing involves two phases: inspiration (inhalation) and expiration (exhalation). Both are brought about by pressure changes in the thorax. CIE IGCSE Biology requires detailed knowledge of the muscular and pressure changes involved.
Inspiration (breathing in):
- External intercostal muscles contract
- Ribs move upward and outward
- Diaphragm contracts and flattens (moves downward)
- Volume of the thorax increases
- Pressure inside the thorax decreases (below atmospheric pressure)
- Air moves into the lungs down the pressure gradient
Expiration (breathing out):
- External intercostal muscles relax
- Ribs move downward and inward
- Diaphragm relaxes and domes upward
- Volume of the thorax decreases
- Pressure inside the thorax increases (above atmospheric pressure)
- Air moves out of the lungs down the pressure gradient
At rest, expiration is mostly a passive process relying on the elastic recoil of the lungs and relaxation of muscles. During exercise, internal intercostal muscles contract actively to force air out more rapidly.
The bell jar model is often used to demonstrate breathing mechanics: the bell jar represents the ribcage, the rubber sheet represents the diaphragm, and the balloon represents the lungs. Pulling the rubber sheet down increases volume and decreases pressure, inflating the balloon—just as the diaphragm contracting causes inspiration.
Structure and adaptations of alveoli
The alveoli are the functional units where gas exchange occurs. A typical human lung contains approximately 300-500 million alveoli, providing an enormous surface area (approximately 70 m²) for gas exchange.
Structural features of alveoli:
- Walls are one cell thick (made of squamous epithelium)
- Surrounded by a dense network of capillaries with walls also one cell thick
- Coated internally with a thin layer of moisture to dissolve gases
- Elastic tissue allows them to stretch during inspiration and recoil during expiration
Adaptations for efficient gas exchange:
- Large surface area — millions of alveoli provide extensive area for diffusion
- Thin barrier — the alveolar wall and capillary wall together are only two cells thick (approximately 0.5 μm), minimizing diffusion distance
- Rich blood supply — each alveolus is surrounded by capillaries that continuously bring deoxygenated blood and remove oxygenated blood, maintaining concentration gradients
- Ventilation — breathing constantly refreshes the air in the alveoli, keeping oxygen concentration high and carbon dioxide concentration low
- Moist surface — gases must dissolve before diffusing across membranes
These adaptations ensure rapid diffusion rates according to Fick's Law, which states that the rate of diffusion is proportional to surface area and concentration gradient, and inversely proportional to diffusion distance.
The process of gas exchange
Gas exchange occurs by diffusion across the respiratory surface. This is a passive process requiring no energy input from cells.
Oxygen exchange:
- Oxygen concentration is higher in alveolar air (~21% in inhaled air, ~14% in alveoli) than in deoxygenated blood arriving from tissues
- Oxygen dissolves in the moisture lining the alveolus
- Oxygen diffuses across the alveolar epithelium, through the tissue fluid, and across the capillary endothelium
- Oxygen enters red blood cells and binds to haemoglobin to form oxyhaemoglobin
- Oxygenated blood travels via pulmonary veins to the left side of the heart, then to body tissues
Carbon dioxide exchange:
- Carbon dioxide concentration is higher in deoxygenated blood (returning from respiring tissues) than in alveolar air
- Carbon dioxide diffuses from blood plasma across the capillary wall and alveolar wall
- Carbon dioxide dissolves in the moisture lining and diffuses into the alveolar air space
- Carbon dioxide is removed from alveoli during expiration
The diffusion gradient for both gases is maintained by continuous ventilation of the lungs and circulation of blood. If either stopped, the gradients would disappear and gas exchange would cease.
Comparison of inspired and expired air
CIE IGCSE Biology students must know the approximate composition of inhaled and exhaled air. This is frequently tested in data handling questions.
| Gas | Inspired air (%) | Expired air (%) |
|---|---|---|
| Oxygen | 21 | 16 |
| Carbon dioxide | 0.04 | 4 |
| Nitrogen | 78 | 78 |
| Water vapour | Variable (low) | High (saturated) |
Key observations:
- Oxygen decreases by approximately 5% (some is absorbed into blood)
- Carbon dioxide increases by approximately 4% (added from blood)
- Nitrogen remains constant (it is not used by the body)
- Water vapour increases because exhaled air is saturated with moisture from the respiratory surfaces
- Exhaled air is warmer than inhaled air due to body temperature
The presence of oxygen in exhaled air (16%) means expired air can still support life—this is why mouth-to-mouth resuscitation works.
Effect of exercise on breathing
During exercise, muscles respire more rapidly, consuming more oxygen and producing more carbon dioxide. The respiratory system responds to meet these increased demands.
Changes during exercise:
- Breathing rate increases (more breaths per minute)
- Depth of breathing increases (tidal volume—the volume of air in each breath—increases)
- Both changes increase ventilation rate (volume of air breathed per minute)
- Faster, deeper breathing maintains steep diffusion gradients at the alveoli
- Increased heart rate ensures oxygen is delivered to muscles and carbon dioxide is removed more rapidly
The increase in breathing rate is triggered by receptors in the brain that detect rising carbon dioxide levels in the blood. Carbon dioxide dissolved in blood plasma forms carbonic acid, lowering blood pH—this is detected by chemoreceptors that signal the breathing control centre in the medulla oblongata of the brain.
Worked examples
Example 1: Alveolar adaptations (4 marks)
Question: Explain how the structure of alveoli is adapted for efficient gas exchange.
Answer:
- Large surface area / many alveoli present, which increases the rate of diffusion (1)
- Walls are one cell thick / thin barrier, which decreases diffusion distance (1)
- Good blood supply / surrounded by capillaries, which maintains concentration gradient (1)
- Moist surface / lined with moisture, which allows gases to dissolve (1)
Examiner tip: Questions asking for adaptations typically award one mark per distinct point. Use the format "feature + function" to ensure you explain how each adaptation helps gas exchange.
Example 2: Breathing mechanism (6 marks)
Question: Describe what happens to cause inspiration (breathing in).
Answer:
- External intercostal muscles contract (1)
- Ribs move up and out (1)
- Diaphragm contracts (1)
- Diaphragm flattens / moves down (1)
- Volume of thorax increases (1)
- Pressure inside thorax decreases / becomes lower than atmospheric pressure (1)
- Air moves into lungs down pressure gradient (1)
(Award maximum 6 marks from the above points)
Examiner tip: Mechanism questions require sequences. Structure your answer logically: muscle contractions → rib/diaphragm movement → volume change → pressure change → air movement.
Example 3: Data interpretation (3 marks)
Question: A student measured the volume of oxygen and carbon dioxide in inspired and expired air. Explain why the percentage of oxygen in expired air is lower than in inspired air.
Answer:
- Some oxygen diffuses from alveoli into blood (1)
- Oxygen is used in (aerobic) respiration in body cells / tissues / muscles (1)
- Less oxygen remains to be breathed out (1)
Examiner tip: Data questions often test understanding of underlying processes, not just recall of percentages.
Common mistakes and how to avoid them
Mistake: Stating that "air is forced into the lungs" during inspiration. Correction: Air moves into lungs passively down a pressure gradient; it is not actively forced in. The diaphragm and intercostal muscles create lower pressure, and atmospheric pressure pushes air in.
Mistake: Confusing breathing with respiration. Correction: Breathing (ventilation) is the physical movement of air into and out of lungs. Respiration is the chemical process in cells that releases energy from glucose. These are not interchangeable terms.
Mistake: Writing that oxygen is "absorbed" or "taken in" without mentioning diffusion. Correction: Always state that oxygen diffuses from alveoli into blood. Diffusion is the specific mechanism and must be named for marks in explain questions.
Mistake: Describing expired air as having "no oxygen." Correction: Expired air still contains approximately 16% oxygen (compared to 21% in inspired air). Only about 5% of oxygen is absorbed per breath.
Mistake: Stating that carbon dioxide "is breathed out" without explaining the exchange process. Correction: Carbon dioxide diffuses from blood into alveoli because of a concentration gradient, then is removed by expiration. The diffusion step must be included.
Mistake: Writing that alveoli are surrounded by blood. Correction: Alveoli are surrounded by capillaries (blood vessels). Blood is contained within capillaries; it does not directly contact alveolar walls.
Exam technique for Gas exchange in humans
"Describe" and "Explain" questions: CIE mark schemes award marks for distinct points. In describe questions, state what happens. In explain questions, give reasons or mechanisms (often requiring the word "because" or stating a consequence). A 4-mark explain question typically requires four separate points.
Use correct anatomical terms: Words like alveoli (not "air sacs"), trachea (not "windpipe" unless defining it), and intercostal muscles earn marks. Vague terms like "breathing tubes" will not.
Structure answers about breathing mechanics: Follow the sequence: muscle action → bone movement → volume change → pressure change → air movement. This ensures you include all mark scheme points.
Adaptation questions: Always link structure to function. State the feature, then explicitly explain how it improves gas exchange (increases surface area, decreases distance, maintains gradient).
Quick revision summary
Gas exchange in humans occurs in the alveoli, tiny air sacs with walls one cell thick surrounded by capillaries. Oxygen diffuses from alveolar air into blood; carbon dioxide diffuses from blood into alveolar air. Breathing is controlled by intercostal muscles and the diaphragm, which alter thoracic volume and pressure. Inspiration involves muscle contraction, increased volume, and decreased pressure; expiration involves muscle relaxation, decreased volume, and increased pressure. Alveoli are adapted with large surface area, thin walls, rich blood supply, and moist surfaces. Expired air contains less oxygen and more carbon dioxide than inspired air.