Gas Exchange in Humans
Summary: The human respiratory system (nasal cavity, trachea, bronchi, bronchioles, alveoli) delivers oxygen to the blood and removes carbon dioxide. The trachea is supported by cartilage rings and lined with cilia and mucus to trap particles. Alveoli are adapted for efficient gas exchange with a very large surface area, one-cell-thick walls, dense capillary network, moist surface, and good blood supply. Ventilation involves the intercostal muscles and diaphragm: inhalation is active (muscles contract), exhalation at rest is passive (elastic recoil). Inhaled vs exhaled air differ in O2 (21% vs 16%), CO2 (0.04% vs 4%), and water vapour. Tags: igcse biology gas-exchange respiratory-system alveoli ventilation breathing Created: 2026-07-16 Last Updated: 2026-07-16
1. The Respiratory System
The respiratory system is responsible for delivering oxygen to the blood and removing carbon dioxide from the blood.
| Structure | Description and Function |
|---|---|
| Nasal cavity | Air enters. Lined with hair and mucus — filters, warms, and moistens the air before it reaches the delicate lung tissue |
| Pharynx | Passage at the back of the throat shared by the respiratory and digestive systems |
| Larynx (voice box) | Contains vocal cords; produces sound |
| Trachea (windpipe) | Tube carrying air from pharynx to bronchi. Supported by C-shaped cartilage rings that keep the airway open and prevent collapse during inhalation. Lined with cilia and goblet cells that secrete mucus |
| Bronchi (singular: bronchus) | Two tubes branching from the trachea — one leading to each lung. Also supported by cartilage rings and lined with cilia and mucus |
| Bronchioles | Smaller branches from the bronchi that spread throughout the lungs. No cartilage — walls contain smooth muscle that can constrict or dilate |
| Alveoli (singular: alveolus) | Microscopic air sacs at the ends of bronchioles. The site of gas exchange — oxygen diffuses from air into blood; carbon dioxide diffuses from blood into air. Millions of alveoli in each lung |
| Ribs | Bony cage protecting the lungs and heart |
| Intercostal muscles | Muscles between the ribs. External intercostals contract during inhalation (lift ribs up and out). Internal intercostals contract during forced exhalation |
| Diaphragm | Large, dome-shaped sheet of muscle separating the thoracic (chest) cavity from the abdominal cavity. Flattens (contracts) during inhalation; relaxes and domes upward during exhalation |
2. The Trachea: Cartilage, Cilia, and Mucus
Cartilage rings (C-shaped):
- Keep the trachea and bronchi permanently open so air can always pass through
- Prevent the airway from collapsing during inhalation when the pressure inside the chest drops
- The rings are C-shaped (not complete circles) — the gap at the back allows the oesophagus (behind the trachea) to expand when food is swallowed
Cilia and mucus:
- Goblet cells in the lining of the trachea and bronchi secrete sticky mucus
- Mucus traps dust, pollen, bacteria, and other particles from the inhaled air
- Cilia are microscopic hair-like projections on the surface of epithelial cells. They beat in a coordinated, rhythmic way, sweeping the mucus upward toward the throat
- The mucus and trapped particles are swallowed (destroyed by stomach acid) or coughed out
Smoking damages cilia — they stop beating effectively → mucus and trapped pathogens accumulate in the airways → increased risk of respiratory infections and “smoker’s cough”.
3. Alveoli: Adaptations for Gas Exchange
The alveoli are the functional units of the lungs where gas exchange occurs by diffusion.
| Adaptation | How It Promotes Efficient Gas Exchange |
|---|---|
| Very large surface area | Hundreds of millions of alveoli in each lung. If flattened out, the total surface area would be ~70 m2 (about the size of a tennis court). Enormous area for gas exchange |
| One-cell-thick walls (thin squamous epithelium) | The alveolar wall is composed of a single layer of flattened epithelial cells, providing a very short diffusion distance (approximately 0.5 um) for O2 and CO2 |
| Surrounded by a dense network of blood capillaries | Each alveolus is wrapped in capillaries. The capillary wall is also one cell thick. The combined distance from alveolar air → blood is only 2 cell layers. Blood constantly flows through, maintaining a steep concentration gradient |
| Moist surface | The inner surface of the alveoli is lined with a thin film of water/fluid (with surfactant). Gases must dissolve in this fluid before they can diffuse across — moist surface facilitates this |
| Good blood supply | The heart pumps all the blood through the lungs with each circuit (pulmonary circulation). This maintains a steep concentration gradient — oxygen is constantly being carried away, CO2 is constantly being delivered |
| Highly elastic | Alveoli stretch during inhalation and recoil during exhalation. This elastic recoil helps push air out during exhalation |
Gas exchange at the alveoli:
- Oxygen: High concentration in alveolar air → diffuses across alveolar wall → across capillary wall → into red blood cells → binds to haemoglobin (forming oxyhaemoglobin)
- Carbon dioxide: High concentration in blood plasma (dissolved, or from hydrogencarbonate ions) → diffuses across capillary wall → across alveolar wall → into alveolar air → exhaled
4. Ventilation (Breathing)
Ventilation is the movement of air into and out of the lungs. It is driven by pressure changes in the thoracic cavity.
Inhalation (Inspiration) — Active Process
| Structure | Action |
|---|---|
| External intercostal muscles | Contract — pull the ribs upward and outward |
| Diaphragm | Contracts — flattens and moves downward |
| Volume of thoracic cavity | Increases |
| Pressure inside thoracic cavity | Decreases (below atmospheric pressure) |
| Air movement | Air rushes into the lungs (from higher pressure outside to lower pressure inside) down the pressure gradient |
Exhalation (Expiration) — Passive at Rest
| Structure | Action (at rest) |
|---|---|
| External intercostal muscles | Relax — ribs move downward and inward |
| Diaphragm | Relaxes — returns to its domed shape |
| Volume of thoracic cavity | Decreases (elastic recoil of lungs and rib cage) |
| Pressure inside thoracic cavity | Increases (above atmospheric pressure) |
| Air movement | Air is pushed out of the lungs (from higher pressure inside to lower pressure outside) |
Forced Exhalation (e.g. during exercise/coughing) — Active
During forced exhalation:
- Internal intercostal muscles contract, pulling ribs further downward and inward
- Abdominal muscles contract, pushing the diaphragm upward more forcefully
- This produces a stronger, faster exhalation
5. Composition of Inhaled vs Exhaled Air
| Gas / Component | Inhaled Air (Atmospheric) | Exhaled Air | Reason for Change |
|---|---|---|---|
| Oxygen (O2) | ~21% | ~16% | O2 is absorbed into the blood at the alveoli for use in aerobic respiration |
| Carbon dioxide (CO2) | ~0.04% | ~4% (100x more) | CO2 is a waste product of respiration in body cells; it diffuses from blood into alveoli |
| Nitrogen (N2) | ~78% | ~78% (unchanged) | Nitrogen is an inert gas — the body does not use or produce it |
| Water vapour | Variable (depends on humidity) | Saturated (more) | Water evaporates from the moist lining of the alveoli and airways |
| Temperature | Variable | Warmer (~37 degrees C) | Air is warmed as it passes through the respiratory passages close to body temperature |
6. Testing for Carbon Dioxide
Limewater test:
- Limewater (calcium hydroxide solution, Ca(OH)2) is a colourless solution
- When CO2 is bubbled through limewater, it reacts to form calcium carbonate (CaCO3) — an insoluble white precipitate
- Result: Limewater turns milky/cloudy/white → carbon dioxide is present
- This test can be used to compare CO2 levels in inhaled vs exhaled air by breathing through limewater using a delivery tube
Chemical equation (for reference): Ca(OH)2 + CO2 → CaCO3 + H2O
Related Notes
- Respiration — O2 is used in aerobic respiration; CO2 is a waste product
- Transport in Animals — Blood transports O2 (bound to haemoglobin in RBCs) and CO2 (dissolved in plasma)
- Excretion in Humans — CO2 is an excretory product removed by the lungs
- Diseases and Immunity — Mucus and cilia as body defences against pathogens
- IGCSE-Bio-Index — Full IGCSE Biology index
Sources
- BBC Bitesize GCSE Biology — Respiratory system / Gas exchange, BBC (free educational resource)
- OpenStax Biology 2e — Chapter 39: The Respiratory System, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Topic 11: Gas Exchange in Humans, Cambridge Assessment International Education
- CK-12 Biology for High School — Respiratory System chapter, CK-12 Foundation (free, CC BY-NC 3.0)
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Gas exchange happens in the trachea or bronchi” | Gas exchange only occurs in the alveoli — the trachea, bronchi, and bronchioles are airways that conduct air but have walls too thick for gas exchange |
| ”Inhalation happens because air pushes the ribs out” | Inhalation is active — the intercostal muscles and diaphragm contract, expanding the thorax, which reduces pressure and draws air in. Air does not push the ribs out |
| ”Exhalation is always active/muscular” | At rest, exhalation is passive — muscles relax and the elastic recoil of the lungs/rib cage pushes air out. Only forced exhalation (e.g. coughing, exercise) is active |
| ”We breathe in only oxygen, breathe out only carbon dioxide” | Air is ~78% nitrogen, ~21% oxygen, ~0.04% CO2. Exhaled air still contains ~16% oxygen and ~4% CO2. Nitrogen is unchanged |
| ”The diaphragm moves upward during inhalation” | During inhalation, the diaphragm contracts and flattens (moves downward). It moves upward (domes) when it relaxes during exhalation |
| ”Limewater turning clear means CO2 is present” | Limewater turns milky/cloudy in the presence of CO2. It starts as a clear solution |