9.1.19 · Gas Exchange

The Human Gas Exchange System — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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9.1.1 The Human Gas Exchange System — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure of the human gas exchange system, how ventilation works and how the airways defend the lungs.Size — humans are large, active organisms with a small surface are…

Limits of the skin
Features of an efficient surface
Air pathway
Bronchioles
Pleural membranes
Intercostal muscles

Part 1 of 8Why a gas exchange system is needed

Why a gas exchange system is needed

Size

Size humans are large, active organisms with a small surface area to volume ratio.

Limits of the skin

Limits of the skin oxygen cannot diffuse from the air to every cell quickly enough, so a specialised surface is needed.

Jobs

Jobs it supplies oxygen for aerobic respiration and removes carbon dioxide.

Features of an efficient surface

Features of an efficient surface a large surface area, thinness, moisture so gases dissolve,
- and a steep concentration gradient from ventilation and blood flow.
- (short diffusion distance)

Part 2 of 8Gross structure

1Air pathway

Air pathway air enters through the nose or mouth and travels through a branching series of tubes.

2Trachea

Trachea a wide tube about 10 to 12 cm long, held open by C-shaped rings of cartilage.

3Bronchi

Bronchi the trachea divides into two, one entering each lung.

4Bronchioles

Bronchioles each bronchus divides repeatedly into smaller tubes with no cartilage.

5Alveoli

Alveoli the finest bronchioles end in clusters of tiny air sacs where gas exchange takes place.

Part 3 of 8Thorax and lungs

Position

Position the lungs lie inside the thorax, protected by the rib cage.

Pleural membranes

Pleural membranes each lung is covered by a thin membrane, and the space between the layers holds pleural fluid.
The fluid lubricates movement.
It lets the lungs follow the movements of the chest wall.

Diaphragm

Diaphragm a dome-shaped sheet of muscle forming the floor of the thorax.

Intercostal muscles

Intercostal muscles external and internal intercostals lie between the ribs and work as an antagonistic pair.

Part 4 of 8Ventilation

Feature	Inspiration	Expiration (at rest)
Diaphragm	Contracts and flattens	Relaxes and domes upwards
External intercostals	Contract, raising ribs up and out	Relax
Thorax volume	Increases	Decreases
Lung pressure	Falls below atmospheric	Rises above atmospheric
Air movement	Into the lungs	Out of the lungs

Purpose

Purpose ventilation keeps alveolar oxygen high and carbon dioxide low compared with blood.

Quiet expiration

Quiet expiration it is passive: the ribs fall under gravity and the elastic fibres in the alveolar walls recoil.

Forced expiration

Forced expiration in exercise the internal intercostals pull the ribs down and in, and the abdominal muscles push the diaphragm up.

Part 5 of 8Measuring lung function

Measuring lung function

Spirometer

Spirometer it records volumes of air breathed.

Tidal volume

Tidal volume air taken in with each normal breath, about 500 cm³ at rest.

Vital capacity

Vital capacity the maximum volume exhaled after a maximum inhalation.

Residual volume

Residual volume air that always remains because airways and alveoli cannot collapse completely.

Breathing rate

Breathing rate breaths per minute.

Pulmonary ventilation

Pulmonary ventilation tidal volume × breathing rate.

Part 6 of 8Protection of the lungs

Protection of the lungs

Inhaled particles

Inhaled particles each day a person breathes in dust, pollen, spores, bacteria and viruses.

Defences

Defences nasal hairs filter large particles, mucus traps smaller ones and cilia carry the mucus away.

Phagocytes

Phagocytes white blood cells patrol the alveoli and engulf particles that get through.

Part 7 of 8Related disorders

Asthma

Asthma an allergic or irritant trigger makes bronchiole smooth muscle contract and the lining secrete extra mucus.
The airway narrows and airflow falls, so the person wheezes and finds it hard to breathe out.

Chronic bronchitis

Chronic bronchitis smoking causes inflamed airways, excess mucus and damaged cilia, giving a persistent cough.

Emphysema

Emphysema alveoli break down and merge, giving a smaller surface area and lung tissue that has lost elasticity.

Part 8 of 8More from the specification

1Submucosal bronchial mucous glands

Submucosal bronchial mucous glands respiratory epithelium contains goblet cells and mucous glands secreting viscous mucus that traps inhaled airborne particulates.

2Ciliated Epithelium and Mucus Production

The trachea and bronchi are lined with pseudostratified ciliated epithelium containing mucus-secreting goblet cells that trap inhaled dust particles and pathogenic microorganisms.

3Alveolar Squamous Epithelium and Capillaries

In lung micrographs or electron micrographs, alveoli are seen to be lined with extremely thin squamous epithelium surrounded by a dense meshwork of blood capillaries, providing a minimal diffusion distance for gas exchange.

4Recognising Airway Tissues in Histology

On a stained photomicrograph, histological identification allows distinguishing between the cartilage rings, smooth muscle, elastic fibres,
and goblet cells lining different tiers of the mammalian respiratory tract.

5Electron micrograph

A photograph taken through an electron microscope.

6Mucous gland

A gland that secretes mucus into the airway.

7Epithelium

A tissue made of closely packed cells covering surfaces or lining cavities.

8Capillary

Vessel one cell thick where exchange takes place.

9Photomicrograph

A photograph taken through a light microscope.

10Micrograph

A photograph taken through a microscope.

11Microorganism

A living organism so small that it can only be seen clearly with a microscope.

12Ciliated epithelium

The tissue with cilia that lines the tubes leading to the lungs.

13Goblet cells

Cells in the airway lining that secrete mucus.

14Squamous epithelium

A single layer of very flat cells forming the alveolar wall.

Key terms

Term	Meaning
Ventilation	Movement of air into and out of the lungs.
Tidal volume	Volume of air in a normal breath, about 500 cm^{3} at rest.
Vital capacity	Maximum volume that can be exhaled after a maximum inhalation.
Residual volume	Air that always remains in the lungs.
Pleural membrane	Thin membrane covering the lung that contains lubricating fluid.
Pulmonary ventilation	Tidal volume multiplied by breathing rate.

Avoid these mistakes

Saying the lungs contain muscle that pulls air in — the diaphragm and intercostals change thorax volume.

Saying quiet expiration is an active process — it is passive, helped by elastic recoil.

Saying air rushes in because the lung pressure rises — pressure falls below atmospheric on inspiration.

Confusing the windpipe (trachea) with a bronchus — the trachea divides into two bronchi.

Last updated: 15 October 2026