B6.1B6. Respiration and Gas Exchange

Gas exchange in humans — Combined Science 0653 - Biology Revision Notes & Topical Questions

10 min read 22 parts Lesson 2 of 8 1 quiz

📚 Combined Science 0653 - Biology Hub / Topical Revision

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Lesson summary

B6.1 Gas exchange in humans — COMBINED SCIENCE notes: Identify the parts of the breathing system, state the features of gas exchange surfaces and compare inspired and expired air.StructureDescription and functionLungsOrgans in the thorax in which gas exchange happensRibsBone stru…

Large surface area
Thin walls
Good ventilation with air
Good blood supply
Moist lining
Carbon dioxide and water vapour

Part 1 of 9Parts of the breathing system

Structure	Description and function
Lungs	Organs in the thorax in which gas exchange happens
Ribs	Bone structure that protects internal organs such as the lungs
Intercostal muscles	Muscles between the ribs that control their movement and cause inhalation and exhalation
Diaphragm	Sheet of connective tissue and muscle at the bottom of the thorax that changes its volume
Larynx	The voice box, where air passing across makes sounds
Trachea	The windpipe connecting the mouth and nose to the bronchi
Bronchi	Large tubes branching off the trachea, one for each lung
Bronchioles	Smaller tubes into which the bronchi split in the lungs
Alveoli	Tiny air sacs where gas exchange takes place, with associated capillaries

Part 2 of 9Path of air

Air enters the nose or mouth.

It passes down the trachea.

It enters the bronchi.

It moves into the bronchioles.

It reaches the alveoli.

Part 3 of 9Features of gas exchange surfaces

Features of gas exchange surfaces

Large surface area

Large surface area it allows faster diffusion of gases across the surface.

Thin walls

Thin walls they keep the diffusion distance short.

Good ventilation with air

Good ventilation with air it maintains the diffusion gradient.

Good blood supply

Good blood supply it maintains a high concentration gradient so diffusion is faster.

Part 4 of 9The alveolus

Alveolus

Alveolus the gas exchange surface in humans.

Walls

Walls the walls of the alveoli and capillaries are one cell thick.

Moist lining

Moist lining gases dissolve in the moisture before they diffuse.

Part 5 of 9Gas exchange in the alveoli

1Oxygen

Oxygen it diffuses from the alveolar air into the blood.

2Carbon dioxide and water vapour

Carbon dioxide and water vapour they diffuse from the blood into the alveolar air.

Part 6 of 9Inspired and expired air

Gas	Inspired air (%)	Expired air (%)
Oxygen	21	16
Carbon dioxide	0.04	4
Water vapour	0.2 to 4	1 to 5
Nitrogen	78	78

Part 7 of 9Investigating inspired and expired air

Investigating inspired and expired air

Breathe in through boiling tube A, which contains limewater.

Breathe out through boiling tube B, which contains limewater.

Compare the limewater in the two tubes.

Result
- the limewater in A stays clear and in B turns cloudy, so expired air has more carbon dioxide.

Part 8 of 9Investigating exercise and breathing

Count the breaths in 1 minute at rest.

Measure the average chest expansion over five breaths with a tape measure round the chest.

Exercise for at least 3 minutes.

Immediately count the breaths in one minute and measure the chest expansion again.

Result
- breathing rate and chest expansion are both greater after exercise.

Part 9 of 9More from the specification

1Gas Exchange Surface Adaptations

Human alveoli provide a tremendous total surface area, an ultra-thin barrier,
- and a dense capillary meshwork maintaining steep concentration gradients.
- (one cell thick squamous epithelium)

2Mechanism of Inhalation and Thoracic Pressure

Contraction of external intercostal muscles raises the ribcage while the diaphragm flattens downward, enlarging thoracic volume and dropping intrapleural pressure below atmospheric pressure to draw air into the lungs.

3Mucus Clearance Mechanism

Goblet cells continuously secrete protective mucus to trap airborne contaminants,
- and coordinated beating of ciliated epithelial cells sweeps the mucus escalator upward towards the larynx.

4Activity

The rate at which the nuclei in a sample decay.

5Pressure

Force acting on a unit area.

6Respiration

Release of energy from food inside cells.

Key terms

Term	Meaning
Alveolus	A tiny air sac in the lung where gas exchange takes place.
Trachea	The windpipe carrying air to the bronchi.
Bronchus	One of two large tubes branching from the trachea.
Bronchiole	A small tube carrying air to the alveoli.
Intercostal muscle	A muscle between the ribs that moves the rib cage.
Diaphragm	The sheet of muscle that changes the volume of the thorax.
Gas exchange surface	A surface across which gases diffuse.
Limewater	A solution that turns cloudy with carbon dioxide.

Avoid these mistakes

Saying nitrogen is used in gas exchange — its percentage does not change.

Saying expired air has no oxygen — about 16% remains.

Saying exhaled air is pure carbon dioxide — it contains about 4%.

Confusing the trachea with the bronchi — the trachea divides into two bronchi.

Last updated: 14 October 2026