5.8Chapter 5: Respiration and the Respiratory System

The structure of the alveoli — Alhussan Biology - Stage 8 Revision Notes & Topical Questions

5 min read 6 parts Lesson 8 of 12 2 quizzes

📚 Alhussan Biology - Stage 8 Hub / Topical Revision

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Lesson summary

Biology 5.8 The structure of the alveoli — Adaptations for Gas Exchange — Alhussan International Academy — Biology (Stages 7–9) notes: Explain how the structure of the alveoli and their blood supply are adapted for efficient gas exchange and how COPD damages them.Alveoli — micros…

Lung structure
Carbon dioxide
Steep gradients
Maintaining the gradients
If breathing slows or blood flow falls
COPD (chronic obstructive pulmonary disease)

Part 1 of 4What the alveoli do

What the alveoli do

Alveoli

Alveoli microscopic air sacs at the ends of the bronchioles, with hundreds of millions in each lung.

Lung structure

Lung structure the lungs are more like a sponge made of millions of alveoli than a simple balloon like the bell jar model.

Function

Function they are the site of gas exchange, where oxygen enters the blood and carbon dioxide leaves it.

Principle

Principle the same design features are found in other exchange surfaces such as villi and fish gills.
- (large surface area, thin wall, good supply, steep gradient)

Part 2 of 4Adaptations of the alveoli

Adaptation	How it helps gas exchange
Huge number of alveoli	A very large surface area, around 70 to 100 m² in total
Wall one cell thick	A short diffusion distance, so gases cross quickly
Capillary wall one cell thick, close to the alveolus	An even shorter distance between air and blood
Rich network of capillaries around each alveolus	Blood carries oxygen away and brings carbon dioxide, keeping the gradient steep
Moist lining	Gases dissolve before diffusing across
Constant ventilation by breathing	Keeps oxygen high and carbon dioxide low in the alveolar air

Part 3 of 4Gas exchange and concentration gradients

1Oxygen

Oxygen it moves from the air in the alveoli into the blood in the capillaries by diffusion down a concentration gradient.

2Carbon dioxide

Carbon dioxide it moves from the blood into the alveolar air and is breathed out.

3Steep gradients

Steep gradients they exist between the oxygen in inhaled air and the blood,
- and between carbon dioxide in the blood and the air in the lungs,
- so diffusion is quick.

4Maintaining the gradients

Maintaining the gradients breathing movements bring fresh air in and take stale air out, and blood flow through the capillaries carries gases away.
Blood arriving at the alveolus is low in oxygen because cells have used it, and oxygen then combines with haemoglobin, which keeps the gradient steep.
Blood arriving is high in carbon dioxide and alveolar air is low, because breathing out removes it.

5If breathing slows or blood flow falls

If breathing slows or blood flow falls the gradients are less steep and less gas is exchanged,
- which is why exercise makes you breathe and your heart beat faster.

Part 4 of 4COPD and the alveoli

1COPD (chronic obstructive pulmonary disease)

COPD (chronic obstructive pulmonary disease) a lung disease that affects the structure of the alveoli.

2Damage

Damage the thin walls of the alveoli break down, leaving fewer, bigger air sacs instead of many tiny ones.
The big sacs have a smaller surface area, and gases have farther to move.

3Effects

Effects less gas exchange takes place, so body cells do not get enough oxygen and carbon dioxide builds up in the blood.

4Symptoms

Symptoms people feel breathless very easily, cannot walk far and find it difficult to run or climb stairs.
The body tries to get more air into the lungs.

5Other damage

Other damage in emphysema the walls break down, in pneumonia fluid and white blood cells fill the alveoli, and tar from smoking damages the lining.

6Explaining disease

Explaining disease link the effect to surface area, distance and gradient.

Key terms

Term	Meaning
Alveolus	A microscopic air sac in the lung where gas exchange occurs.
Capillary	The smallest blood vessel, with a wall one cell thick.
Concentration gradient	A difference in concentration between two places.
Haemoglobin	A protein in red blood cells that binds oxygen.
Emphysema	A disease in which the walls of the alveoli break down.
COPD	A lung disease in which alveoli break down into fewer, bigger air sacs.

Avoid these mistakes

Saying alveoli have thick walls to protect them — thin walls make diffusion faster.

Forgetting to mention the blood supply — capillaries keep the concentration gradient steep.

Writing that gases are exchanged in the bronchi — exchange occurs in the alveoli.

Describing a feature without saying why it helps — state its effect on diffusion.

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Last updated: 13 October 2026