BIOLOGY · Stage 8 · Chapter 5: Respiration and the Respiratory System
Biology 5.8 The structure of the alveoli — Adaptations for Gas Exchange — Alhussan International Academy - Biology (Stages 7-9) Revision Notes & Topical Questions
5 min read 4 parts Lesson 8 of 12
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)
Section 1 of 4What the alveoli do
- Alveoli
- microscopic air sacs at the ends of the bronchioles, with hundreds of millions in each lung.
- Lung structure
- the lungs are more like a sponge made of millions of alveoli than a simple balloon like the bell jar model.
- Function
- they are the site of gas exchange, where oxygen enters the blood and carbon dioxide leaves it.
- 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)
- the same design features are found in other exchange surfaces such as villi and fish gills.
Section 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 |
Section 3 of 4Gas exchange and concentration gradients
- Oxygen
- it moves from the air in the alveoli into the blood in the capillaries by diffusion down a concentration gradient.
- Carbon dioxide
- it moves from the blood into the alveolar air and is breathed out.
- 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.
- 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.
- 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.
Section 4 of 4COPD and the alveoli
- COPD (chronic obstructive pulmonary disease)
- a lung disease that affects the structure of the alveoli.
- 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.
- Effects
- less gas exchange takes place, so body cells do not get enough oxygen and carbon dioxide builds up in the blood.
- 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.
- 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.
- Explaining disease
- link the effect to surface area, distance and gradient.
Last updated: