Adaptations of mammalian lungs

Organisms (Form and function) · Gas exchange · note 2 of 5

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Adaptations of mammalian lungsSpec B3.1.4

In short

The alveolar lungs of a mammal are adapted for gas exchange by a branched network of bronchioles ending in millions of alveoli, giving a very high surface area. Extensive capillary beds surround each alveolus, the walls are thin, and surfactant stops the moist alveoli sticking together and collapsing when air is breathed out.

Air enters through the trachea, which divides into two bronchi, one to each lung. Each bronchus divides again and again into a branched network of bronchioles, which end in clusters of tiny air sacs called alveoli.

Adaptations of the alveolar lungs for gas exchange
Adaptation	How it helps gas exchange
Branched network of bronchioles	Carries air to and from hundreds of millions of alveoli spread through the whole lung.
High surface area	The very large number of small alveoli gives a huge total surface for diffusion.
Extensive capillary beds	Each alveolus is covered in capillaries, so blood flows close to every part of the surface and keeps the gradient steep.
Thin walls	The alveolus wall and the capillary wall are each one layer of very thin, flattened cells, so the diffusion distance is short.
Surfactant	A fluid secreted by cells in the alveolus wall reduces the surface tension of the water lining, so the alveoli do not stick together and collapse when air is breathed out.
Moist lining	Oxygen dissolves in the film of moisture before it diffuses into the blood.

Oxygen diffuses from the air in the alveolus into the blood, and carbon dioxide diffuses from the blood into the alveolus. Ventilation and continuous blood flow keep both gradients steep.

An alveolus cut open at the end of a bronchiole, with a capillary wrapped around it; labels show the alveolus wall of one thin layer of cells, the film of moisture with surfactant, the capillary wall, red blood cells, oxygen diffusing from the air into the blood and carbon dioxide diffusing into the alveolus, deoxygenated blood arriving from the pulmonary artery and oxygenated blood leaving to the pulmonary vein; an inset shows branched bronchioles ending in clusters of alveoli. — Gas exchange in an alveolus: two thin layers of cells separate the air from the blood, and blood flow keeps the gradients steep.

Common mistake:

Surfactant does not help gases dissolve or diffuse. Its job is to lower surface tension so the alveoli stay open.

Written and checked against the IB Biology SL specification · Updated October 2026

Frequently asked questions

Why do larger organisms need specialised gas exchange surfaces?

Larger organisms need specialised gas exchange surfaces because their surface area-to-volume ratio is smaller and the distance from their centre to the outside is greater. Diffusion across the body surface alone would be far too slow, so they need large, thin, moist, permeable surfaces such as lungs or gills and a transport system.

How are alveoli adapted for gas exchange?

Alveoli are adapted for gas exchange by their huge total surface area, walls one thin layer of cells thick, a moist lining and a dense capillary network around each one. Surfactant lowers surface tension so alveoli do not collapse, and ventilation with continuous blood flow keeps the concentration gradients steep.

What happens to the diaphragm and intercostal muscles when you breathe in?

When you breathe in, the diaphragm contracts and flattens and the external intercostal muscles contract, pulling the ribs up and out. The volume of the thorax increases, so the pressure inside falls below atmospheric pressure and air flows into the lungs.

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