The pathway of air
Air is drawn in through the nose and mouth, passes down the trachea, and enters the two bronchi, one to each lung. The bronchi divide into smaller bronchioles, which end in tiny air sacs called alveoli. It is at the alveoli that gas exchange takes place: oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli to be breathed out.
Key idea
Gas exchange occurs by diffusion across the alveolar wall: oxygen moves into the blood and carbon dioxide moves out, each moving down its concentration gradient.
Adaptations of the alveoli
The alveoli are extremely well suited to gas exchange. They provide a very large surface area; their walls are only one cell thick, giving a short diffusion distance; they have a rich blood supply of capillaries to maintain a steep concentration gradient; and their surfaces are moist so gases dissolve before diffusing.
Breathing (ventilation)
Ventilation moves air in and out. During inhalation, the diaphragm and external intercostal muscles contract, the ribcage moves up and out, the chest volume increases and pressure falls, so air rushes in. During exhalation these muscles relax, chest volume decreases, pressure rises and air is pushed out.
Remember
- Order of air: trachea → bronchi → bronchioles → alveoli.
- Alveoli are adapted with large area, thin walls, good blood supply and moist surfaces.
- Inhaling: diaphragm contracts and flattens; chest volume up, pressure down.