The pathway of air
The respiratory system is the set of organs that lets air move in and out of the body so that oxygen can reach the blood and waste carbon dioxide can leave. Air enters through the nose, where it is warmed, moistened and cleaned by tiny hairs and mucus, then passes down the trachea (windpipe). The trachea is held open by C-shaped rings of cartilage. It branches into two bronchi, one to each lung. Inside the lungs each bronchus divides into smaller bronchioles that end in millions of tiny air sacs called alveoli.
Key idea
Air pathway: nose → trachea → bronchus → bronchiole → alveolus. The alveolus is where gas exchange happens.
The breathing mechanism
Breathing is controlled by the diaphragm (a sheet of muscle below the lungs) and the intercostal muscles between the ribs.
| Inhalation | Diaphragm contracts and flattens; ribs move up and out; chest volume increases; air pressure inside falls; air rushes in. |
| Exhalation | Diaphragm relaxes and domes up; ribs move down and in; chest volume decreases; air pressure rises; air is pushed out. |
Built for gas exchange
Alveoli are perfectly designed for their job: there are millions of them to give a huge surface area, their walls are only one cell thick so gases cross quickly, and each one is wrapped in tiny blood capillaries and kept moist. These features make the lungs very efficient at taking in oxygen and removing carbon dioxide with every breath.
Remember
- Inhale: diaphragm contracts and flattens.
- Exhale: diaphragm relaxes and domes up.