The pathway of air
- Air enters through the mouth and nose, then passes down the trachea (windpipe).
- The trachea splits into two bronchi, one to each lung, which divide into smaller bronchioles.
- The bronchioles end in millions of tiny air sacs called alveoli, where gas exchange happens.
Breathing in and out
- Breathing in (inspiration): the diaphragm contracts and flattens, and the external intercostal muscles pull the ribs up and out. The chest gets bigger, the pressure inside falls, and air rushes in.
- Breathing out at rest (expiration): the diaphragm and intercostals relax, the chest gets smaller, the pressure rises and air is pushed out. This is passive.
- During exercise, extra muscles help: the pectorals and sternocleidomastoid lift the ribs further when breathing in, and the abdominals pull the ribs down to force air out.
Lung volumes
- Tidal volume: the amount of air breathed in or out in one normal breath. It increases during exercise.
- Inspiratory reserve volume: the extra air that can be breathed in after a normal breath in. It decreases during exercise.
- Expiratory reserve volume: the extra air that can be breathed out after a normal breath out. It decreases slightly during exercise.
- Residual volume: the air left in the lungs after breathing out as hard as possible, so the lungs don't collapse.
- Vital capacity: the most air that can be breathed out after the deepest breath in. A spirometer trace shows these volumes.
Gaseous exchange at the alveoli
- Oxygen diffuses from the alveoli (high concentration) into the blood in the capillaries (low concentration). Carbon dioxide diffuses the other way.
- Alveoli are suited to this: a huge surface area, walls one cell thick for a short diffusion distance, a moist lining, and a rich supply of capillaries.
Key terms
- Alveoli
- Tiny air sacs in the lungs where gas exchange happens.
- Diaphragm
- The sheet of muscle below the lungs that contracts to help breathing in.
- Tidal volume
- The amount of air breathed in or out in one normal breath.
- Vital capacity
- The most air that can be breathed out after the deepest breath in.
- Residual volume
- The air that stays in the lungs after breathing out as hard as possible.
- Gaseous exchange
- The swapping of oxygen and carbon dioxide between the alveoli and the blood.
- Diffusion
- The movement of a gas from a high to a low concentration.
- Spirometer
- A device that measures and records lung volumes.