How we breathe

Breathing has two stages: inspiration (drawing air in) and expiration (letting air out). Air always flows from higher pressure to lower pressure, so breathing works by changing the pressure inside the lungs.

• For inspiration, the pressure inside the lungs must fall below the pressure of the air outside, so air rushes in.
• For expiration, it must rise above it, so air is pushed out.

How the chest changes its size:

• The diaphragm (a dome-shaped muscle under the lungs) contracts and flattens, making the chest longer from front to back (the antero-posterior axis).
• The external intercostal muscles (between the ribs) contract, lifting the ribs and breastbone, making the chest deeper (the dorso-ventral axis).
• The chest gets bigger, so the lungs get bigger, the pressure inside falls, and air enters.

When the diaphragm and intercostal muscles relax, the chest and lungs return to their smaller size. The pressure inside rises slightly above the outside pressure, and air is pushed out. At rest, breathing out needs no effort. During exercise, abdominal muscles can add force to both breathing in and out.

A healthy person breathes 12 to 16 times a minute. The amounts of air breathed in and out can be measured with a spirometer.