Air flows downhill
The most useful idea in all of breathing mechanics is also the simplest: gas always flows from higher pressure to lower pressure. Your lungs do not reach out and grab air. Instead, your body makes the inside of the chest a little roomier, the pressure inside drops below the pressure of the air around you, and air flows in to even things out. To breathe out, the chest gets smaller, inside pressure rises above the outside, and air flows back out. That single seesaw of pressure, repeated about twelve to sixteen times a minute at rest, is what we call ventilation.
The muscles that do the work
The star of breathing in is the diaphragm, a broad dome of muscle slung underneath the lungs. When it contracts it flattens and drops, like a piston pulling down, and that alone accounts for most of a quiet breath. The muscles between your ribs, the external intercostals, swing the rib cage up and out at the same time, adding a little more room. Together they enlarge the chest in every direction.
Quiet breathing out is the easy part: you simply relax. The stretched lung tissue and rib cage spring back on their own, like letting go of a pulled rubber band, and squeeze the air out. We will give this springiness its proper name, elastic recoil, in a later guide. Only when you breathe hard, during exercise or illness, do you recruit extra muscles such as the abdominal wall to actively push air out.
The breath in order
- The brain sends a signal; the diaphragm contracts and drops while the rib cage lifts.
- The chest grows larger, so the pressure inside the lungs falls just below the air outside.
- Air flows in down that pressure gradient until inside and outside are equal: that is inspiration.
- The muscles relax; elastic recoil shrinks the chest and raises the inside pressure above the outside.
- Air flows back out down the new gradient: that is expiration, and the cycle begins again.
Notice that the lungs themselves are passive throughout. They have no muscle to inflate themselves; they are stretchy bags that follow the chest wall wherever it goes. Every other idea in this track, from compliance to the work of breathing, is built on this one picture of a pressure pump moving a passive balloon.