Concept preview

What you will be able to do

1. 01Explain how the diaphragm and intercostal muscles change the volume of the chest cavity during inhalation and exhalation.
2. 02Use Boyle's law to relate volume changes to pressure changes and hence to the direction of airflow.
3. 03Distinguish quiet breathing from forced breathing and identify the muscles involved in each.
4. 04Define and use the main lung volumes and capacities (tidal volume, vital capacity, residual volume).

The single most important idea in the mechanics of breathing is deceptively simple: air always flows from where the pressure is higher to where the pressure is lower. You cannot suck air into your lungs the way a straw pulls up a drink — there is no force reaching out to grab the air. Instead, your body plays a clever trick. By enlarging the sealed space that holds the lungs, it lowers the pressure of the air inside the lungs until that pressure drops below the pressure of the atmosphere outside; air then rushes in of its own accord to even things out. To breathe out, the body does the reverse, shrinking the space so the pressure inside rises above the outside and air flows back out. Breathing, in other words, is not really about moving air directly at all — it is about changing volumes to create pressure differences, and letting the air follow. Everything else in this topic is a consequence of that one principle.