What ventilation is, and why pressure matters
Breathing is a pump: change the volume of the chest, and air follows the pressure.
Ventilation is just the scientific word for breathing — the movement of air in and out of the lungs. Do not confuse it with gas exchange (oxygen and carbon dioxide swapping across the alveoli) — ventilation is the air movement that keeps fresh air arriving.
The key idea you must understand is pressure:
- Air, like all gases, flows from a region of high pressure to a region of low pressure.
- Your lungs cannot pull air in by themselves. Instead, muscles change the volume of the thorax (the chest cavity).
- Bigger volume → lower pressure (the same air spreads out, so it presses less). Air then flows in from outside.
- Smaller volume → higher pressure. Air is squeezed, so it flows out.
The two structures that change the chest volume are the intercostal muscles (between the ribs) and the diaphragm (a sheet of muscle below the lungs).
Exam tip. Examiners want a clear chain: muscles → volume change → pressure change → air movement. Skipping the pressure step is the most common way to lose marks.
- Ventilation = breathing (air in/out); gas exchange is separate.
- Air flows from high to low pressure.
- Change volume → change pressure → air moves.
See the full worked example for role of the intercoastal muscles and diaphragm →