Boyle’s law
Go to the experiment ↓Before we add lungs or airways, start with a fixed amount of gas. Boyle’s law describes how its pressure changes when you change the space it occupies, while keeping temperature constant.
Pressure and volume vary inversely: compress the gas and pressure rises; expand it and pressure falls.
P₁V₁ = P₂V₂
P is absolute pressure. V is gas volume. Temperature and amount of gas stay fixed.
Sealed cylinder
This cylinder starts with 1 litre of gas at atmospheric pressure. The piston moves, but the cylinder is sealed: no gas can enter or leave.
Before you start: If you halve the volume, what do you expect to happen to the absolute pressure?
- Compress
- Use the volume slider to move from 1.0 L to 0.5 L. Compare the absolute pressure before and after.
- Expand
- Move to 2.0 L. Watch the pressure-volume curve and the P × V readout.
- Look closer
- Switch to Breath-sized. Explore how a small volume change affects pressure near atmosphere.
About this model. An ideal gas at constant temperature. Particle motion is illustrative, and piston travel is exaggerated 25× in Breath-sized mode. A sealed cylinder does not model an entire breath: real lungs exchange gas through an airway.
What happened?
At 0.5 L, absolute pressure doubles from 1033 to 2066 cmH₂O. At 2.0 L, it halves to 516.5 cmH₂O. The amount of gas has not changed. At the same temperature, compression increases the frequency of molecular collisions with the walls.
Changing volume can change pressure before any gas flows.
Check your understanding
At the starting position, the large pressure readout is zero. Does that mean there is no pressure inside?
Reveal explanation
No. That readout is pressure relative to the atmosphere. Zero means the cylinder and the room have equal pressure. Boyle’s law uses absolute pressure, which starts at 1033 cmH₂O in this model.
Further reading: OpenStax, The Process of Breathing.