From volume to flow

Now bring the first two ideas together. A change in volume can create a pressure difference. If the gas has a path to the room, that difference can drive flow.

Expansion lowers pressure inside, drawing air in. Compression raises pressure inside, pushing air out. Flow reduces the difference between the chamber and the room.

Flow in = (P_room − P_inside) / R

The room is the pressure reference: 0 cmH₂O relative pressure. Positive flow is into the cylinder.

Open cylinder

The piston chamber is connected to the room through an airway. Begin with Open airway. Move the piston yourself, or use Breathe to repeat a 500 mL excursion at 12 cycles per minute.

Before you start: After pulling the piston out and holding it still, will the chamber stay below room pressure?

Pull and hold: With Open airway selected, increase the volume using the slider. Hold it steady and watch pressure return toward zero.
Repeat a breath: Press Breathe. Compare the pressure trace with the direction of airway flow as the piston moves.
Change the connection: Try Narrow airway, then Sealed. Watch how the pressure changes for the imposed piston motion.

Open cylinder · drag the piston, or breathe

Volume

2.40

litres

rests at 2.4 L, like a lung at FRC

Pressure inside

0.0

cmH₂O above the room

1033 cmH₂O absolute

Flow through the airway

No flow · no gradient

Still

Piston

1.9 L2.4 L · rest3.4 L

12 breaths a minute · 500 mL

Resistance 1.5 cmH₂O per L/s, about a healthy airway.

Pressure and flow · last 12 s

About this model. A prescribed piston motion with ideal gas and a linear airway resistance. This demonstrates pressure-driven ventilation, not a complete alveolus: compliance, pleural pressure, gas exchange, and muscle effort are not modeled.

What happened?

When the piston expands the chamber, pressure dips below the room and air enters. Once you hold the piston still, continued inflow brings pressure back toward the room’s pressure. A narrower airway requires a larger pressure difference for a similar flow. When sealed, no gas crosses the boundary.

Volume change creates a gradient; the gradient drives flow.

Check your understanding

With a narrow airway, this model keeps taking the same size breath. Would a person always do that?

Reveal explanation

No. The piston prescribes volume here, so the required pressure can rise. In a person, respiratory muscle effort and lung and chest-wall mechanics also determine the volume achieved. The same effort against more resistance may produce a smaller breath.

Further reading: OpenStax, The Process of Breathing.