IGCSE Biology / Breathing and gas exchange
Ventilation- ribs, intercostals and diaphragm
Edexcel International GCSE Biology (4BI1), chapter 3: Breathing and gas exchange, specification point 2.47. Free lesson.
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Ventilation is the movement of air into and out of the lungs, caused by changes in thorax volume and pressure. Knowing the order of these changes helps you explain breathing and evaluate the bell jar model.
How breathing in moves air into the lungs
During inspiration, muscle movements make the thorax larger. This lowers the pressure inside the lungs below atmospheric pressure, so air moves in from the higher pressure outside. The external intercostal muscles contract, moving the ribs up and out. The diaphragm contracts, flattens and moves down. These movements increase the thorax volume. The pressure inside the lungs then falls below atmospheric pressure. Air flows into the lungs until the pressure inside and outside is equal.
How breathing out moves air out of the lungs
During expiration, the changes happen in reverse. At rest, breathing out is mostly passive because the muscles relax. The external intercostal muscles relax, so the ribs move down and in. The diaphragm relaxes, returning to its domed shape and moving up. The thorax volume decreases, so pressure inside the lungs rises above atmospheric pressure. Air flows out, from higher pressure inside to lower pressure outside, until the pressures are equal.
Why volume changes cause air to move
The order matters: muscles change volume, the volume change causes a pressure change, and the pressure difference moves the air. Air always moves from higher pressure to lower pressure. Breathing in: muscles contract, volume increases, pressure falls, air flows in. Breathing out: muscles relax, volume decreases, pressure rises, air flows out. The lungs have no muscle of their own, so they cannot expand themselves. Air is pushed in by the atmosphere when pressure inside the lungs is lower.
How to describe the bell jar model
The bell jar model shows how changing volume can change pressure and move air. The balloons represent the lungs, while the rubber sheet represents the diaphragm. A glass tube passes through a bung at the top of a bell jar. Two balloons are attached to its branches inside the jar. A rubber sheet is stretched across the open bottom. Pulling the sheet down increases the volume inside the jar. Pressure falls, so air flows into the balloons and they inflate. Releasing the sheet lets it move back up. The balloons deflate.
What the bell jar model cannot show
The model is useful for showing the diaphragm's effect, but it does not represent every part of breathing accurately. The model has no ribs or intercostal muscles. The glass jar cannot change shape. Real lungs are not hollow balloons.