The mechanism of breathing to include the role of the diaphragm and the…

The idea

Breathing works by changing the volume of the thoracic cavity and letting pressure changes move the air. During inspiration the external intercostal muscles contract while the internal intercostal muscles relax, an antagonistic pair, pulling the ribcage upwards and outwards; at the same time the diaphragm muscles contract, so the diaphragm flattens from its domed shape. Both changes increase the volume of the thoracic cavity, so the pressure inside falls below atmospheric pressure and air moves in down the pressure gradient. Inspiration is therefore an active process requiring ATP. In normal expiration at rest the external intercostals and the diaphragm relax, the ribcage falls under gravity and the elastic recoil of the lung tissue reduces the volume of the thoracic cavity, so the pressure rises above atmospheric and air is forced out; this is largely passive.

What earns the marks

• External intercostal muscles contracting while internal intercostal muscles relax during inspiration, described as antagonistic.
• The diaphragm muscles contracting so that the diaphragm flattens.
• The volume of the thoracic cavity increasing so the pressure falls below atmospheric pressure.
• Air moving in down the pressure gradient, with the converse sequence for expiration.
• Expiration at rest being largely passive, using elastic recoil, with the internal intercostals contracting only in forced expiration.

Common mistakes and how to put them right

• Saying the diaphragm moves down without stating that its muscles contract and that it flattens.

• Writing that the lungs expand and pull air in, when the volume change is in the thoracic cavity and air moves down a pressure gradient.

• Describing both sets of intercostal muscles contracting at the same time, which is not antagonistic.

• Claiming air is sucked in, which mark schemes do not credit.

• Treating normal expiration as an active process driven by the internal intercostals.

Exam tips

• Use the same four-step chain every time: muscles contract, volume increases, pressure decreases, air moves in down the pressure gradient.
• State which muscle relaxes as well as which contracts, because the antagonistic pairing is itself a marking point.
• Say 'below atmospheric pressure' rather than 'low pressure'; the comparison is what earns the mark.