Mechanisms of Breathing: Inhalation and Exhalation

The pressure–volume principle behind breathing

Breathing works on a simple physical principle: in a closed space, if you increase the volume, the pressure inside falls; if you decrease the volume, the pressure inside rises. Muscles in the chest change the volume of the thoracic cavity, and air then moves passively along the pressure difference this creates, always flowing from higher pressure to lower pressure.

This means muscles never pull air in directly, they only change the shape and volume of the chest, and normal atmospheric pressure does the rest.

What happens during inhalation

1. The external intercostal muscles contract while the internal intercostal muscles relax, pulling the ribcage upward and outward.
2. The diaphragm contracts, changing from a domed shape to a flatter shape and moving downward.
3. These two changes together increase the volume of the thoracic cavity.
4. Because the same amount of air now occupies a larger volume, the air pressure inside the thoracic cavity falls below atmospheric pressure.
5. Air flows into the lungs from the higher-pressure atmosphere outside, down the pressure gradient, until the pressures are equal.

What happens during exhalation

1. The external intercostal muscles relax and the ribcage moves downward and inward under gravity and elastic recoil.
2. The diaphragm relaxes and returns to its domed shape, moving upward.
3. These changes decrease the volume of the thoracic cavity.
4. The air pressure inside the thoracic cavity rises above atmospheric pressure.
5. Air is pushed out of the lungs to the lower-pressure atmosphere outside, until the pressures are equal again.
6. During forced exhalation, such as blowing hard, the internal intercostal muscles and abdominal muscles also contract to push more air out faster.

How it is examined

This standard is usually tested with a table to complete or a diagram to label, filling in the muscle, rib, diaphragm, volume and pressure changes for each stage. You may also be asked to explain why air moves in or out, referring to the pressure gradient rather than saying the lungs 'suck' air in.

Worked exam-style question

Question. A student presses on the sides of a bell-jar model of the thorax, in which a balloon inside represents a lung and a rubber sheet at the base represents the diaphragm. When the rubber sheet is pulled downward, the balloon inflates; when the sheet is pushed upward, the balloon deflates.

(a) State which part of the model represents the ribcage and intercostal muscles. (b) Explain, in terms of volume and pressure, why the balloon inflates when the rubber sheet is pulled down.

(c) State one way in which this model does not accurately represent real breathing. (d) Explain why the balloon would fail to inflate if a hole were made in the wall of the bell-jar.

Model answer. (a) The rigid glass wall of the bell-jar represents the ribcage, since the intercostal muscles are not modelled by any moving part here. (b) Pulling the rubber sheet down increases the volume inside the sealed bell-jar, which lowers the pressure around the balloon below atmospheric pressure, so air flows from the higher-pressure atmosphere into the balloon, inflating it.

(c) The model does not show the ribcage moving upward and outward under muscle action, since only the diaphragm's movement is represented. (d) A hole in the bell-jar wall would let atmospheric air enter directly, so pulling the sheet down would no longer lower the pressure around the balloon, and the balloon would not inflate, this mirrors why a punctured chest cavity prevents normal lung inflation.

Practice question

Try this. A person takes a deep breath in and holds it, keeping the glottis closed so no air can escape. Explain what would happen to the pressure inside the thoracic cavity if the diaphragm then relaxed and moved back upward while the airway remained sealed.

Exam tip

Key terms

Revise these linked terms to answer breathing mechanism questions precisely:

• Diaphragm, the dome-shaped muscle whose contraction and relaxation changes thoracic volume.
• Gaseous exchange, the process that breathing movements make possible by supplying fresh air to the alveoli.
• Alveolus, the air sac where the oxygen brought in by breathing actually enters the blood.
• Trachea, the airway that carries air between the throat and the bronchi.
• Bronchus, one of the two branches carrying air from the trachea into each lung.

Source:SRC-DSKP-EN

Frequently asked questions

More for Respiratory System in Humans and Animals