Role of the Intercostal Muscles & Diaphragm (Edexcel IGCSE Science (Double Award))

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Intercostal Muscles & Diaphragm

  • Muscles are only able to pull on bones, not push on them

  • This means that there must be two sets of intercostal muscles to work antagonistically to facilitate breathing

    • External intercostal muscles, pull the rib cage up

    • Internal intercostal muscles pull the ribcage down

  • The diaphragm is a thin sheet of muscle that separates the chest cavity from the abdomen and also facilitates in the breathing process

The lungs, IGCSE & GCSE Biology revision notes

There are two sets of intercostal muscles: the external, on the outside of the rib cage, and the internal, on the inside of the rib cage. The diaphragm also works with these to facilitate breathing.

Ventilation

  • During inhalation

    • The diaphragm contracts and flattens

    • The external set of intercostal muscles contract to pull the ribs up and out:

    • This increases the volume of the chest cavity (thorax)

    • Leading to a decrease in air pressure inside the lungs relative to outside the body

    • Air is drawn in

  • During exhalation

    • The diaphragm relaxes it moves upwards back into its domed shape

    • The external set of intercostal muscles relax so the ribs drop down and in

    • This decreases the volume of the chest cavity (thorax)

    • Leading to an increase in air pressure inside the lungs relative to outside the body

    • Air is forced out

Inhalation, IGCSE & GCSE Biology revision notes
Exhalation, IGCSE & GCSE Biology revision notes

Changes in the thorax during ventilation

Forced Exhalation

  • The external and internal intercostal muscles work as antagonistic pairs 

  • When we need to increase the rate of gas exchange (for example during strenuous activity) the internal intercostal muscles will also work to pull the ribs down and in to decrease the volume of the thorax more, forcing air out more forcefully and quickly – this is called forced exhalation

    • There is a greater need to rid the body of increased levels of carbon dioxide produced during strenuous activity

  • This allows a greater volume of gases to be exchanged

Examiner Tips and Tricks

You may see the terms inhalation OR inspiration (breathing in), and exhalation OR expiration (breathing out). Both sets of terms mean exactly the same thing, so don’t let them confuse you! This sequence of events is a common exam question and you should be able to explain in detail what is happening to the external and internal intercostal muscles, the rib cage, the diaphragm, the volume and the pressure-volume of the lungs when breathing in and out. Remember, if you learn one, the other is almost exactly the opposite.

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Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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