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

Intercostal Muscles & Diaphragm

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

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

  • External intercostal muscles contract to pull the rib cage up during inhalation

  • Internal intercostal muscles contract to pull the ribcage down during forced exhalation

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

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

• Inhalation

  • The diaphragm contracts and flattens

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

  • This increases the volume of the chest cavity, or thorax

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

  • Air is drawn in

• Normal exhalation

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

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

  • This decreases the volume of the chest cavity

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

  • Air is forced out

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, e.g. during strenuous activity, the internal intercostal muscles will also work to pull the ribs down and in; this decreases the volume of the thorax further, forcing air out more 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