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

• 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

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

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