2.9. Movements of the Thorax During Respiration

Increasing the Volume of the Thoracic Cavity

After studying the intercostal muscles and the diaphragm, it is worth writing a couple of sentences to explain how these muscles facilitate ventilation. As already mentioned, ventilation has two distinct phases: inspiration and expiration. During inspiration, the bones, joints and cartilage of the thorax need to increase the volume of the thoracic cavity. This thoracic cavity can expand along three different axes to maximise the volume it can hold. These axes are noted below.

Three Axes of Movement

  • Anteroposterior axis - Basically front-to-back, increased by the external intercostal muscles moving the ribs and sternum outwards.
  • Vertical axis - Increased when the diaphragm contracts and flattens.
  • Transverse/horizontal axis - Again, caused by the external muscles pulling the ribs outwards.