The lungs themselves neither draw in air nor push it out. The diaphragm, abdominal muscles and the intercostals muscles accomplish the expansion and contraction of the lungs. The diaphragm (meaning, partition) is a large dome of skeletal muscle that separates the thoracic cavity from abdominal cavity. Theta are two sets of intercostal, muscles between each pair of ribs: the external intercostals and the internal intercostals.

The muscle fibres run diagonally but in opposite direction in the two sets of muscles. Breathing takes place in two phases i.e., inspiration and expiration.



Inspiration

Inspiration is taking in of air, it is the active phase of breathing. During inspiration contraction of the diaphragm causes its dome shape to flatten (less dome shape) whereas contraction of the external intercostals and relaxation of the internal intercostals causes the rib cage to move upward and forward. Both these events result in increase of inner space of thoracic cavity. Consequently, the pressure in the thorax and hence in the lungs, is reduced to less than atmospheric pressure. Air therefore enters the lungs and alveoli become inflated.

Process of Inspiration
  • Diaphragm – flattens, extending the superior/inferior dimension of the thoracic cavity.
  • External intercostal muscles – elevates the ribs and sternum, extending the anterior/posterior dimension of the thoracic cavity.

The action of the inspiratory muscles results in an increase in the volume of the thoracic cavity. As the lungs are held against the inner thoracic wall by the pleural seal, they also undergo an increase in volume.

As per Boyle’s law, an increase in lung volume results in a decrease in the pressure within the lungs. The pressure of the environment external to the lungs is now greater than the environment within the lungs, meaning air moves into the lungs down the pressure gradient.

Expiration

Expiration is the removal of air out of the lungs, it is the passive phase of breathing. During expiration relaxation of the diaphragm causes it to become more dome shape whereas relaxation of the external intercostals and contraction of the internal intercostals causes the rib cage to move downward and backward. Both these events result in decrease of inner space of thoracic cavity. Consequently, the pressure in the thorax and hence in the lungs, is increased to more than atmospheric pressure, therefore, air is forced to expel from the lungs.

Process of Passive Expiration
  • Diaphragm– relaxes to return to its resting position, reducing the superior/inferior dimension of the thoracic cavity.
  • External intercostal muscles – relax to depress the ribs and sternum, reducing the anterior/posterior dimension of the thoracic cavity.

The relaxation of the inspiratory muscles results in a decrease in the volume of the thoracic cavity. The elastic recoil of the previously expanded lung tissue allows them to return to their original size.

As per Boyle’s law, a decrease in lung volume results in an increase in the pressure within the lungs. The pressure inside the lungs is now greater than in the external environment, meaning air moves out of the lungs down the pressure gradient.

Forced Breathing

Forced breathing is an active mode of breathing which utilises additional muscles to rapidly expand and contract the thoracic cavity volume. It most commonly occurs during exercise.



Active Inspiration

Active inspiration involves the contraction of theaccessory muscles of breathing (in addition to those of quiet inspiration, the diaphragm and external intercostals). All of these muscles act to increase the volume of the thoracic cavity:
  • Scalenes – elevates the upper ribs.
  • Sternocleidomastoid – elevates the sternum.
  • Pectoralis major and minor – pulls ribs outwards.
  • Serratus anterior – elevates the ribs (when the scapulae are fixed).
  • Latissimus dorsi elevates the lower ribs.
Active Expiration

Active expiration utilises the contraction of several thoracic and abdominal muscles. These muscles act to decrease the volume of the thoracic cavity:
  • Anterolateral abdominal wall – increases the intra-abdominal pressure, pushing the diaphragm further upwards into the thoracic cavity.
  • Internal intercostal – depresses the ribs.
  • Innermost intercostal – depresses the ribs.