1.3 Mechanics of inhalation and expiration
Movement of the diaphragm and intercostal muscles acts to expand and decrease the size of the thoracic cavity, creating pressure gradients that draw air into and force air out of the lungs, as described in Video 4.
Transcript: Video 4 Mechanics of inhalation and expiration.
Expiration is generally a passive event brought about by relaxation of the diaphragm and external intercostal muscles. The ribcage, diaphragm and lung tissue itself return by elastic recoil to their original pre-inspiratory positions. The consequent retraction of the chest wall forces air out of the lungs. Forced expiration is mainly achieved by contraction of the internal intercostal muscles, aided to some extent by contraction of the abdominal muscles.
Most of the time, you will be unaware of the contraction and relaxation of the muscles that control respiration. They become much more noticeable when you cough or develop a bout of hiccups. In fact, hiccups are caused by a spasm of the diaphragm and intercostal muscles in response to increased activity of the phrenic nerve and vagus nerve (which innervates the muscles of the abdomen). The spasms cause the floor of the thoracic cavity to drop suddenly, which pulls air quickly and forcefully into the airways. Movement of the air past the closed vocal cords creates the characteristic ‘hic’ sound.
Activity 3 Lung in a bottle
You can explore the relationship between movement of the diaphragm and lung volume directly by making your own ‘lung in a bottle’ as shown in Video 5.
Transcript: Video 5 ‘Lung in a bottle’ experiment.
According to the video, what factors are responsible for the inflation and deflation of the balloon ‘lung’ inside the bottle?
Pulling down on the blue balloon ‘diaphragm’ caused the air pressure in the ribcage to drop lower than the air pressure in the atmosphere. This drove air to flow down its pressure gradient into the balloon, causing it to inflate. Pressing up on the diaphragm increased the air pressure in the ribcage, driving air out of the balloon lung and causing it to collapse.