- The diaphragm is a thin sheet of muscle that separates the chest cavity from the abdomen; it is ultimately responsible for controlling ventilation in the lungs
- When the diaphragm contracts it flattens and this increases the volume of the chest cavity (thorax), which consequently leads to a decrease in air pressure inside the lungs relative to outside the body, drawing air in.
- When the diaphragm relaxes it moves upwards back into its domed shape and this decreases the volume of the chest cavity (thorax), which consequently leads to an increase in air pressure inside the lungs relative to outside the body, forcing air out
- The external and internal intercostal muscles work as antagonistic pairs (meaning they work in different directions to each other)
- During inhalation the external set of intercostal muscles contract to pull the ribs up and out:
- This also increases the volume of the chest cavity (thorax), decreasing air pressure, drawing air in
- During exhalation, the external set of intercostal muscles relax so the ribs drop down and in:
- This decreases the volume of the chest cavity (thorax) increasing air pressure, forcing air out
- 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 actually 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
You may see the terms inhalation OR inspiration (breathing in), and exhalation OR expiration (breathing out). Both sets of terms mean exactly the same thing, so don’t let them confuse you!
This sequence of events is a common exam question and you should be able to explain in detail what is happening to the external and internal intercostal muscles, the rib cage, the diaphragm, the volume and the pressure-volume of the lungs when breathing in and out.
Remember, if you learn one, the other is almost exactly the opposite.