What are the muscles involved and pressures found in the lungs under rest, quiet inspiration, forced inspiration, quiet expiration, and forced expiration?
Answer
@ Rest:
Intrapulmonary pressure (aka alveolar pressure) = 0 mmHg
Intraplueral pressure = -4 mmHg or 756 mmHg
Atmospheric pressure = 760 mmHg
Air flows from an area of high pressure to one of low pressure. This is how the lungs remain inflated even under negative pressures.
@ Quiet inspiration:
Depends upon external intercostals and diaphragm --> External intercostals and diaphragm contract to increase the thoracic cavity volume. Thereby decreasing intrapleural pressure.
Intrapulmonary pressure = -1 mmHg or 759 mmHg
Intraplueral pressure = -6 mmHg or 754 mmHg
Atmospheric pressure = 760 mmHg
@ Forced inspiration:
Sternocleidomastoid, scalenes, and pectoralis minor muscles are at work. All with the aim to increase thoracic cavity volume.
Intrapulmonary pressure = -2 mmHg or 758 mmHg
Intrapleural pressure = -7 mmHg or 753 mmHg
Atmospheric pressure = 760 mmHg
@ Quiet expiration
No muscles are involved. This is a passive process. Depends on the elasticity of the lungs. Elasticity = change in pressure / change in volume
The diaphragm and external intercostals are relaxing.
Intrapulmonary pressure (aka alveolar pressure) = +1 mmHg or 761 mmHg
Intrapleural pressure = -4 mmHg or 756 mmHg
Atmospheric pressure = 760 mmHg
@ Forced expiration:
Involves the external oblique, internal oblique, transverse abdominis, rectus abdominis, and internal intercostals. The main goal is to decrease the thoracic cavity volume thereby increasing intraplerual pressure.
Intrapulmonary pressure (aka alveolar pressure) = +2 mmHg or 762 mmHg
Intrapleural pressure = -3 mmHg 0r 757 mmHg
Atmospheric pressure = 760 mmHg
Therefore, there will be a lot more air rushing out of the lungs under forced expiration than normal expiration.
Or you may prefer to see the video. Click on the link to Mechanisms of Breathing II.