I have read here that the two major inhalation muscles are the (1) diaphragm and the (2) external intercostals.

Additionally, inhalation can also be caused by (1) expansion of the abdominal cavity, or (2) movement of the pelvic floor.

I thought that inhalation was caused by muscles that essentially cause the intra-thoracic pressure to drop, thus causing air to flow into the lungs to fill the absence, like a straw. This is what I thought caused us to inhale.


If my understanding is correct, then do the muscles cause this drop in pressure? Which spaces within the trunk experience this drop in pressure? If not, how exactly do these muscles facilitate inhalation?

  • $\begingroup$ there are two main muscle groups. The diaphragm and the rib cage assists with breathing by using a pair of muscles located between each rib known as the internal intercostals and external intercostals. which are mostly unconcious and which change depending on hormones and air intake. The upper group is for high oxygen intake and it is very mysterious because it is a passive muscle group which allows the ribcage to become flexible and expand to a higher lung volume, try hyperventilating using only the diaphragm! The diaphragm is for rest and sleeping. $\endgroup$ – com.prehensible Feb 26 at 10:56

Other than the branching, gas filled spaces within the lungs themselves, there are no other spaces within the trunk and above the diaphragm that contain gas - for a normal healthy lung. All the other space is filled with either tissue or fluid that serve to extend any pressure applied by the muscles you mentioned to the pleural sac that surrounds the lungs - like a hydraulic device. It's only the gas space that expands in volume and this is what causes air to move into that space through the upper airway. Exhalation is caused by elastic recoil - the springing back of this compartment.


Either the diaphragm or the rib lifting muscles dependently or together can enlarge the lungs and cause a vacuum. The vacuum is caused by enlargement of the the chamber like for a bike pump.

The less known muscles raise and lower the ribs: enter image description here https://en.wikipedia.org/wiki/Intercostal_muscle enter image description here enter image description here enter image description here Here's a video of diaphragm movement: https://www.youtube.com/watch?v=hp-gCvW8PRY enter image description here

The best is to research the muscle groups on google images and check out graphs of them to see lists of studies.

The pressure drop is actually an expansion of the lung cavity, with the diaphragm being very good at drawing air in like a serynge and the expiration groups being stronger for exhaling, they can both expand, contract and compress for activities like swimming and coughing. You may want to check graphs and studies of air pressure, then it gets complicated with trans-thoracic and trans-pulmonary pressure: enter image description here


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