Along with bringing the body into a parasympathetic “rest and digest” mode, it is the body’s most efficient way to expand the lungs while also preventing unnecessary neck tension.
The diaphragm is a parachute shaped muscle that occupies our entire midsection and can be thought of as shaped like two merged umbrellas which attach from our lumbar spine around the entire bottom and front of the rib cage.
The lungs are shaped like two jelly fish that start at the tip of the shoulder where it meets the neck, just under the scalenes muscles, and then each lung drapes down over the double umbrella shape of the diaphragm. When the lungs are expanded the diaphragm is contracted down so belly breathing contracts the diaphragm, which pushes down on the soft abdominal organs, expanding the abdomen. This draws the lungs downward, because they’re draped over the diaphragm, and thanks to negative pressure in the pleural cavity.
The close relationship between neck tension and breathing is due to the location of the lungs, which sit rather high in the chest cavity and only extend as low as about the 8th rib, leaving two rib spaces worth of empty pleural cavity.
When we are not breathing with the abdomen we are chest breathing and using our accessory muscles of breathing, the scalenes and sternocleidomastoid
These overworked muscles attach onto our upper two ribs from a large portion of the sides of the cervical spine and from the bottom portion of the back of the skull to the collar bone, respectively.
These muscles can easily get chronically tight when chest breathing is predominate. Not to mention that all that prime abdominal real estate space of pleural cavity that is underused down there at the 9th through 11th ribs when chest breathing is in action. So, relax, take a breath, and notice the difference.
Living Anatomy class notes of Dr. Joe Coletto N.D.
Kendall, F., McCreary, E., Provance, P., Rodgers, M., Romai, W. (2005). Muscles testing and function with posture and pain (5th ed.). PA, USA: Lippincott Williams & Wilkins.