Now we’ll move on, to look at the muscles that are involved in breathing in, and breathing out.
The act of breathing in is called inspiration, the act of breathing out is expiration. The whole process of breathing is called respiration. In looking at the muscles of respiration, we’ll look first at the diaphragm, then at the muscles that produce movements of the ribs. The best way to understand the diaphragm is to look at it from below.
Here are the lower ribs. The muscles between them are the intercostals, which we’ll see shortly. Here’s the left costal arch, here’s the xiphoid process. The anterior abdominal wall, and all the abdominal organs have been removed. Now we’ll take a look from below.
Here’s the diaphragm. It’s a thin, continuous sheet of muscle, with fibers that converge from all around the circumference, to insert on this flat tendon, the central tendon of the diaphragm. The diaphragm arises from a line that goes right around the inside of the lower thoracic aperture, with one interruption, here.
To see the line of attachment, we’ll remove one half of the thorax, and look from inside. The line of attachment of the diaphragm goes from here on the back of the sternum, along the inside of the costal arch, and round to the tip of the twelfth rib.
Between the twelfth rib and the body of the second lumbar vertebra, the diaphragm arises on each side from the fascia which overlies the two big muscles of the posterior abdominal wall. These are the quadratus lumborum, and psoas major muscles.
Three important structures pass through the diaphragm: the esophagus, and the two main blood vessels of the lower half of the body, the inferior vena cava, and the descending aorta. This is the opening for the inferior vena cava, the vena caval foramen. This is the opening for the esophagus, the esophageal hiatus. This is the opening for the aorta.
On each side of these two openings there’s a thickening of the diaphragm called a crus, the plural of which is crura. The left crus arises all the way down here, on the body of L2 The right crus arises even further down, on L3. The two crura arch over the aortic opening, forming the median arcuate ligament. Fibers of the two crura cross over, to surround the esophageal hiatus.
When the diaphragm contracts, the whole sheet of muscle, together with the central tendon, moves downward, expanding the lungs, and causing us to breathe in.
As the diaphragm contracts, the structures below it, the contents of the upper part of the abdominal cavity are pushed downwards, which leads to this bulging of the abdominal wall when we take a quiet breath in.
When we’re at rest and breathing quietly, inspiration is produced almost entirely by the downward pull of the diaphragm, with little or no movement of the ribs. In quiet expiration, the upward, return movement of the diaphragm is produced passively by elastic forces, notably by elastic contraction of the lungs themselves.
When we’re breathing vigorously, the diaphragm is pushed upward actively by contraction of the muscles of the abdominal wall. These raise the pressure in the abdomen, forcing the upper abdominal organs, and the diaphragm, upward.