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Diaphragm
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The thoracic diaphragm is a sheet of muscle and tendon that separates the chest from the abdomen. Fibromuscular means just that: part muscle, part fibrous tissue. Its upper surface is the floor of the chest cavity, where the lungs sit.
It is a piston: as it drops it creates a vacuum and air flows in. When the diaphragm contracts, it pulls down its central tendon, the fibrous plate in its middle, and that enlarges the chest cavity. As the cavity enlarges, intrathoracic pressure falls, meaning there is less pressure inside the chest than outside, and the lung expands and fills its alveoli, the tiny sacs where air meets blood.
It is not a smooth, closed sheet: the esophagus, the phrenic and vagus nerves, the descending aorta and the inferior vena cava all pass through it, since they have to travel from the chest to the abdomen. And it is not symmetric: the left side sits a little lower than the right, mostly because of the liver.
In brief
The thoracic diaphragm is a fibromuscular sheet that separates the chest from the abdomen and forms the floor of the chest cavity.
As it contracts it pulls its central tendon down, enlarging the chest cavity and lowering intrathoracic pressure, so the lung expands and fills its alveoli.
It is a piston: as it drops it creates a vacuum and air flows in.
The esophagus, phrenic and vagus nerves, descending aorta and inferior vena cava pass through it, and it is asymmetric: the liver leaves the left side slightly lower.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingAnatomy, Thorax: Diaphragm, StatPearlsPublished
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