The abdominopelvic cavity is a closed system filled with a fluid (water) continuum. The abdominopelvic cavity is enclosed by essentially muscular barriers. The inferior end is closed off by the pelvic diaphragm. The cylindrical walls of the abdomen are composed of three muscular sheets.
Their orientation is similar to plywood. These muscles are kept taut by their intrinsic tone, but they are capable of additional contraction.
Forming the top of the abdominopelvic cavity is the thoracic diaphragm. We discuss the thoracic diaphragm in the next paragraph.
The thoracic diaphragm is attached to the inferior margin of the rib cage and to the bodies of the lumbar vertebrae behind. As a muscular membrane, it domes upward into the thoracic cavity.
Upon contraction, the fibers of the thoracic diaphragm shorten and pull downward. This downward motion produces a piston-like pressure on the contents of the abdominopelvic cavity.