The diaphragm acts like a second heart. Its piston-like movements expand the base of the lungs, allowing them to suck in more venous blood. The increase in the venous circulation improves the general circulation. The abdominal organs are massaged by the up and down movements of the diaphragm. Low breathing has a beneficial effect on the solar plexus, a very important nerve center.
Middle breathing is a little harder to describe since the limits of variability are more indefinite.
Yet it is breathing in which mainly the middle parts of the lungs are filled with air. It exhibits some of the characteristics of both high breathing, since the ribs rise and the chest expands somewhat, and low breathing, since the diaphragm moves up and down and the abdomen in and out a little. It has been called thoracic or intercoastal or rib breathing. But too often it also remains a shallow type of breathing. With this form of breathing, the ribs and chest are expanded sideways.