Diaphragm-lung demonstration. Image 1 of 2. Bell jar, balloons (red) and a rubber seal (green) being used to demonstrate the principles of the human lungs and diaphragm. The balloons represent the two human lungs, and the rubber seal represents the diaphragm, a sheet of tissue as the base of the chest cavity between the chest and the abdomen. It is the downwards movement of the diaphragm (combined with movement of the chest muscles) that expands the chest cavity and allows the lungs to inflate with air. The inflation and deflation of the lungs as the diaphragm moves down and then up, is seen in images H465/352 and H465/353.