in this procedure, we will use a bell-jar model of the lungs, shown in figure 23.3, to view the effects of pressure and volume on ventilation. the bell-jar model has two balloons, each representing one lung, and a flexible membrane on the bottom, which represents the diaphragm muscle. when you have completed the activity, answer check your understanding questions 1 and 2 (p. 625). 1 apply upward pressure to the membrane. this represents how the diaphragm muscle looks when it is relaxed. what has happened to the pressure of the system (has it increased or decreased)? what happened to the volume of the lungs? explain.