Like a bellows. The diaphragm is a muscle. When it contracts it moves downward creating negative pressure in the lungs and causing air to enter the lungs. When it relaxes, positive pressure is created in the lungs, causing air to be expelled. When part of the diaphragm does not work it is difficult to breathe.
Diaphragm. The diaphragm is a muscle that separates the respiratory system from the abdomen. It is a muscle that contracts and relaxes. When you inspire, it relaxes. When you expire, it contracts. For the most part, it is an involuntary action, but can be voluntary as well. It is the main muscle associated with respiration.