The breathing mechanism can be explained by the syringe model: If one pulls the plunger, air is drawn in, supporting the motion of the plunger. During inhalation in the human body, diaphragm and accessory muscles of respiration contract and force is transmitted to the rib cage. This increases the space in the rib cage, into which the lungs expand: air is pulled in. At the end of an inhalation process, breathing pauses for a brief moment, the rib cage stays in "inhaling position".

In contrast to the illustrated syringe-model, it is the tendency of the lung, being a tissue, to contract back into its original shape after having been stretched. At the end of the inhalation process, relaxation of the respiratory muscles causes the lung (filled with air) to contract. The rib cage reacts to this pulling force, contracts, space is reduced and air is forced out of the lungs. A short pause also occurs after exhalation. The rib cage is in "exhalation position".

For people with healthy lungs, exhalation at rest is mostly a "passive" process, meaning they need little strength or muscle movement to perform it. If having a respiratory or lung disease or under exertion, exhalation needs to be actively supported by the accessory muscles of respiration.