When you inhale, the diaphragm and the muscles between the ribs contract and expand the chest cavity. This expansion lowers the pressure in the chest cavity below the outside air pressure. Air then flows in through the airways (from high pressure to low pressure) and inflates the lungs. When you exhale, the diaphragm and muscles between the ribs relax and the chest cavity gets smaller. The decrease in volume of the cavity increases the pressure in the chest cavity above the outside air pressure. Air from the lungs then flows out of the airways to the outside air. The cycle then repeats with each breath, passing about 100 millilitres oxygen to the lungs. The oxygen is then transported to all parts of the body by blood. Also during the sleep the body is supplied with oxygen constantly. To a certain degree respiration can be controlled - by taking a deep breath or holding your breath.

When breathing stops for some time, critical injuries can be caused. Even after a few minutes of an oxygen deficiency, irreversible damage can be caused.

Illnesses or accidents which damage the nervous system can impair the breathing technique, and people affected can then be dependant on mechanical ventilation. Also diseases impairing the oxygen supply from the lungs into blood may make an occasional, mechanical ventilation necessary.

Long-term mechanical ventilation requires a tracheostomy (an opening to allow for the new breathing passage to be inserted below the voice box) for placement of a tube into the windpipe to deliver air directly from the mechanical ventilation equipment into the lungs. For that purpose the surgeon makes an incision low in the neck.

Although a mechanical ventilation equipment has a lot of safety features to ensure safe operation, it has to be observed by qualified nursing staff. Additionally, most of the patients requiring assisted breathing are prone to diseases caused by immobility. Professional care avoids infections and needless weakening of the body.