ABSTRACT
The lungs are composed of several lobes, three in the right lung (upper, middle and lower), and two in the left lung (upper and lower). Each of these lobes is supplied with air by its own tube, the lobar bronchus. The chest cavity is separated from the abdominal cavity by a sheet of muscle known as the diaphragm. Normal quiet breathing is often called 'diaphragmatic breathing'. When the diaphragm contracts and moves downwards, the reduced pressure in the chest cavity (intrathoracic pressure) causes air to rush into the lungs from the atmosphere. When the diaphragm relaxes, intrathoracic pressure rises and air leaves the lungs. Other muscles, such as those between the ribs (intercostal muscles), in the back (e.g. quadratus lumborum), and in the neck (e.g. sternocleidomastoid) can be used to aid breathing. As such, these muscles are often collectively termed the 'accessory' muscles of breathing.
