Air is moved into an exchange chamber containing the lungs, and is there brought into close contact with blood vessels containing red blood cells in order for diffusion to occur in both directions. Breathing air into the lungs is accomplished through the action of the diaphragm , a muscular sheet at the bottom of the thoracic cavity, and the intercostal muscles in the rib cage. The rib cage is pushed out at the same time, and the volume of the thoracic cavity is increased. This increase in volume causes air pressure within the cavity to decrease relative to the outside atmospheric pressure, forcing air in from the outside of the body to equalize the pressure.
Air rushes in through the nostrils and the mouth.
The air is moistened and filtered here, and then descends through the pharynx , the cavity behind the nose and mouth. The air passes the larynx , or voice box, descends down the cartilaginous tube called the trachea , and then through the tree-like configuration of the lung. During exhalation of air, or expiration , the diaphragm and rib muscles relax and the abdominal muscles contract, decreasing the size of the thoracic cavity and forcing air out.