The lungs also have a cleaning system that traps, ejects, or destroys irritants and other harmful substants that travel in with the air. In quiet breathing a person inhales about a pint of air with each breath. The lungs are inside the chest, in the cavity framed by the rib cage. The lungs are covered by a thin membrane called the pelura. In normal breathing the diaphragm and the muscles between the ribs automaticaly contract and expand in a rhythmic cycle.
Humans breathe by first taking air into the lungs and then expelling it.
These two acts make up a complete breath. The average adult normally takes in about 16 breaths per minute when awake, and about 6 to 8 breaths when asleep. Under strees that rate may increase to 100 per minute. During inhalation, the rib mucles elevate the ribs and the diaphragm moves downward, enlarging the chest cavity. Air pressure in the lungs is reduced, and air flows into them.