During normal quiet respiration, the average person breathes 15 times per minute. Breathing is controlled by movements in the thorax brought about by the contracting and relaxing of the diaphragm and the intercostals muscles. Upon inspiration the diaphragm and the external intercostals muscles contract and the internal intercostals muscles relax, moving the ribs upwards and outwards. As the volume of the thorax increases, the elastic tissue of the lungs allows them to expand and air rushes into the lungs due to pressure inside the lungs being less than the atmospheric pressure outside. During expiration the diaphragm and the external intercostals muscles relax and the internal intercostals muscles contract, moving the ribs downwards and inwards. The decrease in volume of the thorax decreases the size of the lungs and air rushes out due to the increased internal pressure.
Normal breathing is a reflex action that occurs automatically without the person being consciously aware that it is taking place. The movement of