Human Rib Cage - Respiratory Function

Respiratory Function

The human rib cage is a component of the human respiratory system. It encloses the thoracic cavity, which contains the lungs. An inhalation is accomplished when the muscular diaphragm, at the floor of the thoracic cavity, contracts and flattens, while contraction of intercostal muscles lift the rib cage up and out.

Expansion of the thoracic cavity is driven in three planes; the vertical, the anteroposterior and the transverse. The vertical plane is extended by the help of the diaphragm contracting and the abdominal muscles relaxing to accommodate the downward pressure that is supplied to the abdominal viscera by the diaphragm contracting. A greater extension can be achieved by the diaphragm itself moving down, rather than simply the domes flattening. The second plane is the anteroposterior and this is expanded by a movement known as the 'pump handle.' The downward sloping nature of the upper ribs are as such because they enable this to occur. When the external intercostal muscles contract and lift the ribs, the upper ribs are able also to push the sternum up and out. This movement increases the anteroposterior diameter of the thoracic cavity, and hence aids breathing further. Finally, you have the transverse. In this situation, it involves mainly the lower ribs (some say it is the 7th-10th ribs in particular) with the diaphragm's central tendon acting as a fixed point. When the diaphragm contracts, the ribs are able to evert and produce what is known as the 'bucket handle' movement, facilitated by gliding at the costovertebral joints. In this way, the transverse diameter is expanded and the lungs can fill.

Breathing may be assisted by other muscles that can raise the ribs, such as sternocleidomastoid, pectoralis major and minor as well as the scalenes. Whilst under most circumstances, individuals respire via eupnea, exercise and other forms of physiological stress can cause the body to require forced expiration, rather than the simple elastic recoil of the thoracic cage, lungs and diaphragm. In this case, muscles are recruited which can help depress the ribs and raise the diaphragm - such as the anterior abdominal wall muscles, excluding the transversus abdominis muscle. Latissimus dorsi can also aid deep, forced expiration.

Another way the thoracic cavity can expand during inhalation is called belly breathing. This also involves a contraction of the diaphragm, but the lower ribs are stabilized so that when the muscle contracts, rather than the central tendon remaining stable and lifting the ribs up, the central tendon moves down, compressing the sub-thoracic cavity and allowing the thoracic cavity and lungs room to expand downward.

These actions produce an increase in volume, and a resulting partial vacuum, or negative pressure, in the thoracic cavity, resulting in atmospheric pressure pushing air into the lungs, inflating them. An exhalation results when the diaphragm and intercostal muscles relax, and elastic recoil of the rib cage and lungs expels the air.

The circumference of the normal adult human rib cage expands by 3 to 5 cm during inhalation.