Respiration is a general term used to describe gas exchange within the body and can be categorized as either external respiration or internal respiration. Basic terms are described here but an in-depth discussion of respiratory physiology, including diffusion and perfusion, goes well beyond the scope or purpose of this chapter. The reader is referred to several references for further study.

External respiration describes the exchange of gas at the alveolar-capillary membrane and the pulmonary capillaries. When a person inhales and oxygen is delivered to the alveoli via the tracheobronchial tree, oxygen diffuses through the alveolar wall and interstitial space and into the bloodstream through the pulmonary capillary walls. The opposite occurs with carbon dioxide transport. Internal respiration describes the exchange of gas between the pulmonary capillaries and the cells of the surrounding tissues. Internal respiration occurs when oxygen in arterial blood diffuses from red blood cells into tissues requiring oxygen for function. The reverse occurs with carbon dioxide transport.

Ventilation, as it refers to the respiratory system, is the mass exchange of air to and from the body during inspiration and expiration. This cyclic process requires coordinated ventilatory muscle activity, rib cage movements, and appropriate structure and function of the upper and lower respiratory tracts.

Thorax and Chest Wall: Structure and Function

The main functions of the thoracic cage, also referred to as the chest wall, are to protect the internal organs of respiration, circulation, and digestion and to participate in ventilation of the lungs. The thoracic cage provides the site of attachment for the muscles of ventilation to mechanically enlarge the thorax for inspiration or to compress the thorax for expiration. It is also the site of attachment of upper extremity muscles, which function during lifting, pulling, or pushing activities. These activities usually are carried out in conjunction with inspiratory effort. Along the posterior aspect of the thorax, the dorsal portions of the ribs articulate with the 12 thoracic vertebrae at the costotransverse and costovertebral joints. Along the anterior aspect of the thoracic cage, the first to seventh ribs articulate directly with the sternum via the costal cartilage. The eighth to tenth ribs have cartilaginous attachments to the rib above, whereas the eleventh and twelfth are floating ribs.

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