Respiration is the vital exchange of oxygen and carbon dioxide as blood circulates through the lungs. The thoracic skeleton, thoracic wall muscles, bronchial tree, and pulmonary circulation all aid in this process. Breathing is a mechanical process resulting from volume changes in the thoracic cavity with inverse changes in pleural pressure. Pressure changes lead to gas flow.
The process of inspiration (inhalation) is easily understood if you visualize the thoracic cavity as a closed box with a single opening at the top called the trachea. The trachea allows air to move in and out of the thoracic cavity (“the box”). The volume of the thoracic cavity is changeable and can be increased by enlarging all its diameters (superior to inferior, anterior to posterior, and medial to lateral), thereby decreasing the pressure in the pleural space (pleural pressure) (Figure 3-4A and B). This, in turn, causes air to rush from the atmosphere (positive pressure relative to the lungs) into the lungs (negative pressure relative to the atmosphere) because gas flows down its pressure gradient. The muscles that primarily expand the thoracic cavity during inspiration are the diaphragm and the intercostal muscles.
- Diaphragm. In the relaxed state, the diaphragm is dome shaped. When the diaphragm contracts, it flattens, increasing the vertical dimensions and thus the volume of the thoracic cavity.
- Intercostal muscles. Contraction of the external intercostal muscles lifts the rib cage and pulls the sternum anteriorly. Because the ribs curve downward as well as forward around the chest wall, the broadest lateral and anteroposterior dimensions of the rib cage are normally directed downward. However, when the ribs are raised and drawn together, they also swing outward, expanding the diameter of the thorax both laterally and in the anteroposterior plane. This is similar to the action that occurs when a curved bucket handle is raised away from the bucket (Figure 3-4C).
Movements of the thoracic wall during inhalation and exhalation in the anterior (A) and axial superior (B) views. C. Thoracic wall movements during respiration. The bucket and water-pump handle are analogies for the movement of the rib cage when acted upon by respiratory muscles.
Although these actions expand the thoracic dimensions by only a few millimeters along each plane, this expansion is sufficient to increase the volume of the thoracic cavity by approximately 0.5 L, the approximate volume of air that enters the lungs during normal inhalation. The diaphragm is by far the most important structure that brings about the pressure, gas flow, and volume changes that lead to normal inhalation.
As the thoracic dimensions increase during inspiration, pleural pressure becomes more negative and “pulls” on the lungs as thoracic volume increases. The consequence is that the lungs expand (fill with gas) and intrapulmonary volume increases. Inspiration ends when thoracic volume ceases to increase, resulting in no further reduction in pleural pressure. Gas flow ceases and thus lung volume does not change.
During the deep or forced inspirations that occur during vigorous exercise, the volume of the thoracic cavity is further increased by activation of the accessory muscles. Accessory respiratory muscles, including the scalenes, sternocleidomastoid, and pectoralis minor, elevate the ribs more than occurs during quiet inspiration.
Quiet expiration (exhalation) is largely a passive process that depends more on the natural elasticity of the thoracic wall and lungs than on muscle contraction. In contrast, forced expiration is an active process (Figure 3-4A–C).
- Quiet expiration. As the inspiratory muscles relax, the diaphragm ascends, the rib cage descends, and the stretched elastic tissue of the lungs recoils. Thus, both thoracic and lung volumes decrease. Decreased lung volume compresses the alveoli, resulting in increases above atmospheric pressure, thereby forcing gas flow out of the lungs. For example, when the diaphragm relaxes, it passively moves superiorly. Consequently, the vertical dimension of the thorax is decreased and thus the volume of the thoracic cavity decreases.
- Forced expiration. When the expiratory muscles (e.g., the external and internal oblique and transverse and rectus abdominis) contract, they increase intra-abdominal pressure. This forces the abdominal organs superiorly against the diaphragm, raising it. The same muscles depress the rib cage. Both actions forcibly reduce the volume in the thoracic cavity, increasing pleural volume and thence pressure in the lungs, forcing air to move from the lungs and out of the trachea.