Skip to Main Content

++

Ventilation of the lungs allows exchange of gas between blood and atmospheric air. This chapter will describe the anatomy of the lungs and airways and explain the mechanical properties of the lungs and chest wall that affect the amount of air that is exchanged between the atmosphere and alveoli, how this air is distributed within the lungs, diffusion of alveolar gas into the pulmonary circulation, and control of this process. Mechanical properties of the lungs will be described in the context of the elastic and resistive forces that need to be overcome during inspiration to ventilate the lungs, expiratory flow limitation, and the work and energetics of breathing.

++

Major elements of the respiratory system include the chest wall, airways, alveolar–capillary units, pulmonary and bronchial circulations, nerves, and lymphatics. To complete the structure, the pleura is a mesenchymal lining that consists of a visceral layer adhered intimately to the lungs and a parietal layer lining the mediastinum and chest wall.

++

Chest Wall

++

The chest wall is composed of the diaphragm and rib cage. The rib cage is composed of the spine posteriorly, the sternum anteriorly, and the ribs and their cartilaginous attachments to the sternum. The rib cage serves as a framework for attachment of respiratory muscles and their ligaments. Respiratory muscles are classified as inspiratory or expiratory although some of them contribute to both actions (Figure 1–1). Inspiratory muscles include the diaphragm, external intercostals, parasternal internal intercostals, scalenes, and sternocleidomastoids. Expiratory muscles include the abdominal wall muscles and the interosseus internal intercostals.

++
Figure 1–1.
Graphic Jump Location

Schematic of the rib cage and the intercostal muscles. The parasternal intercostal and external intercostal muscles are important in inspiration. The interosseus internal intercostal muscles are expiratory.

++

The diaphragm is the major muscle of inspiration and separates the thoracic and abdominal cavities (Figure 1–2). It is composed of a fibrous central tendon and a muscular layer that inserts into the lower rib cage and spine. The component of the diaphragm that inserts anteriorly and laterally into the rib cage is referred to as the costal diaphragm, whereas the portion that inserts posteriorly is the crural diaphragm. The zone of apposition is that part of the diaphragm that is closely apposed to the inner surface of the lower rib cage; it is sandwiched between the internal surfaces of the rib cage and the abdominal cavity. The zone of apposition represents an area in which the lower rib cage is exposed to abdominal pressure. The diaphragm is innervated by the phrenic nerve that originates from the third, fourth, and fifth cervical nerve roots.

++
Figure 1–2.
Graphic Jump Location

Drawing of the chest and abdomen depicting the position of the diaphragm and the zone of apposition.

++

Contraction ...

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.