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