The principal physiologic role of the lungs is to make oxygen available
to tissues for metabolism and to remove the main byproduct of that
metabolism, carbon dioxide. The lungs perform this function by moving
inspired air into close proximity to the pulmonary capillary bed
to enable gas exchange by simple diffusion. This is accomplished
at a minimal workload, is regulated efficiently over a wide range
of metabolic demand, and takes place with close matching of ventilation
to lung perfusion. The extensive surface area of the respiratory
system must also be protected from a broad variety of infectious
or noxious environmental insults.
Humans possess a complex and efficient respiratory system that
satisfies these diverse requirements. When injury to components
of the respiratory system occurs, the integrated function of the
whole is disrupted. The consequences can be profound. Airway injury
or dysfunction results in obstructive lung diseases, including bronchitis
and asthma, whereas parenchymal lung injury can produce restrictive
lung disease or pulmonary vascular disease. To understand the clinical presentations
of lung disease, it is necessary first to understand the anatomic
and functional organization of the lungs that determines normal
- 1. What are the two principal
physiologic roles of the lungs?
- 2. What are the requirements for successful
The mature respiratory system consists of visceral pleura-covered
lungs contained by the chest wall and diaphragm, the latter serving
under normal conditions as the principal bellows muscle for ventilation.
The lungs are divided into lobes, each demarcated by intervening
visceral pleura. Each lung possesses an upper and lower lobe; the
middle lobe and lingula are the third lobes in the right and left
lungs, respectively. At end expiration, most of the volume of the
lungs is air (Table 9–1), whereas
almost half of the mass of the lungs is accounted for by blood volume.
It is a testament to the delicate structure of the gas-exchanging region
of the lungs that alveolar tissue has a total weight of only 250
g but a total surface area of 75 m2.
Table 9–1 Components of Normal Human Lung. |Favorite Table|Download (.pdf)
Table 9–1 Components of Normal Human Lung.
|Component||Volume (mL) or Mass (g)||Thickness (μm)|
|Gas (functional residual capacity)||2400|
Connective tissue fibers and surfactant serve to maintain the
anatomic integrity of this large and complex surface area. The connective
tissue fibers are highly organized collagen and elastic structures.
They radiate into the lungs, dividing segments, investing airways
and vessels, and supporting alveolar walls with a very elastic and
delicate fibrous network. The ...