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 asthma and chronic obstructive pulmonary disease (COPD). Injury to the pulmonary parenchyma can produce restrictive lung diseases, such as idiopathic pulmonary fibrosis, acute respiratory distress syndrome, and 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 function.
NORMAL STRUCTURE & FUNCTION OF THE LUNGS
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 9–1Components of a normal human lung. ||Download (.pdf) TABLE 9–1 Components of a normal human lung.
|Component ||Volume (mL) or Mass (g) ||Thickness (μm) |
|Gas (functional residual capacity) ||2400 mL || |
|Tissue ||900 g || |
| Blood ||400 g || |
| Lung ||500 g || |
| Support structures ||225 g || |
| Alveolar walls ||275 g || |
| Epithelium ||60 g ||0.18 |
| Endothelium ||50 g ||0.10 |
| Interstitium ||110 g ||0.22 |
| Alveolar macrophages ||55 g || |