The surfactant system of the adult lung: physiology and clinical perspectives

Abstract

Pulmonary surfactant is synthesized and secreted by alveolar type II cells and constitutes an important component of the alveolar lining fluid. It comprises a unique mixture of phospholipids and surfactant-specific proteins. More than 30 years after its first biochemical characterization, knowledge of the composition and functions of the surfactant complex has grown considerably. Its classically known role is to decrease surface tension in alveolar air spaces to a degree that facilitates adequate ventilation of the peripheral lung. More recently, other important surfactant functions have come into view. Probably most notable among these, surfactant has been demonstrated to enhance local pulmonary defense mechanisms and to modulate immune responses in the alveolar milieu. These findings have prompted interest in the role and the possible alterations of the surfactant system in a variety of lung diseases and in environmental impacts on the lung. However, only a limited number of studies investigating surfactant changes in human lung disease have hitherto been published. Preliminary results suggest that surfactant analyses, e.g., from bronchoalveolar lavage fluids, may reveal quantitative and qualitative abnormalities of the surfactant system in human lung disorders. It is hypothesized that in the future, surfactant studies may become one of our clinical tools to evaluate the activity and severity of peripheral lung diseases. In certain disorders they may also gain diagnostic significance. Further clinical studies will be necessary to investigate the potential therapeutic benefits of surfactant substitution and the usefulness of pharmacologic manipulation of the secretory activity of alveolar type II cells in pulmonary medicine.

Identification of Early Acute Lung Injury at Initial Evaluation in an Acute Care Setting Prior to the Onset of Respiratory Failure.

“…Because up to 30% of patients with ALI may have concomitant volume overload, 5 patients with suspected left atrial hypertension (by the criteria just listed) were eligible if they had these admission diagnoses: pneumonia (defined by focal airspace opacities on CXR or purulent sputum and an abnormal temperature [< 36°C or > 38°C] or WBC count > 12,000, <4,000, or > 10% bands) or sepsis (defined by criteria for the systemic inflammatory response syndrome [SIRS] 21 and a known infectious etiology. Additional details about our inclusion and exclusion criteria are available in the online supplement.…”

Immunological functions of the pulmonary epithelium

“…Their apical surface is covered with microvilli and their cytoplasm is notable for numerous lamellated inclusions. The lamellated inclusions are composed of lipids (predominantly phospholipids) and proteins [37][38][39][40]. They are secreted onto the apical surface of the alveolar epithelium to form surfactant.…”

Surface rheology of monolayers of phospholipids and cholesterol measured with axisymmetric drop shape analysis

“…Rheological properties of phospholipid monolayers are of fundamental importance for the physiology of breathing, since the fluid layer on the alveolar lining is covered by monolayers consisting of almost 90% phospholipids [1]. Consequences of surface rheology on respiratory performance have been discussed in detail [2][3][4][5].…”