Pulmonary surfactant
Introduction
Pulmonary surfactant is a complex mixture of lipids and proteins that is secreted into the alveolar space by type II alveolar cells. The primary function of pulmonary surfactant is to lower the surface tension at the air/liquid interface within the lung, thereby preventing end-expiratory alveolar collapse. This article will delve into the composition, synthesis, and function of pulmonary surfactant, as well as its clinical significance.
Composition
Pulmonary surfactant is composed primarily of lipids (approximately 90% by weight), with the remainder being proteins. The lipid component is predominantly phospholipids, with phosphatidylcholine (specifically dipalmitoylphosphatidylcholine or DPPC) being the most abundant. Other phospholipids present include phosphatidylglycerol and phosphatidylinositol. The protein component of surfactant consists of four surfactant-associated proteins, SP-A, SP-B, SP-C, and SP-D. SP-B and SP-C are small, hydrophobic proteins that play a crucial role in the adsorption and spreading of surfactant at the air-water interface, while SP-A and SP-D are large, hydrophilic proteins involved in the regulation of surfactant metabolism and host defense.
Synthesis and Secretion
Pulmonary surfactant is synthesized and secreted by type II alveolar cells. The synthesis of surfactant begins with the synthesis of its lipid and protein components in the endoplasmic reticulum. The newly synthesized surfactant lipids and proteins are then packaged into specialized organelles called lamellar bodies. These lamellar bodies are secreted into the alveolar space via exocytosis, where they unroll to form a lipid-protein film at the air-water interface.
Function
The primary function of pulmonary surfactant is to lower the surface tension at the air/liquid interface within the lung. This is achieved by the adsorption and spreading of surfactant lipids and proteins at the air-water interface, which results in a reduction in surface tension. This reduction in surface tension prevents end-expiratory alveolar collapse, thereby maintaining alveolar stability and facilitating gas exchange. In addition to its role in maintaining alveolar stability, pulmonary surfactant also plays a role in host defense. The surfactant proteins SP-A and SP-D, in particular, have been shown to have immunomodulatory properties, including the ability to enhance phagocytosis and to modulate inflammatory responses.
Clinical Significance
Pulmonary surfactant plays a critical role in lung function, and deficiencies or abnormalities in surfactant can lead to respiratory distress. One of the most well-known conditions associated with surfactant deficiency is neonatal respiratory distress syndrome (NRDS), which occurs in premature infants due to the insufficient production of surfactant. Treatment for NRDS often includes the administration of exogenous surfactant. Surfactant abnormalities can also occur in adults, leading to conditions such as acute respiratory distress syndrome (ARDS) and pulmonary alveolar proteinosis.