Pulmonary surfactant is a substance within the respiratory system that plays a role in breathing. This complex mixture facilitates the exchange of oxygen and carbon dioxide. Without its proper function, the lungs would struggle to perform their duties efficiently. Understanding pulmonary surfactant provides insight into how the lungs inflate and deflate with ease.
Composition and Location of Lung Surfactant
Lung surfactant is a complex biological substance primarily composed of lipids and proteins. Approximately 90% of its content consists of lipids, with dipalmitoylphosphatidylcholine (DPPC) being the most abundant and functionally important phospholipid, making up about 40% of the surfactant by weight. DPPC, along with other phospholipids and neutral lipids like cholesterol, forms a film at the air-liquid interface within the lungs.
The remaining 10% of surfactant is made up of specialized surfactant proteins (SPs), specifically SP-A, SP-B, SP-C, and SP-D. SP-B and SP-C contribute to the surface tension-lowering properties, while SP-A and SP-D are involved in the lung’s defense mechanisms against pathogens. Surfactant is produced and secreted by Type II alveolar cells, located in the walls of the alveoli. These cells store surfactant in structures called lamellar bodies before releasing it to line the inner surface of the alveoli.
The Role of Surfactant in Breathing
The primary function of lung surfactant is to reduce the surface tension at the air-liquid interface within the alveoli. Surface tension is an inward-pulling force exerted by liquid molecules that would otherwise cause the air sacs to collapse. By interfering with these attractive forces, surfactant lowers the surface tension from approximately 70 dyn/cm (70 mN/m) to very low, near-zero levels during exhalation.
This reduction in surface tension is important for efficient breathing. It prevents the complete collapse of the alveoli at the end of exhalation, ensuring they remain slightly open and stable. Surfactant also makes it easier for the lungs to inflate during inhalation, as less pressure is needed to overcome the collapsing forces. This increased ease of inflation, known as compliance, reduces the muscular effort required for breathing.
Health Conditions Linked to Surfactant Problems
Problems with lung surfactant can lead to respiratory difficulties. An example is Infant Respiratory Distress Syndrome (IRDS), often seen in premature infants. Premature babies are vulnerable because their lungs may not have developed sufficiently to produce adequate amounts of surfactant. Without enough surfactant, their alveoli collapse with each breath, requiring effort to re-inflate, leading to symptoms like rapid breathing, grunting, and nasal flaring.
While IRDS is a neonatal condition, surfactant dysfunction can also contribute to respiratory issues in adults. Acute Respiratory Distress Syndrome (ARDS), a lung injury, can involve changes in surfactant composition and function. In ARDS, inflammation and fluid accumulation can inactivate or dilute existing surfactant, impairing its ability to reduce surface tension and leading to widespread alveolar collapse. Rare genetic mutations affecting surfactant proteins can also cause lung diseases in both newborns and adults, ranging from interstitial lung disease to pulmonary fibrosis.
Medical Interventions for Surfactant Deficiency
Medical interventions for surfactant deficiency involve surfactant replacement therapy (SRT). This treatment delivers exogenous (from outside the body) surfactant directly into the lungs. The administered surfactant can be derived from animal sources, such as bovine or porcine lungs, or be synthetically manufactured to mimic natural surfactant’s properties.
SRT is administered through a tube inserted into the trachea, allowing the liquid surfactant to spread throughout the lungs and coat the alveoli. This therapy improves lung function by reducing surface tension, which helps the alveoli remain open and facilitates gas exchange. For premature infants with IRDS, SRT has reduced mortality rates and the need for mechanical ventilation, improving overall outcomes. While primarily used in neonates, it has also been explored, though with inconsistent results, for conditions like ARDS in adults.