Question

Describe the formation, composition, and function of pulmonary surfactant. What happens when surfactant is absent? How is this condition treated?

Short answer

Pulmonary surfactant, a mixture of lipids and proteins, is synthesized and secreted by type II alveolar cells and reduces surface tension in the alveoli to prevent collapse and facilitate breathing. Composed of 70-80% lipids and 20-30% proteins, its main lipid component is dipalmitoylphosphatidylcholine (DPPC), while proteins are divided into hydrophilic (SP-A, SP-D) and hydrophobic (SP-B, SP-C) groups. When surfactant is absent, as in neonatal respiratory distress syndrome (NRDS), alveolar collapse and breathing difficulties occur. The primary treatment is exogenous surfactant administration, along with possible mechanical ventilation, oxygen therapy, and medications to support lung function.

Step-by-step solution

Introduction to Pulmonary Surfactant

Pulmonary surfactant is a naturally occurring mixture of lipids and proteins that is produced by specialized alveolar cells in the lungs. It plays a crucial role in reducing the surface tension at the air-liquid interface of the alveolar spaces, allowing for easier breathing and preventing the collapse of the alveoli.

Formation of Pulmonary Surfactant

Pulmonary surfactant is synthesized and secreted by type II alveolar cells (also called pneumocytes). These cells are found in the epithelial lining of the alveoli. Surfactant formation begins during fetal development and continues throughout life. The surfactant precursors are packaged into lamellar bodies within the type II alveolar cells and are secreted into the alveolar space, where they form a thin layer on the surface of the alveolar lining fluid.

Composition of Pulmonary Surfactant

Pulmonary surfactant is composed of approximately 70-80% lipids and 20-30% proteins. The lipids mainly consist of phospholipids, with dipalmitoylphosphatidylcholine (DPPC) being the primary component. The proteins in the surfactant are divided into two major groups: hydrophilic surfactant-associated proteins (SP-A and SP-D) and hydrophobic surfactant-associated proteins (SP-B and SP-C).

Function of Pulmonary Surfactant

The primary function of pulmonary surfactant is to reduce the surface tension at the air-liquid interface in the alveoli. By performing this function, surfactant prevents the alveoli from collapsing during exhalation, and it ensures that the lungs remain expanded, allowing for efficient gas exchange. Surfactant also plays a role in innate immunity by enhancing the clearance of inhaled pathogens and foreign particles.

Surfactant Absence: Consequences and Treatments

When pulmonary surfactant is absent or insufficient, the surface tension in the alveoli becomes very high, leading to alveolar collapse and difficulties in breathing. This condition is known as neonatal respiratory distress syndrome (NRDS) or hyaline membrane disease in premature infants. The primary treatment for NRDS is the administration of exogenous surfactant, which can be extracted from animal sources or created synthetically. Other interventions may include mechanical ventilation support, oxygen therapy, and the use of medications to support lung function.

Key concepts

Neonatal Respiratory Distress Syndrome

Neonatal Respiratory Distress Syndrome (NRDS), also known as hyaline membrane disease, primarily affects premature infants whose lungs haven't yet fully developed. In a healthy full-term baby, pulmonary surfactant is produced in sufficient quantity by type II alveolar cells. However, premature babies may lack this critical substance, leading to NRDS.

Surfactant reduces alveolar surface tension, which is essential for keeping alveoli open during the breathing cycle. Without it, alveoli are prone to collapse, creating a challenge for gas exchange and leading to difficulty in breathing. This can manifest as rapid, shallow breathing and a blue tinge to the skin due to lack of oxygen.

Treatment for NRDS includes the administration of exogenous surfactant and supportive care, such as oxygen therapy or mechanical ventilation, to help the infant breathe while their lungs mature and begin producing their own surfactant.

Alveolar Surface Tension

Alveolar surface tension refers to the force exerted by the liquid lining the alveoli, which can cause the alveoli to contract and potentially collapse—especially when the lungs exhale. Surfactant plays a pivotal role in reducing this tension, promoting lung stability and making it easier for the lungs to inflate and deflate with each breath.

Without an adequate surfactant layer, surface tension would increase, leading to greater resistance against the expansion of the lungs during inhalation. This phenomenon is a critical aspect of the challenges seen in NRDS, as it demands more work from the muscles involved in breathing and can severely impair oxygen exchange.

Type II Alveolar Cells

Type II alveolar cells, or pneumocytes, are crucial for respiratory health because they synthesize and secrete pulmonary surfactant. Aside from producing surfactant, these cells also serve critical roles in the repair and regeneration of the alveolar lining following injury.

These specialized cells store surfactant components in structures known as lamellar bodies. Upon receiving the right signals, type II cells release these contents into the alveolar space, where they spread out to form the protective surfactant layer. Scientists continue to study these cells to understand their full range of functions and potential therapeutic applications.

Dipalmitoylphosphatidylcholine

Dipalmitoylphosphatidylcholine (DPPC) is a phospholipid and the most abundant component of pulmonary surfactant. Its unique molecular structure provides surfactant with its surface tension-reducing properties.

DPPC molecules are amphipathic, meaning they have both a hydrophilic 'head' that interacts with water and a hydrophobic 'tail' that repels water. When organized into a layer at the air-liquid interface of the alveoli, these molecules decrease surface tension effectively, which is a deciding factor in keeping the alveoli open during the respiratory cycle.

Surfactant-Associated Proteins

Surfactant-associated proteins (SPs) play various roles in the function and regulation of pulmonary surfactant. There are four main SPs: SP-A, SP-B, SP-C, and SP-D.

SP-A and SP-D are hydrophilic proteins involved in immune defense. They help the body respond to inhaled pathogens and particles. The hydrophobic proteins, SP-B and SP-C, contribute to the spread and stability of the surfactant layer. They assist DPPC molecules to unfurl at the air-liquid interface, ensuring a uniform and effective surfactant layer. The intricate balance and interactions between these proteins and lipids in surfactant are vital for proper lung function.