Gas exchange and the problem of airborne antigens
The lungs perform the vital function of gas exchange: oxygen (O₂) enters the blood from atmospheric air, and carbon dioxide (CO₂) is eliminated. However, along with the air, various antigens—microorganisms, pollen, dust particles, allergens—enter the respiratory tract. A fundamental challenge arises: to ensure the free passage of gases while simultaneously preventing foreign agents from penetrating the body's internal environment.
This challenge is addressed at the microstructural level of the respiratory division of the lungs. Defense is based on two complementary mechanisms: the blood-air barrier and the surfactant system.
Structure of the respiratory division
The terminal structural unit of gas exchange is the alveolus. Its wall is formed by epithelial tissue resting on a basement membrane. The alveolar epithelium contains two types of cells:
- Type I alveolar cells (respiratory) — flat cells with thin cytoplasm through which gas diffusion occurs;
- Type II alveolar cells (secretory) — prismatic cells that produce the liquid part (hypophase) of surfactant.
Adjacent to the alveolus is a blood capillary lined with endothelial cells on their own basement membrane. The basement membranes of the alveolar epithelium and capillary endothelium have a similar structure and can fuse in areas of maximal contact, ensuring efficient gas diffusion.
Air-blood barrier
The blood-air (alveolar-capillary) barrier is the set of structures separating the alveolar air from the capillary blood. Its components include:
- surfactant lining the alveolus from the inside;
- cytoplasm of a type I alveolar cell with its basement membrane;
- common or fused basement membrane;
- cytoplasm of the capillary endothelial cell;
- alveolar macrophage, which performs phagocytosis of antigens that have penetrated the alveolar lumen.
Gas exchange across this barrier occurs via simple diffusion down a partial pressure gradient: O₂ moves into the blood, and CO₂ moves into the alveolar lumen. Alveolar macrophages belong to the innate immunity system and represent the first line of cellular defense in the respiratory tract.
Surfactant: composition and origin
Surfactant is a surface-active substance lining the alveoli from the inside. It consists of two phases:
- Hypophase — the liquid part produced by type II alveolar cells;
- Membrane phase — a denser surface film in the formation of which Clara cells (bronchiolar exocrinocytes) located in terminal bronchioles take part.
Chemically, surfactant is a complex of phospholipids and specific proteins (surfactant-associated proteins).
Functions of surfactant
The surfactant system performs three key functions:
- Barrier (anti-adhesive for antigens): antigens contacting the membrane phase of surfactant cannot penetrate to the surface of the alveolar cell and further into the blood;
- Prevention of plasma transudation: surfactant prevents the liquid part of the blood from leaking into the alveolar lumen, which would otherwise create a nutrient medium for microorganisms and impair breathing;
- Maintenance of alveolar expansion: by reducing surface tension, surfactant prevents alveolar collapse (atelectasis) during expiration.
In addition, surfactant proteins SP-A and SP-D possess opsonic properties, facilitating the phagocytosis of microorganisms by alveolar macrophages.
Key points
- Lung defense against airborne antigens is provided by two mechanisms: the blood-air barrier and the surfactant system.
- The blood-air barrier includes surfactant, type I alveolar cells, the common basement membrane, capillary endothelial cells, and alveolar macrophages.
- Gas exchange between air and blood is carried out by simple diffusion.
- Type II alveolar cells produce the hypophase of surfactant, and Clara cells produce components of the membrane phase.
- Surfactant prevents antigen penetration, stops plasma from leaking into the alveoli, and prevents their collapse.
- Alveolar macrophages are a key link in the innate immunity of the lungs, performing phagocytosis of foreign particles.
- The structural commonality of the basement membranes of alveolar cells and endothelial cells is the morphological basis for efficient gas exchange.