Morphological Basis and Air Conditioning
Protection of the bronchopulmonary system is based on structural tissue diversity, various epithelial types, drainage systems, and complex neuroendocrine regulation.
The first defense stage is air conditioning, which takes place in the upper respiratory tract and large bronchi. The inhaled air flow is warmed (or cooled) and humidified.
Simultaneously, mechanical filtration occurs. Large particles are filtered out in the nasal cavity. The depth of penetration of foreign agents strictly depends on their diameter:
- Nasal cavity: particles larger than 50 µm are deposited.
- Trachea: elements from 30 to 50 µm are retained.
- Bronchi: capture dust sized 10–30 µm.
- Bronchioles: filter particles 3–10 µm in size.
- Alveoli: reached by particles 1–3 µm in size.
Larger deposited elements are cleared via the sneezing and coughing reflexes.
Bronchial Mucociliary Clearance
This is the primary clearance mechanism at the level of the bronchi and bronchioles. It consists of two components: mucus secretion and the beating of surface epithelial cilia. Mucus is produced by bronchial glands, goblet cells, and Clara cells.
The mucous blanket is 5–7 µm thick and divided into two functional phases:
- Sol (fluid phase): located at the level of the cilia, providing them with space for unhindered movement.
- Gel (dense phase): located on the surface; inhaled foreign particles adhere precisely to this layer.
The cilia beat rhythmically, propelling the gel layer along with trapped debris toward the trachea, from which secretions are eliminated by coughing. The secretion contains glycoproteins, surfactant, proteases, and non-specific defense factors (interferon, lysozyme, lactoferrin) produced by macrophages and leukocytes.
Impairment of this mechanism (due to cold air, tobacco smoke, drugs, allergens, or Kartagener syndrome) leads to mucus stasis and the development of acute pneumonia. Prostaglandins (PGE1, PGE2), leukotrienes, and the slow-reacting substance of anaphylaxis stimulate ciliary activity.
Alveolar Clearance and Immunity
The respiratory zones (alveoli) lack a mucociliary system and secretions with humoral factors. Here, cellular non-specific defense operates. The primary "sentinels" of this zone are alveolar macrophages, polymorphonuclear leukocytes, eosinophils, and mast cells. They phagocytose small particles and destroy microbes.
Specific immune defense is provided by lymph nodes and bronchus-associated lymphoid tissue (BALT):
- IgA (present in secretions): neutralizes toxins, agglutinates bacteria, and protects the mucosa from viruses.
- IgG (operates in the lower respiratory tract): opsonizes bacteria, lyses gram-negative microflora, neutralizes viruses, and activates the complement system, which accelerates macrophage and granulocyte chemotaxis.