Respiratory organs

Diagnostic search strategy

Any examination begins with a thorough patient history. The clinician's main goal at this stage is topical diagnosis, i.e., establishing the precise localization of the pathological process. Physicians distinguish three main lesion sites:

• Airways.
• Lung parenchyma.
• Pleura.

In practice, a combined lesion of several compartments is frequently encountered. For example, in focal pneumonia, inflammation usually starts in the bronchi (bronchitis develops) and then spreads to the peribronchial tissue, forming bronchopneumonia. In lobar (croupous) pneumonia, involvement of an entire parenchymal lobe is almost always accompanied by inflammation of the pleural leaves—pleurisy.

In addition to searching for organic changes, the modern approach requires mandatory analysis of functional disorders, for which pulmonary function testing (PFT) is performed.

The role of traditional physical examination

Despite enormous technological progress, classical diagnostic methods developed by R. Laennec retain paramount importance. They have serious advantages over instrumental approaches.

• What X-rays miss: Dry pleurisy, chronic bronchitis, or bronchial patency disorders (bronchobstructive syndrome) may not be visualized on images. However, these pathologies are successfully detected physically, particularly via auscultation.
• Limitations of the method: Physical methods are powerless in the early stages of certain dangerous diseases. For example, in tuberculosis or bronchogenic carcinoma, classical symptoms appear only in late stages; therefore, highly sensitive instrumental examinations are necessary for their early diagnosis.

Anatomical and physiological defense barriers

Understanding the pathogenesis of pulmonary diseases is impossible without knowing how the respiratory organs protect themselves against dust, microorganisms, and toxins. There are several key defense levels:

1. Anatomical and physiological barriers: The larynx, epiglottis, and architectonics of the bronchial tree (gradual narrowing of lumens and multiple branchings) prevent deep particle penetration. The epithelial barrier and rich vascularization of the mucous membrane play an important role.
2. Reflex mechanisms: Cough and sneeze reflexes provide emergency evacuation of mucus and foreign bodies.
3. Mucociliary clearance: The ciliated epithelium beats rhythmically, moving mucus with trapped pathogens upward (toward the upper respiratory tract). If pathology occurs (e.g., bronchitis), the ciliary beating activity increases to ensure the clearance of excess sputum.
4. Secretory and humoral factors: Production of tracheobronchial secretion containing lysozyme and alpha-1-antitrypsin. Local immune defense is realized through the synthesis of immunoglobulins (predominantly IgA) by plasma cells.

Cellular immunity and the role of surfactant

At the level of terminal bronchioles, alveolar ducts, and alveoli, a specialized defense system operates. Its central element is the alveolar macrophage. It is the first to interact with inhaled agents, performing phagocytosis and releasing aggression factors: proteases, cytokines, and oxygen radicals. Macrophages possess both antimicrobial and antitumor activity.

In addition to macrophages, the following participate in defense:

• Neutrophils: Migrate from the capillaries of the interalveolar septa directly into the alveoli to destroy bacteria.
• Antigen-presenting cells (APCs): Dendritic cells and Langerhans cells.
• T lymphocytes: Release lymphokines that enhance macrophage bactericidal activity.
• B lymphocytes: Differentiate into plasma cells to synthesize antibodies (IgG, IgE).

A special place is occupied by surfactant. It performs a barrier function (envelops dust and infectious agents, preventing their direct contact with alveolar epithelial cells), activates alveolar macrophages, regulates their migration from the interstitium, and participates in opsonization (preparing bacteria for phagocytosis).

Five pathways of microorganism elimination

According to the pulmonary defense scheme, the removal of pathogens is realized through five main processes:

1. Mechanical removal: Deposition of microorganisms in the mucous film and their evacuation via mucociliary clearance.
2. Macrophage phagocytosis: Uptake of foreign agents by alveolar macrophages with subsequent removal of "loaded" cells via the clearance system.
3. Neutrophil phagocytosis: Uptake of pathogens by polymorphonuclear leukocytes directly in the alveolar lumen.
4. Humoral support (opsonization): Activation of complement components from the interalveolar septa via the alternative pathway. The C3b opsonin is formed, facilitating the capture of microbes by phagocytes.
5. Initiation of immune response (sensitization): Transport of phagocytosed microorganisms inside macrophages or neutrophils to lymph nodes, where specific immune reactions are triggered.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Methodology for assessing pulmonary function testing (PFT) and its clinical interpretation.
• The role of surfactant in opsonization and regulation of macrophage migration.
• Interaction of T lymphocytes and B lymphocytes in the pulmonary defense system.
• Mechanisms of alternative pathway complement activation in the alveoli.
• Features of the production and function of secretory IgA.

Other topics in the "Respiratory System" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.