Anatomy of the Respiratory System

Anatomical division of the airways

The conducting airways deliver air to the lungs while performing air conditioning: purification, warming, and humidification. Topographically, they are typically divided into two major sections:

• Upper respiratory tract: includes the nasal cavity and pharynx (nasal and oral parts). The boundary of this section is the laryngeal inlet.
• Lower respiratory tract: begins at the larynx, transitions into the trachea, and continues as a branching system of extra- and intrapulmonary bronchi (bronchi) down to the terminal bronchioles.

Morphofunctional zones and pharmacology

From a functional and histological standpoint, the respiratory system is divided into conducting and gas-exchange parts. This division is crucial for the design of inhalation drugs.

Conducting airways are lined with ciliated epithelium and a well-defined mucous layer. Absorption of substances here is limited. This zone serves as a target for locally acting inhalers (bronchodilators, glucocorticosteroids). Large aerosol particles (over 10 µm) settle precisely here, minimizing systemic effects.

Respiratory zone is responsible for gas exchange. Its structural and functional unit is the pulmonary acinus. Here, bronchioles transition into alveolar ducts (ductuli alveolares) and alveoli (alveoli pulmonis).

Alveolar-capillary barrier

Gas exchange and systemic absorption occur across an ultrathin barrier, the structure of which includes three layers:

1. Type I alveolar cells (thin squamous epithelium).
2. Epithelial basement membrane, closely apposed or fused with the capillary basement membrane.
3. Blood capillary endothelium.

Due to the minimal diffusion thickness, vast surface area, and rich pulmonary circulation, the absorption rate here is comparable to intravenous administration. Particles sized 1–5 µm reach the alveoli, ensuring rapid systemic action while avoiding first-pass hepatic metabolism.

Lymphatic system of the lungs and mediastinum

Lymph drainage from the lungs (vasa lymphatica pulmonum) is carried out via two interconnected networks:

• Superficial vessels: lie within the visceral pleura. They drain lymph from the narrow- and wide-mesh pleural capillary networks. Some vessels head toward the lymph nodes of the pulmonary hilum (hilum pulmonis), while others penetrate the parenchyma and anastomose with the deep network.
• Deep vessels: originate in the interalveolar septa and the submucosa of the bronchi, forming plexuses along the blood vessels and bronchial tree.

Regional lymph nodes include bronchopulmonary, paratracheal, brachiocephalic, and deep lateral cervical nodes. Notably, lymph from the left side of the esophagus collects in the thoracic duct (ductus thoracicus), which empties into the left venous angle, while lymph from the right side drains into the right lymphatic duct.

Age-related features in children

In infants and young children, the anatomy and biomechanics of the respiratory system have several critical features that determine the severity of diseases:

• Nasal passages are anatomically short and narrow. This creates a high risk of nasal breathing obstruction even with mild rhinitis.
• Bronchioles have a significantly smaller lumen compared to adults. Mucosal edema rapidly causes bronchial obstruction and collapse of lung tissue (atelectasis).
• Biomechanics: the ribs are positioned horizontally, and the respiratory muscles are poorly developed. This severely limits the child's ability to increase tidal volume during physical exertion or pathology.

Class and exam questions

What else to study on this topic

• Non-respiratory functions of the respiratory system
• Topography of the mediastinal and cervical lymph nodes
• Anastomoses between the superficial and deep lymphatic vessels of the lungs
• Respiratory biomechanics and the role of respiratory muscles
• Histological structure of the bronchial mucosa

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