Boundary between the upper and lower respiratory tract
Air inhaled through the nasal or oral cavity sequentially passes through the pharynx and larynx, after which it enters the trachea. The larynx serves as a conventional border: everything located at its level and above belongs to the upper respiratory tract, while structures below the larynx form the lower respiratory tract. It is the lower division that includes the trachea, the bronchial tree, and the respiratory structures of the lung.
Division of the trachea into main bronchi
The trachea descends in the mediastinum and, at the level of the V thoracic vertebra, divides in the bifurcation area into two main bronchi:
- Right main bronchus — bronchus principalis dexter: shorter, wider, and positioned almost vertically;
- Left main bronchus — bronchus principalis sinister: longer, narrower, and runs more horizontally.
The anatomical features of the right main bronchus are of clinical significance: in foreign body aspiration, the foreign body is much more likely to enter the right main bronchus.
Orders of branching of the bronchial tree
The main bronchi give rise to successive dichotomous branching, where each subsequent level is designated by a specific order:
- Lobar bronchi (bronchi lobares) — second-order bronchi. Their number corresponds to the number of lobes: three on the right, two on the left.
- Segmental bronchi (bronchi segmentales) — third-order bronchi. Each lung has 10 bronchopulmonary segments and, accordingly, 10 segmental bronchi.
- Subsegmental bronchi (bronchi subsegmentales) — branches approximately from the 4th to the 14th order.
- Lobular bronchi (bronchi lobulares) — 15th-order bronchi entering a lung lobule.
- Terminal bronchioles (bronchioli terminales) — the final structures of the conducting zone.
Difference between bronchi and bronchioles
The main histological feature distinguishing a bronchus from a bronchiole is the presence of a cartilaginous framework in the wall. In the trachea and main bronchi, cartilage is represented by C-shaped rings; as the caliber of the airways decreases, the cartilage tissue fragments into individual plates, and its amount progressively decreases. Starting from the level of the terminal bronchiole, cartilage is completely absent in the wall: the wall is formed by smooth muscle elements, epithelium, and the basement membrane.
Conducting and respiratory zones
The lower respiratory tract is functionally divided into two parts:
- Conducting zone — ensures only the conduction, warming, and purification of air. Includes all structures up to and including the terminal bronchiole. Gas exchange does not occur here.
- Respiratory zone — responsible for gas exchange. Begins below the terminal bronchiole.
The terminal bronchiole divides into respiratory bronchioles, which lead into alveolar ducts (ductuli alveolares), ending in alveolar sacs (sacculi alveolares).
Acinus and alveolar structure
The structural and functional unit of the lung is the acinus. It comprises a respiratory bronchiole with all its branches, alveolar ducts, and alveolar sacs. The acinus is densely entwined with a network of blood capillaries, providing the conditions for gas exchange.
Alveoli (alveoli pulmonis) are outpouchings of the walls of respiratory bronchioles, alveolar ducts, and sacs. They significantly increase the surface area for respiration. The alveolar wall contains:
- Type I alveolar cells (type I pneumocytes) — flat cells through which gas diffusion takes place;
- Type II alveolar cells (type II pneumocytes) — secretory cells producing surfactant, a surface-active agent that prevents alveolar collapse during expiration and facilitates their expansion during inspiration.
Air-blood barrier
Air-blood barrier — the structure through which oxygen and carbon dioxide diffuse between the alveolar air and capillary blood. It includes:
- type I pneumocyte on the alveolar side;
- shared (fused) basement membrane;
- capillary endothelial cell on the vascular side.
The small thickness and large total surface area of the barrier ensure efficient gas exchange in the lungs.
Key points
- The boundary between the upper and lower respiratory tract runs at the level of the larynx.
- The trachea divides into the right and left main bronchi at the level of the V thoracic vertebra; the right one is shorter, wider, and more vertical.
- Bronchial branching: main → lobar → segmental → subsegmental → lobular → terminal bronchioles.
- Cartilage disappears in the wall at the level of the terminal bronchiole — this is the anatomical landmark between a bronchus and a bronchiole.
- The conducting zone ends with the terminal bronchiole; the respiratory zone begins with the respiratory bronchiole.
- The acinus is the structural and functional unit of the lung providing gas exchange.
- Type II pneumocytes produce surfactant, preventing alveolar collapse.
- The air-blood barrier is formed by the type I pneumocyte, basement membrane, and capillary endothelium.