Organization of Air-Conducting Pathways
The respiratory tract sequentially conducts air, purifying, humidifying, and warming it throughout its length. The system is divided into two major blocks:
1. Extrapulmonary pathways:
- Nasal cavity: begins with the vestibule, and the proper cavity is divided into respiratory and olfactory zones.
- Pharynx: the site of intersection with the digestive tract (includes nasal, oral, and laryngeal parts).
- Tubular organs: larynx, trachea, and main bronchi.
- Large bronchi: lobar and zonal (1st and 2nd order). Zonal bronchi partially lie within the lung tissue.
2. Intrapulmonary pathways (bronchial tree): Classification is based on the caliber of structures:
- Medium bronchi: segmental (10 on the left, 11 on the right) and subsegmental (diameter 5–2 mm).
- Small bronchi: diameter 2–1 mm.
- Terminal bronchioles: the concluding link with a diameter of about 0.5 mm.
Respiratory Division (Acinus)
The acinus is the structural and functional unit of the lung, whose main task is gas exchange. The key distinguishing feature of the respiratory division from the air-conducting pathways is the presence of alveoli in the walls.
Air inside the acinus moves in a strictly sequential manner:
- Respiratory bronchioles (have three orders of branching).
- Alveolar ducts.
- Alveolar sacs.
- Alveoli (terminal gas-exchange elements).
Features of Lung Blood Supply
The lungs have a unique feature: two vascular systems function simultaneously within them, anastomosing with each other at the level of arterioles and venules.
- System of bronchial vessels (systemic circulation): provides trophics. Bronchial arteries carry arterial blood from the aorta, nourishing the bronchial walls and parenchyma. Venous blood outflow goes via bronchial veins into the superior vena cava system.
- System of pulmonary vessels (pulmonary circulation): ensures gas exchange. Pulmonary arteries carry venous (oxygen-poor) blood from the right ventricle to the alveolar capillaries. After oxygenation, arterial blood flows out through four pulmonary veins into the left atrium.
Development and Histological Structure
The respiratory system develops from the ventral wall of the pharyngeal part of the foregut. Development proceeds from proximal to distal zones. Epithelial cords grow into the mesenchyme, branch dichotomously, and only then do lumens form within them.
Sources of tissues:
- Endoderm: epithelial lining.
- Mesenchyme: cartilaginous framework, connective tissue, smooth muscles.
- Myotomes: skeletal musculature of the larynx.
- Splanchnotome: pleura mesothelium.
General structural plan of the pathways: Up to the level of small bronchi, the wall has common features. The inner lining is represented by the mucous membrane with pseudostratified ciliated epithelium. This is followed by the submucosa, and the cartilaginous (or bony) skeleton performs the supporting function.
Functions of the Respiratory System
In addition to conducting air and gas exchange, the system performs a number of vital tasks:
- Conditioning: nasal hairs and mucociliary clearance (ciliary action) purify the air. Goblet cells and submucosal glands humidify it, while the dense vascular network warms it.
- Immune defense: pharyngeal tonsils (Waldeyer's ring), bronchial-associated lymphoid tissue (BALT), and immunoglobulins (IgA) on the epithelial surface.
- Detoxification: Clara cells in terminal bronchioles neutralize toxins and possess antimicrobial activity.
- Metabolic functions of the lungs: vascular endothelial cells participate in hemostasis (synthesis of thromboplastin and heparin). The breathing pattern regulates heat exchange and the acid-base balance of the blood.
- Specific functions: voice production (larynx), olfaction (upper part of the nasal cavity), and regulation of bronchial lumen by endocrinocytes.
Briefly: answers to topical questions
What cell types are part of the pseudostratified ciliated epithelium of the trachea and bronchi?
The pseudostratified ciliated epithelium of the trachea and bronchi includes several cell types.
Main cells of the proximal regions:
- Ciliated cells — the ciliary apparatus is characteristic of cells whose nuclei form the most superficial row; cilia are directed toward the tracheal lumen.
- Goblet (mucous) cells.
- Intercalated cells, including long intercalated cells — transitional forms of differentiation; develop from basal cells into ciliated or goblet cells, their nuclei forming the middle row.
Additional (minor) cells:
- Endocrinocytes.
- Langerhans cells.
- M cells.
What structures make up the air-blood barrier in the alveoli?
The air-blood barrier in the alveoli ensures gas exchange and consists of the following structures (from the alveolar lumen to the capillary lumen):
- Surfactant complex (membrane phase + hypophase).
- Type I alveolar cell (thinned cytoplasmic part).
- Basement membrane, which is common to both the epithelium and endothelium (sometimes separated by a layer of loose connective tissue).
- Capillary endothelial cell (thinned cytoplasmic part).
What types of alveolar cells form the alveolar wall and what are their functions?
The alveolar wall is formed by two main types of alveolar cells (pneumocytes):
- Type 1 alveolar cells (respiratory) — flat cells covering up to 95% of the inner alveolar surface. They perform gas exchange and are a key component of the blood-air barrier.
- Type 2 alveolar cells (secretory/large) — cover 5% of the surface area. They do not participate directly in gas exchange. Their functions include secretion (releasing phospholipid body contents to form surfactant) and cambial activity (serving as a source for alveolar epithelium regeneration).
Neuroendocrine cells (type III pneumocytes) are also present in the epithelium.
The acronym RAAA helps to remember the sequence of acinus structures: Respiratory bronchioles -> Alveolar ducts -> Alveolar sacs -> Alveoli.
Class and exam questions
Where do the air-conducting passages end and gas exchange begin?
The air-conducting passages end at the terminal bronchioles (about 0.5 mm in diameter). Respiratory bronchioles begin immediately beyond them, with alveoli appearing in their walls—this is where gas exchange starts.
What is mucociliary clearance?
This is a mechanism of "fine" purification of inhaled air, carried out through the coordinated beating of epithelial cilia that propel mucus with trapped particles outward.
What causes the formation of bronchovascular bundles in the lungs?
In the lungs, 6 tubular systems run in parallel: bronchi, systemic vessels (bronchial), pulmonary vessels, and lymphatic vessels. Running side by side, they unite into single anatomical bundles.
What else to study on this topic
- Cellular composition of pseudostratified ciliated epithelium
- Histophysiology of Clara cells (club cells)
- Morphology of the blood-air barrier
- Structure and functions of bronchial lymphoid tissue (BALT system)
- Mechanisms of endocrine regulation of the bronchial lumen
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