Organization of the Conducting Airways
The respiratory tract sequentially conducts air, cleaning, humidifying, and warming it throughout its length. The system is divided into two major blocks:
1. Extrapulmonary airways:
- Nasal cavity: begins with the vestibule, and the nasal cavity proper is divided into respiratory and olfactory regions.
- Pharynx: the intersection with the digestive tract (includes nasal, oral, and laryngeal parts).
- Tubular organs: larynx, trachea, and main bronchi.
- Large bronchi: lobar and segmental (1st and 2nd orders). Segmental bronchi are partially embedded in the lung tissue.
2. Intrapulmonary airways (bronchial tree): Classification is based on the caliber of the 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 final conducting 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 conducting airways is the presence of alveoli in their walls.
Air inside the acinus passes strictly sequentially:
- Respiratory bronchioles (have three branching orders).
- Alveolar ducts.
- Alveolar sacs.
- Alveoli (the ultimate gas exchange elements).
Peculiarities of Pulmonary Blood Supply
The lungs possess a unique feature — two vascular systems function simultaneously within them, anastomosing with each other at the level of arterioles and venules.
- Bronchial vessel system (systemic circulation): provides nutrition (trophics). Bronchial arteries carry oxygenated blood from the aorta, nourishing the bronchial walls and parenchyma. Venous blood drains via bronchial veins into the superior vena cava system.
- Pulmonary vessel system (pulmonary circulation): provides gas exchange. Pulmonary arteries carry deoxygenated (oxygen-poor) blood from the right ventricle to the alveolar capillaries. After oxygenation, oxygenated blood flows via four pulmonary veins into the left atrium.
Development and Histological Structure
The respiratory system develops from the ventral wall of the pharyngeal foregut. Development proceeds from proximal to distal zones. Epithelial cords grow into the mesenchyme, branch dichotomously, and only later do lumens form within them.
Tissue sources:
- Endoderm: epithelial lining.
- Mesenchyme: cartilaginous framework, connective tissue, smooth muscles.
- Myotomes: skeletal musculature of the larynx.
- Splanchnotome: pleura mesothelium.
General structural plan of the airways: Down to the level of small bronchi, the wall shares common features. The inner lining is represented by a mucous membrane with pseudostratified ciliated epithelium. This is followed by the submucosa, while the supporting function is performed by a cartilaginous (or bony) skeleton.
Functions of the Respiratory System
In addition to air conduction and gas exchange, the system performs a number of vital tasks:
- Conditioning: nasal hairs and mucociliary clearance (ciliary beating) clean the air. Goblet cells and submucosal glands humidify it, and a 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: club cells in terminal bronchioles neutralize toxins and exhibit antimicrobial activity.
- Metabolic functions of the lungs: vascular endothelial cells participate in hemostasis (synthesis of thromboplastin and heparin). Breathing patterns regulate heat exchange and the acid-base balance of the blood.
- Specific functions: phonation (larynx), olfaction (upper part of the nasal cavity), and regulation of bronchial lumen by endocrinocytes.