Detailed Anatomy of the Respiratory System
Airway Generations and Branching Architecture
The respiratory system follows a dichotomous branching pattern with the trachea designated as generation 0, progressing through an average of 23 generations to reach the alveolar level, where the number of branches in any generation Z equals 2^Z 1.
Conducting vs. Respiratory Zones
Conducting airways (generations 0-16) possess multilayered walls containing mucous membrane, smooth muscle, and cartilage, serving purely for air transport without gas exchange 1, 2.
Respiratory/acinar airways (generations 17-23) begin at the first respiratory bronchiole and are intimately associated with gas-exchanging alveoli, with the gas exchange apparatus forming a sleeve of alveoli over approximately eight generations of the most distal airways 1, 3.
The transition from conducting to respiratory zones marks a critical structural change where airway walls become reduced to networks of alveolar entrance rings as part of the axial fiber system 1.
Airway Dimensions
The trachea has a cross-sectional area of approximately 2.5 cm², expanding to a massive alveolar gas exchange surface of approximately 100 m² in adult humans 1.
Human acini measure approximately 5 mm in diameter and contain the terminal gas exchange units 1.
Bronchopulmonary Segments
Each lung is divided into functionally independent bronchopulmonary segments, each supplied by a segmental bronchus and corresponding pulmonary artery branch 4.
Segmental and subsegmental pulmonary arteries parallel the bronchi and are named according to the bronchopulmonary segments they supply 4.
Considerable anatomic variations exist, particularly in the upper lobes, with variations in number or presence of accessory arteries from adjacent segments 4.
International agreement on bronchopulmonary segment nomenclature and anatomy was formalized in 1949 at the International Congress of Otorhinolaryngology and subsequently adopted by the first Nomina Anatomica in 1955 5.
Blood Supply
Pulmonary Circulation
Pulmonary arteries follow airways in a similar dichotomous branching pattern but branch over approximately five more generations than airways (averaging 28 generations) before reaching capillaries 1, 3, 6.
At nearly all levels, small "supernumerary" branches arise from main pulmonary artery vessels to perfuse nearby parenchyma, making the arterial branching pattern more complex than the airway tree 1, 3.
Pulmonary veins course independently of airways in intermediate positions related to interlobular septa, running within interlobular septa and not paralleling the segmental or subsegmental pulmonary artery branches 1, 4.
Pulmonary veins converge on the left atrium in four main stems (right and left superior and inferior pulmonary veins) 1, 3, 4.
Bronchial Circulation
- Bronchial arteries provide systemic arterial blood supply to the lung tissue itself, supplying the bronchial walls, visceral pleura, and supporting structures 4.
Nerve Supply
While the provided evidence does not contain detailed information about respiratory system innervation, the functional implications are evident:
The diaphragm requires intact phrenic nerve innervation for proper function as the principal inspiratory muscle 7.
Upper airway muscles have complex neural control that modulates respiratory airflow throughout the respiratory cycle and coordinates with speech and swallowing functions 8.
Diaphragm
The diaphragm is the thin dome-shaped muscle separating the thoracic cavity from abdominal contents and serves as the principal inspiratory muscle in humans 7.
Unique Characteristics
The diaphragm is anatomically unique compared to locomotor muscles and is the only skeletal muscle that is chronically active 7.
It is essential for achieving adequate pulmonary ventilation and gas exchange across the blood-gas interface 7.
Beyond breathing, the diaphragm contributes to non-breathing functions including coughing and sneezing 7.
Clinical Significance
The diaphragm exhibits plasticity in response to both increased and decreased contractile activity 7.
Ventilator-induced diaphragm dysfunction represents an important clinical problem in critically ill patients 7.
Asynchronous and paradoxic motion of both rib cage and abdomen may predict ventilatory failure 2.
Lining of the Respiratory System
Conducting Airways
The epithelial reticular basement membrane serves as the foundational structure, with conducting airways lined by mucous membrane containing epithelial cells, smooth muscle, and supporting cartilage 1.
Airway wall composition includes layered structures of epithelium and smooth muscle sheaths that can be quantified relative to basement membrane surface area 1.
Alveolar-Capillary Interface
The air-blood barrier consists of three layers: alveolar epithelium, capillary endothelium, and their shared basement membrane 3.
The harmonic mean barrier thickness is a critical measure of diffusion resistance for gas exchange 3.
Only 10-15% of lung volume consists of tissue and blood, while the remainder is air space, optimizing the structure for gas exchange 3.
Gas Exchange Surface
The alveolar surface area ranges from 40 to 80 square meters depending on lung size 6.
There are approximately 300 million alveoli, 14 million alveolar ducts, and 280 billion capillary segments in normal adult lungs 6.
The lung diffusing capacity for oxygen (DLO₂) is determined by alveolar capillary blood volume, intra-acinar alveolar and capillary surfaces, and the harmonic mean air-blood barrier thickness 3.
Hilum
While specific hilar anatomy details are limited in the provided evidence:
The hilum represents the entry/exit point where main bronchi, pulmonary arteries, and pulmonary veins enter and leave the lung 4.
Pulmonary veins converge from their interlobular positions to exit through the hilum as four main stems entering the left atrium 1, 3, 4.
Respiratory Mechanics Integration
The respiratory system consists of complex structures including lungs, upper and lower rib cage, diaphragm, and abdominal compartments, each with distinct mechanical properties working together to facilitate gas exchange 2.
Understanding this hierarchical organization is essential for optimizing ventilator settings and preventing ventilator-induced lung injury in critically ill patients 2.
Endotracheal tubes pose substantial flow-dependent resistance that must be overcome during lung inflation 2.