External morphology
Each lung has a narrowed upper part — the apex (apex pulmonis) and a broadened lower part — the base (basis pulmonis).
Anatomically, the organ has three surfaces:
- costal surface (facies costalis) — faces the inner surface of the chest wall;
- mediastinal surface (facies mediastinalis) — faces the mediastinum;
- diaphragmatic surface (facies diaphragmatica) — rests on the diaphragm.
The surfaces are separated from each other by borders: the anterior (margo anterior) and the inferior (margo inferior). On the mediastinal surface lies a critical area — the hilum of the lung (hilum pulmonis). This is the site through which key anatomical structures enter and leave the organ:
- Main bronchus.
- Pulmonary artery (a. pulmonalis).
- Pulmonary veins (vv. pulmonales).
- Lymphatic vessels and nerves.
The totality of these structures passing through the hilum forms the root of the lung (radix pulmonis).
Lobar and segmental structure
The lungs are asymmetrical. The right lung is larger and consists of three lobes: superior (lobus superior), middle (lobus medius), and inferior (lobus inferior). The left lung has only two lobes: superior and inferior.
The lobes are separated by interlobar fissures. The oblique fissure (fissura obliqua) is present in both lungs, while the horizontal fissure (fissura horizontalis) is found only in the right lung, separating the middle lobe.
Within the lobes, smaller anatomical and functional units are distinguished — bronchopulmonary segments (segmentum bronchopulmonale). Each such segment:
- is ventilated by its own segmental bronchus;
- receives blood supply from a separate segmental artery;
- is separated from neighboring areas by connective tissue septa.
There are 10 segments in the right lung (3 in the superior lobe, 2 in the middle, and 5 in the inferior). In the left lung, depending on the classification used, there are 8 to 10 segments.
Bronchial branching and respiratory surface
Upon entering the lung, the main bronchi begin to branch sequentially, forming the bronchial tree. They divide into lobar bronchi (bronchi lobares), which then break down into segmental bronchi (bronchi segmentales).
Within the segment itself, branching continues down to microscopic structures:
- Bronchioles (bronchioli).
- Respiratory bronchioles (bronchioli respiratorii).
- Alveolar ducts (ductuli alveolares).
- Alveolar sacs (sacculi alveolares).
- Alveoli of the lung (alveoli pulmonis).
Alveoli are terminal sacs lined with simple squamous epithelium. They are densely surrounded by a capillary network. It is the totality of all alveoli that forms the massive respiratory surface of the lungs, where gas exchange takes place.
Topography and borders of the lungs
The lungs do not completely fill the pleural cavity, although their borders largely correspond to the borders of the parietal pleura.
Superior border: the apex of the lung projects 2–3 cm above the clavicle.
Anterior border: the anterior margin of the right lung descends to the level of the 6th rib. The anterior margin of the left lung features a specific cardiac indentation — the cardiac notch (incisura cardiaca pulmonis sinistri).
Inferior border is projected onto the chest wall along the following lines:
- midclavicular line (linea medioclavicularis) — 6th rib;
- midaxillary line (linea axillaris media) — 8th rib;
- scapular line (linea scapularis) — 10th rib;
- paravertebral line (linea paravertebralis) — 11th rib.
Briefly: answers to topical questions
What is the superior-to-inferior arrangement of the elements in the root of the right and left lungs (what is their syntopy)?
The elements of the roots of the right and left lungs are arranged asynchronously.
Syntopy of elements in the root of the right lung (from top to bottom):
- Main bronchus (bronchus principalis) — occupies the highest position.
- Pulmonary artery (a. pulmonalis) — lies anterior and inferior to the bronchus.
- Superior pulmonary vein (v. pulmonalis superior) — lies inferior to the artery.
Syntopy of elements in the root of the left lung (from top to bottom):
- Pulmonary artery (a. pulmonalis) — occupies the highest position.
- Main bronchus (bronchus principalis) — lies inferior and posterior to the artery.
- Pulmonary veins (vv. pulmonales) — lie adjacent to the anterior and inferior surfaces of the bronchus and artery.
Which specific bronchopulmonary segments (by name) are distinguished in the superior lobe of the right lung?
Three bronchopulmonary segments are distinguished in the superior lobe of the right lung. A bronchopulmonary segment is ventilated by a single segmental bronchus, supplied by a segmental artery, and separated by connective tissue septa.
According to the anatomical classification, the following segments are distinguished in the superior lobe of the right lung:
- Apical segment (I)
- Posterior segment (II)
- Anterior segment (III)
These segments are ventilated by the branches of the superior lobar bronchus, which originates from the right main bronchus before it enters the hilum of the lung.
What is a pulmonary acinus and which anatomical structures does it include?
The acinus is the structural and functional unit of the respiratory division of the lung, representing the branching system of a single terminal bronchiole where gas exchange takes place.
The acinus sequentially includes the following anatomical structures:
- Respiratory bronchioles — have three branching orders (1st, 2nd, and 3rd).
- Alveolar ducts — are formed by the division of 3rd-order respiratory bronchioles.
- Alveolar sacs — these form the termination of the alveolar ducts.
- Alveoli — terminal elements forming the walls of the aforementioned structures, where gas exchange occurs.
Which blood vessels provide the nutritive blood supply to the lung tissue itself and the bronchi?
The nutritive blood supply to the lung tissue and bronchial walls is provided by the bronchial arteries.
These vessels belong to the systemic circulation. The bronchial arteries (rr. bronchiales), numbering 2–4 branches, arise from the anterior wall of the upper thoracic aorta (or its branches). They travel along and branch within the bronchial walls, providing trophism to the pulmonary parenchyma, bronchi, as well as bronchial lymph nodes and pleura. Venous drainage is carried out via accompanying veins. Bronchopulmonary anastomoses exist between the branches of the pulmonary and bronchial arteries.
At the level of which vertebrae does the trachea bifurcate into the right and left main bronchi?
Sources do not explicitly specify the vertebral level at which the tracheal bifurcation occurs. It is noted that the trachea extends from the level of the VII cervical to the IV thoracic vertebra, while the main bronchi show skeletotopy at the level of the V–VI thoracic vertebrae.
Class and exam questions
What is the difference between the hilum and the root of the lung?
The hilum (hilum pulmonis) is an anatomical depression on the mediastinal surface of the organ. The root (radix pulmonis) is the set of structures (bronchus, vessels, nerves) passing through this hilum.
Why does the left lung have only two lobes instead of three?
This is due to the location of the heart in the left half of the thoracic cavity. Consequently, the left lung has a smaller volume, two lobes instead of three, and a characteristic cardiac notch on its anterior margin.
What separates the bronchopulmonary segments from each other?
The segments are securely separated from each other by thin connective tissue septa, making them relatively autonomous anatomical and functional units.
What else to study on this topic
- Topography of the right and left lung root elements (syntopy of vessels and bronchus)
- Histological structure of the air-blood barrier in the alveoli
- Blood supply to the lung tissue (differences between bronchial and pulmonary vessels)
- Structure of the parietal and visceral pleura, pleural recesses
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