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Lungs

Pulmones

For medical students2 min readUpdated 2026-10-10

Lungs are the primary paired organs of the respiratory system, located within the thoracic cavity. They consist of an elastic and spongy pinkish-gray tissue inside which the vital process of gas exchange takes place.

LocationThoracic cavity
LobesRight — 3 lobes, Left — 2 lobes
Structural unitBronchopulmonary segment
AlveoliLined by simple squamous epithelium

External Morphology

Each lung features a tapered upper part — the apex (apex pulmonis) and a broadened lower part — the base (basis pulmonis).

Anatomically, three surfaces are distinguished on the organ:

The surfaces are separated by margins: the anterior margin (margo anterior) and the inferior margin (margo inferior). The mediastinal surface contains a critical area — the hilum of the lung (hilum pulmonis). This is the site through which key anatomical structures enter and leave the organ:

  1. Main (primary) bronchus.
  2. Pulmonary artery (a. pulmonalis).
  3. Pulmonary veins (vv. pulmonales).
  4. Lymphatic vessels and nerves.

The entire collection 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 more massive 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, whereas the horizontal fissure (fissura horizontalis) is found only in the right lung, separating the middle lobe.

Within the lobes lie smaller anatomical and functional units — bronchopulmonary segments (segmentum bronchopulmonale). Each such segment:

The right lung contains 10 segments (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 branch sequentially, forming the bronchial tree. They divide into lobar bronchi (bronchi lobares), which then branch into segmental bronchi (bronchi segmentales).

Within the segment itself, branching continues down to microscopic structures:

  1. Bronchioles (bronchioli).
  2. Respiratory bronchioles (bronchioli respiratorii).
  3. Alveolar ducts (ductuli alveolares).
  4. Alveolar sacs (sacculi alveolares).
  5. Pulmonary alveoli (alveoli pulmonis).

Alveoli are terminal sacs lined by simple squamous epithelium. They are densely entwined with a capillary network. The collective network of all alveoli forms the vast respiratory surface of the lungs, where gas exchange occurs.

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 extends 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 thoracic cage along the following lines:

Frequently asked questions

How are the elements of the right and left lung roots arranged from top to bottom (their syntopy)?

The elements of the right and left lung roots are arranged asymmetrically.

Syntopy of elements in the right lung root (superior to inferior):

  • 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 left lung root (superior to inferior):

  • Pulmonary artery (a. pulmonalis) — occupies the highest position.
  • Main bronchus (bronchus principalis) — lies inferior and posterior to the artery.
  • Pulmonary veins (vv. pulmonales) — lie against the anterior and inferior surfaces of the bronchus and artery.
Which specific bronchopulmonary segments (by name) are identified in the superior lobe of the right lung?

There are three bronchopulmonary segments 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 anatomical classification, the superior lobe of the right lung contains the following segments:

  • Apical segment (I)
  • Posterior segment (II)
  • Anterior segment (III)

These segments are ventilated by branches of the superior lobar bronchus, which branches off the right main bronchus before it enters the lung hilum.

What is a pulmonary acinus and what anatomical structures does it comprise?

An acinus is the structural and functional unit of the respiratory region of the lung, representing the branching system of a single terminal bronchiole where gas exchange takes place.

The acinus sequentially comprises the following anatomical structures:

  • Respiratory bronchioles — have three orders of branching (1st, 2nd, and 3rd).
  • Alveolar ducts — formed by the division of 3rd-order respiratory bronchioles.
  • Alveolar sacs — terminal endings 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 course along and branch within the bronchial walls, providing trophics to the pulmonary parenchyma, bronchi, bronchial lymph nodes, and pleura. Venous drainage occurs 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?

The text does not explicitly state the vertebral level of tracheal bifurcation. It notes that the trachea extends from the level of the 7th cervical to the 4th thoracic vertebra, while the main bronchi show skeletotopy at the level of the 5th–6th thoracic vertebrae.

What is the difference between the hilum and the root of the lung?

The hilum (hilum pulmonis) is the anatomical depression on the mediastinal surface of the organ. The root (radix pulmonis) is the collection of the structures themselves (bronchus, vessels, nerves) that pass through this hilum.

Why does the left lung have only two lobes instead of three?

This is due to the presence 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 reliably separated from one another by thin connective tissue septa, making them relatively autonomous anatomical and functional units.

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