Internal Features of Lungs – Medical Sciences Hub

Contents

Introduction

The lungs consist of three main components: the bronchial tree, alveoli, and connective tissue containing blood vessels.

The bronchial tree includes the principal bronchi, lobar bronchi, tertiary bronchi, terminal bronchioles, and respiratory bronchioles.

The trachea divides into two principal bronchi, the right and left principal bronchi.
The tracheal bifurcation occurs at the level of the lower border of the fourth thoracic vertebra.

Right Principal Bronchus

The right principal bronchus is shorter, wider, and more vertical than the left principal bronchus.
It is approximately 2.5 cm (1 inch) long.

Left Principal Bronchus

The left principal bronchus is narrower, longer, and more horizontal than the right principal bronchus.
It is approximately 5 cm (2 inches) long.

Foreign body aspiration: The right principal bronchus is more closely aligned with the trachea than the left. Therefore, aspirated foreign bodies are more likely to enter the right principal bronchus. As a result, lung abscesses caused by aspirated material are more common on the right side.
Bronchoscopy: Bronchoscopy is a procedure used to visualize the interior of the trachea and bronchi. A bronchoscope is a flexible fiber-optic instrument fitted with a camera for this purpose.
Carina: The carina is a cartilaginous ridge located at the bifurcation of the trachea.
The right principal bronchus lies more in line with the trachea than the left principal bronchus. Therefore, inhaled foreign bodies usually enter the right principal bronchus.

The lobar bronchi divide into tertiary, or segmental, bronchi.
Each tertiary bronchus supplies a distinct region of the lung known as a bronchopulmonary segment.
There are 10 tertiary bronchi in the right lung and 10 in the left lung.

The tertiary bronchi divide into smaller bronchioles measuring less than 1 mm in diameter.
Bronchioles do not contain cartilage or glands in their walls.
Terminal bronchioles measure approximately 0.5–1 mm in diameter.
Respiratory bronchioles are smaller and usually measure less than 0.5 mm in diameter.

The respiratory bronchiolar unit is the smallest functional unit of the lung.
It consists of the respiratory bronchiole, alveolar ducts, alveolar atria, air saccules, and alveoli.

Alveoli

Alveoli are thin-walled, polyhedral, sac-like terminal air spaces of the respiratory system.
They form the principal sites of gaseous exchange.
Each alveolus is approximately 200 μm in diameter.
Each lung contains about 150–250 million alveoli.

Adjacent alveoli are separated by a thin connective tissue septum containing blood capillaries. The alveolar septum also contains fibroblasts and macrophages.
Interalveolar pores of Kohn are small openings in the alveolar septa that permit communication between adjacent alveoli.
Alveoli are lined mainly by type I pneumocytes, type II pneumocytes, and sensory brush cells.
Type I pneumocytes are mainly involved in gaseous exchange.
Type II pneumocytes produce pulmonary surfactant.

Bronchiectasis: Bronchiectasis is a chronic lung disorder characterized by permanent dilatation of the airways due to progressive destruction of elastic tissue and smooth muscle in their walls.
Emphysema: Emphysema is characterized by abnormal and permanent enlargement of air spaces distal to the terminal bronchioles. Cigarette smoking and long-term exposure to air pollutants are common causative factors.

A bronchopulmonary segment is a well-defined part of the lung aerated by a segmental, or tertiary, bronchus.
Each bronchopulmonary segment is pyramidal or wedge-shaped.
The apex of the segment is directed toward the hilum of the lung.
The base of the segment faces the surface of the lung.
Each segment is supplied by its own segmental bronchus and a segmental branch of the pulmonary artery.
Adjacent bronchopulmonary segments are separated by connective tissue septa called intersegmental planes.
Pulmonary veins lie within the intersegmental planes and drain adjacent segments.
Each bronchopulmonary segment has separate branches of the pulmonary and bronchial arteries but does not have an independent pulmonary vein.
Therefore, a bronchopulmonary segment is not a true bronchovascular segment.
There are classically 10 bronchopulmonary segments in each lung, although some segments may be fused in the left lung.

Segmental involvement: The apical segment of the lower lobe and the posterior segment of the upper lobe are common sites of lung abscess following aspiration. Such involvement is more common on the right because the right principal bronchus is more vertical.
Segmental resection of the lung: Each bronchopulmonary segment has its own segmental bronchus and branch of the pulmonary artery. Therefore, a diseased segment can be surgically removed with minimal involvement of adjacent segments. Segmental resection may be performed for localized lesions such as tumors or tuberculous lesions.
Mendelson’s syndrome: Mendelson’s syndrome refers to aspiration of gastric contents, particularly in unconscious patients. The aspirated material commonly enters dependent lung segments and may cause aspiration pneumonitis or pneumonia.
Segmental atelectasis: Segmental atelectasis is the collapse of a bronchopulmonary segment caused by obstruction of its segmental bronchus and loss of ventilation to that segment.

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