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Synonym: Pulmo
German: Lunge

1 Definition

The lung is a respiration serving organ existing as part of a pair. It takes oxygen from the breath and exports carbon dioxide as the end product of body metabolism from the body. True lungs occur in humans and in air-breathing vertebrates.

2 Anatomy

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2.1 General Structure

The lungs begins in principle at the hilum (hilum pulmonis), lateral to the windpipe (trachea). This transitions via a bifurcation into the two main bronchi (bronchi principales), which together with the pulmonary arteries and the pulmonary veins which arise in the hilum.

Humans have two lungs that lie to both sides of the chest cavity, and are separated from the mediastinum. The left lung (pulmo sinister) is subdivided into two, the right lung (pulmo dexter) into three lobes (lobi pulmonis). The lobes can be further divided into 19 pulmonary segments (segmenta bronchopulmonalia), which are each supplied by a segmental bronchus and segment artery. The right lung consists of 10 segments, the left lung of 9 segments.

2.2 Bronchial tree

The subdivision of the lung is characterised by the branching of the bronchi: the bronchial tree. The main bronchi form the stem of the bronchial tree, which splits within the lung dichotomously. The right main bronchus is called bronchus principalis dexter, the left bronchus is called bronchus principalis sinister.

The main bronchi are divided into lobar bronchi - right in three lobar bronchi: bronchi lobares superior, medius and inferior and left into the bronchi lobares superior and inferior. Thereafter, there follows further division into segmental bronchi - right 10, left 9 segmental bronchi. This is followed by subsegmental bronchi, the bronchioles and finally the bronchioli terminales.

The conductive, air-transporting phase of the bronchial tree ends here. Thereafter follows the sectional formation, which serves for gas exchange and as the lung parenchyma in the narrow sense. These include the bronchioli respiratorii, the ductus alveolaris and sacculi alveolares. The respiratory bronchiole merges into the alveolar duct, which after subsequent divisions opens into the final alveolar saccule. The alveolar saccule contains the pulmonsary alveoles, the actual site of gas exchange. Thus, the alveoli do not collapse during exhalation, they are moistened with surfactant.

The last-mentioned structures, which diverge from a terminal bronchiole, together form the acini of the lung. Several of these acini form the structure of the pulmonary lobules (lobuli) having a diameter of about 2 cm. Characteristic is the macroscopically visible grid pattern, represented by the margins of the lung by interlobular connective tissue septa and is histologically marked by the dust-laden macrophage arranged in networks.

All bronchi have in their wall structural features cartilage, seromucosal glands (glandulae bronchiales) and smooth muscle. In the trachea and the bronchi, the air-conducting cavities are kept open by cartilaginous bracing. In the smaller bronchi one sees only island-like cartilage occurrence, but also a much thicker layer of smooth muscle. The bronchioles and alveoli contain no cartilage.

It may happen that some lung tissue has no function where it is not connected to the tracheal tree. This is called sequestered lung tissue.

2.3 Pleura

Externally the lung is covered with visceral sheets of the pleural cavity (pleura). At the hilum the visceral sheet merge into the parietal sheet of the pleura. The pleura duplicature caudal to the hilum is called ligamentum pulmonale.

3 Histology

3.1 Alveoli

The alveoli are the site of gas exchange (O2 for CO2), forming a total surface area of 140 m2. The superficial alveolar wall is covered by alveolar epithelium, which houses two cell types:

  • pneumocytes type 1
  • pneumocytes type 2 (for the production of surfactant).

The polygonal, air-filled spaces (diameter 250 microns) have a capillary network around them and are separated from one another by interalveolar septa composed of connective tissue. These conduct the capillaries, thereby ensuring the integrity of the blood-air barrier. The blood-air barrier can be reduced to three structural elements:

  • capillary endothelium
  • pneumocytes type 1
  • common basal lamina

3.2 alveolar macrophages

The alveolar macrophages belong to the monocyte-phagocyte system and eliminate dust particles, germs and dead unutilisable cells on the surface of the alveoli. Besides the alveoli there are alveolar macrophages in the terminal, cilia-free airways. After phagocytosis alveolar macrophages migrate to the airways, which are lined with ciliated epithelium, where they are transported by the mucus film. However, alveolar macrophages can also migrate to the connective tissue of the lung parenchyma where they either remain and form the lobe pattern or transfer over to the regional lymph nodes via the regional lymph ducts.

3.3 Clara cells

Clara cells are secretory active cells in the distal airways and secrete antimicrobial and inflammatory proteins. Examples include the surfactant-proteins SP-A and SP-D as well as the clara cell protein CC10.

3.4 Neuroendocrine cells

The neuroendocrine cells of the lung are part of the diffuse neuroendocrine system (DNES). They derive from the neural crest, acting as chemoreceptors and secrete hormones such as serotonin, calcitonin and cholecystokinin. Apart from this they also regulate the development of the lungs.

4 Embryology of the lungs

By the 26th day of pregnancy the future breathing apparatus has already been set in place.

By the end of the first month of pregnancy, five small sacks have already emerged from the first structure. They correspond to the main bronchi of the five lobes (two left, three right). Within a few weeks (by the 17th week of pregnancy) an entire bronchial apparatus has been produced by subbranching in the brachial tree.

Toward the end of pregnancy the first alveoli emerge. By far the greater share of the alveoli (about 80%) originate after birth in the first eight months of life.

In the womb, the lungs perform a specific task. They create working as glands the amniotic fluid. The liquid fills all parts of the unborn lung. During delivery the lungs is veritably squeezed in the birth canal. This facilitates the newborn being able to fill the lungs with air.

5 Diseases of the lungs

Among the diseases of the lung, among others

see also: respiratory diseases

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