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Anatomy and Functions of the Lungs

Q: What is the chemical composition of surfactant?

Surfactant is chemically composed of lipids and proteins. Lipids — make up 90% of the mass, predominantly represented by phospholipids and cholesterol. The main component (40% of mass) is dipalmitoylphosphatidylcholine, containing two saturated palmitic acid molecules.Proteins — make up 10% of the mass and are divided into 4 groups.

*Pulmo*

For medical students2 min readUpdated 2026-10-10

The lungs are the central organs of the respiratory system, providing gas exchange between air and blood. Lung tissue serves as a vast biologically active membrane performing essential metabolic tasks; therefore, any pathology of the organ inevitably affects the entire body.

Parenchyma Surface AreaFrom 80 m² on expiration to 120 m² on inspiration

Barrier ThicknessThe air-blood barrier is only about 0.5 µm thick

Water LossAbout 500 mL of fluid is eliminated daily via respiration

Oncotic Pressure25–30 mm Hg in capillaries protects against edema

Architectonics and Cellular Composition of Alveoli

The respiratory system includes air-conducting pathways (from the nasal cavity to the terminal bronchioles) and the respiratory zone, where direct gas exchange occurs. Supportive roles are played by the rib cage, intercostal muscles, diaphragm, pleural cavities, as well as intrinsic neural and endocrine apparatuses. The vascular network is a crucial component at all stages.

Alveolar walls are formed by a simple squamous epithelium resting on a basement membrane. Its cellular composition includes:

Type I alveolar cells (Type I pneumocytes): Large, flattened cells. They occupy 95% of the entire surface and serve as the main site of gas exchange.
Type II alveolar cells (Type II pneumocytes): Cover the remaining 5% of the surface area. They do not directly participate in gas exchange.
Neuroendocrine cells (Kulchitsky cells, or third-order pneumocytes): Located among the alveolar epithelium, between basal cells in the bronchi, near nerve endings, and in the stroma close to blood vessels.

Air-Blood Barrier

This is the primary site of gas exchange in the body. The barrier is formed by capillaries and closely apposed Type I pneumocytes.

The total thickness of this barrier is only 0.5 µm. It consists of three layers:

Flattened portion of the Type I pneumocyte (0.2 µm).
Fused basement membrane (0.1 µm) — the site of tight adhesion between the epithelial and capillary membranes.
Endothelial cell of the blood vessel (0.2 µm).

An important component of the barrier is also the surfactant film lining the internal surface of the alveoli.

Non-Respiratory Functions

In addition to air conditioning (warming in the nasal passages and humidification), the lungs actively participate in metabolism and homeostasis maintenance. Organ diseases are frequently associated with the impairment of these protective mechanisms.

Metabolism: Regulation of water-salt, protein, and carbohydrate balance. The organ is characterized by a high rate of lipid metabolism (biogenesis of surfactant, leukotrienes, prostaglandins).
Inactivation of biologically active substances (BAS): Serotonin, histamine, bradykinin, epinephrine, and norepinephrine are degraded in the lung tissue. The generation and inactivation of prostaglandins, reactive oxygen species (ROS), as well as the inactivation of angiotensin-converting enzyme (ACE), also take place here.
Hemodynamics and thermoregulation: The lungs act as a blood reservoir, regulate blood clotting, clear the bloodstream of microthrombi and metabolic products. The body loses 5–10% of its heat through exhaled air.

Pathophysiology and Fluid Balance

Pathological processes (tumors, inflammation, pulmonary sclerosis) lead to respiratory hypoxia. Acute oxygen deprivation triggers cor pulmonale and cardiopulmonary failure, while chronic hypoxia causes sclerotic, atrophic, and dystrophic changes both in the lung tissue itself and in internal organs (primarily affecting the cardiovascular system).

Lung tissue is capable of actively absorbing fluid from the alveoli. Normally, capillary oncotic pressure (25–30 mm Hg) significantly exceeds hydrodynamic (hydrostatic) pressure (8–10 mm Hg), which ensures reabsorption. Pulmonary edema develops if hydrodynamic pressure rises sharply (e.g., in acute heart failure) or if blood hypoonquia occurs.

Respiration also directly affects acid-base balance and potassium metabolism:

Respiratory acidosis (in respiratory failure) leads to hyperkalemia.
Respiratory alkalosis causes a drop in plasma potassium concentration (hypokalemia).

Mnemonic

To quickly remember the layers of the air-blood barrier, use the rule "PEB": Pneumocyte (Type I), fused Basement membrane, capillary Endothelium. Surfactant lies on top of this structure.

Frequently asked questions

What are the functions of Type II pneumocytes?

Type II pneumocytes (type 2 alveolar cells) perform secretory and cambial functions while not directly participating in gas exchange.

Secretory function — release of cytophospholiposome contents into the alveolar lumen to form surfactant (they act as the "surfactant factory").
Cambial function — act as stem elements, ensuring the regeneration of the alveolar epithelium.

What pathomorphological changes occur in internal organs during chronic respiratory hypoxia?

During chronic respiratory hypoxia, dystrophic, atrophic, and sclerotic processes develop in internal organs, with the cardiovascular system suffering first. Such processes also develop within the lung tissue itself. Alterations in arterial blood gas composition and sclerotic changes require strain on compensatory mechanisms, leading to morphological remodeling of the respiratory system.

Which cells synthesize surfactant in the lungs?

Surfactant in the lungs is synthesized by type 2 pneumocytes and non-ciliated epithelial cells of respiratory bronchioles. Type 2 pneumocytes are also called secretory or large alveolar cells and serve as the "surfactant factory." They possess a well-developed synthetic apparatus and secrete the contents of specific osmiophilic organelles — cytophospholiposomes — into the alveolar lumen to form surfactant.

What is the chemical composition of surfactant?

Surfactant is chemically composed of lipids and proteins.

Lipids — make up 90% of the mass, predominantly represented by phospholipids and cholesterol. The main component (40% of mass) is dipalmitoylphosphatidylcholine, containing two saturated palmitic acid molecules.
Proteins — make up 10% of the mass and are divided into 4 groups.

What macrophages are present in lung tissue and what is their function?

Alveolar macrophages belonging to the mononuclear phagocyte system are present in lung tissue. Their primary function is protective, mediated through phagocytosis (engulfment and destruction of pathogens, body cells, dust particles, and erythrocytes). When activated, they can generate reactive oxygen species and proteases. In pathologies (e.g., usual interstitial pneumonia), a monocytoid macrophage type appears, which loses phagocytic activity but enhances protein synthesis and contains high amounts of fibronectin.

Which cells cover the majority of the alveolar surface?

Type I pneumocytes. They are large, flat cells covering about 95% of the alveolar area and serve as the main zone for gas exchange.

Why does pulmonary edema occur in acute heart failure?

Due to a sharp increase in intracapillary hydrodynamic pressure. Normally, it is lower than oncotic pressure, allowing the tissue to absorb fluid, but pathology disrupts this balance.

How does respiratory failure affect blood potassium levels?

It triggers respiratory acidosis, which in turn leads to an elevated plasma concentration of potassium ions (hyperkalemia).

Go deeper

Biogenesis of surfactant and its role in pulmonary lipid metabolism
Mechanisms of inactivation of biologically active substances in lung tissue
Pathogenesis of cor pulmonale and cardiopulmonary failure in acute hypoxia
Effects of chronic hypoxia on the cardiovascular system
Neuroendocrine apparatus of the lungs and functions of Kulchitsky cells