During inspiration, due to the enlargement of thoracic cage, the negative pressure is increased in the thoraciccavity. It causes expansion of the lungs. During expiration, the thoracic cavity decreases in size to the preinspiratory position. Pressure in the thoracic cage also comes back to the preinspiratory level. It compresses the lung tissues so that, the air is expelled out of lungs.

 Collapsing Tendency of Lungs

Lungs are under constant threat to collapse even in resting conditions because of certain factors.

Factors Causing Collapsing Tendency of Lungs

Two factors are responsible for the collapsing tendency of lungs:

1. Elastic property of lung tissues: Elastic tissues of lungs show constant recoiling tendency and try to

collapse the lungs

2. Surface tension: It is the tension exerted by the fluid secreted from alveolar epithelium on the surface of alveolar membrane.

Fortunately, there are some factors, which save the lungs from collapsing.

Factors Preventing Collapsing Tendency of Lungs

In spite of elastic property of lungs and surface tension in the alveoli of lungs, the collapsing tendency of lungs is prevented by two factors:

1. Intrapleural pressure: It is the pressure in the pleural cavity, which is always negative (see below).

Because of negativity, it keeps the lungs expanded and prevents the collapsing tendency of lungs

produced by the elastic tissues.

2. Surfactant: It is a substance secreted in alveolar epithelium. It reduces surface tension and prevents the collapsing tendency produced by surface tension.


Surfactant is a surface acting material or agent that is responsible for lowering the surface tension of a fluid. Surfactant that lines the epithelium of the alveoli in lungs is known as pulmonary surfactant and it decreases the surface tension on the alveolar membrane.

Source of secretion of pulmonary surfactant

Pulmonary surfactant is secreted by two types of cells:

1. Type II alveolar epithelial cells in the lungs, which are called surfactant secreting alveolar cells or

pneumocytes. Characteristic feature of these cells is the presence of microvilli on their alveolar surface.

2. Clara cells, which are situated in the bronchioles. These cells are also called bronchiolar exocrine


Chemistry of surfactant

Surfactant is a lipoprotein complex formed by lipids especially phospholipids, proteins and ions.

1. Phospholipids: Phospholipids form about 75% of the surfactant. Major phospholipid present in

the surfactant is dipalmitoylphosphatidylcholine (DPPC).

2. Other lipids: Other lipid substances of surfactant are triglycerides and phosphatidylglycerol (PG).

3. Proteins: Proteins of the surfactant are called specific surfactant proteins. There are four main

surfactant proteins, called SPA, SPB, SPC and SPD. SPA and SPD are hydrophilic, while SPB and SPC are hydrophobic. Surfactant proteins are vital components of surfactant and the surfactant becomes inactive in the absence of proteins.

4. Ions: Ions present in the surfactant are mostly calcium ions.

Formation of surfactant

Type II alveolar epithelial cells and Clara cells have a special type of membrane bound organelles called

lamellar bodies, which form the intracellular source of surfactant. Laminar bodies contain surfactant phospholipids and surfactant proteins. These materials are synthesized in endoplasmic reticulum and stored in laminar bodies. By means of exocytosis, lipids and proteins of lamellar bodies are released into surface fluid lining the alveoli. Here, in the presence of surfactant proteins and calcium, the phospholipids are arranged into a lattice (meshwork) structure called tubular myelin. Tubular myelin is in turn converted into surfactant in the form of a film that spreads over the entire surface of alveoli. Most of the surfactant is absorbed into the type II alveolar cells, catabolized and the products are loaded into lamellar bodies for recycling.

Factors necessary for the formation and spreading of surfactant

Formation of surfactant requires many substances. Formation of tubular myelin requires DPPC, PG and the hydrophobic proteins, SPB and SPC.

Formation of surfactant film requires SPB, SPC and PG.

Type II alveolar epithelial cells occupy only about 5% of alveolar surface. However, the surfactant must

spread over the entire alveolar surface. It is facilitated by PG and calcium ions.

Glucocorticoids play important role in the formation of surfactant.

Functions of surfactant

1. Surfactant reduces the surface tension in the alveoli of lungs and prevents collapsing tendency

of lungs.

2. Surfactant is responsible for stabilization of the alveoli, which is necessary to withstand the collapsing


3. It plays an important role in the inflation of lungs after birth. In fetus, the secretion of surfactant

begins after the 3rd month. Until birth, the lungs are solid and not expanded. Soon after birth, the

first breath starts because of the stimulation of respiratory centers by hypoxia and hypercapnea.

Although the respiratory movements are attempted by the infant, the lungs tend to collapse repeatedly.

And, the presence of surfactant in the alveoli prevents the lungs from collapsing.

4. Another important function of surfactant is its role in defense within the lungs against infection and

inflammation. Hydrophilic proteins SPA and SPD destroy the bacteria and viruses by means of opsonization. These two proteins also control the formation of inflammatory mediators.

Effect of deficiency of surfactant – respiratory distress syndrome

Absence of surfactant in infants, causes collapse of lungs and the condition is called respiratory distress

syndrome or hyaline membrane disease. Deficiency of surfactant occurs in adults also and it is called adult respiratory distress syndrome (ARDS). In addition, the deficiency of surfactant increases

the susceptibility for bacterial and viral infections.

Post a Comment