Surfactant is a lipoprotein complex secreted by type II alveolar cells and Clara cells that lines the alveoli. It is composed mainly of phospholipids like DPPC, proteins, and ions. The surfactant proteins SP-B and SP-C along with phospholipids form a film that reduces surface tension preventing alveolar collapse during exhalation. Surfactant deficiency can cause respiratory distress syndrome in infants and adults by making it difficult to inflate the lungs.

1. SURFACTANT

2. •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 cells.
SURFACTANT

4. 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).

5. 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.
SP-A, SP-D is a glycoprotein. Both SP-A and SP-D are
members of the collectin family are involved in innate
immunity in the conducting airway as well as in the alveoli.
•SP-A is a large glycoprotein and has a collagen-like domain
within its structure. It has multiple functions, including
regulation
of the feedback uptake of surfactant by the type II alveolar
epithelial cells that secrete it

6. 4. Ions: Ions present in the surfactant are mostly calcium
ions.
•SP-B and SP-C are smaller proteins,which are the of the
key protein members of the monomolecular film of
surfactant.
• SURFACTANT becomes inactive in the absence of
proteins.

7. 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.
• It is facilitated by PG and calcium ions.
Glucocorticoids play important role in the formation
of surfactant.

8. 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.

9. •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.

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

12. 2. Surfactant is responsible for stabilization of the alveoli,
which is necessary to withstand the collapsing tendency.
3. It plays an important role in the inflation of lungs after
birth.
•In fetus, the secretion of surfactant begins after the 3rd
month,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.

13. 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.

14. Surfactant acts by the following mechanism:
Phospholipid molecule in the surfactant has two
portions. One portion of the molecule is hydrophilic.
This Portion Dissolves In water and lines The alveoli.
Other portion is Hydrophobic and it is Directed towards
the alveolar air.
This surface of the Phospholipid Along with other portion
spreads over the
alveoli and reduces the surface tension.
SPB and SPC play active role in this process.

16.  It has been calculated that if it were not present,
the unopposed surface tension in the alveoli would
produce a 20mmHg force favouring transudation of
fluid from the blood into the alveoli.
the deficiency of surfactant increases
the susceptibility for bacterial and viral infections.

17. 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).
CLINICAL

18. Overproduction/dysregulation of surfactant proteins
can also lead to respiratory distress and is the cause of
Pulmonary Alveolar Proteinosis (PAP)

19. .
LALIT YADAV