Surfactant

SURFACTANT

Q. 1 Amount of surfactant in the developing lung increases between the 26th and 32nd weeks of gestation. This increase is related to which of the following developmental events?
 A Differentiation of alveoli form embryonic foregut
 B Increased density of pulmonary capillaries
 C Development of ciliated epithelium in airspaces
 D Differentiation of type II alveolar epithelial cells
Q. 1 Amount of surfactant in the developing lung increases between the 26th and 32nd weeks of gestation. This increase is related to which of the following developmental events?
 A Differentiation of alveoli form embryonic foregut
 B Increased density of pulmonary capillaries
 C Development of ciliated epithelium in airspaces
 D Differentiation of type II alveolar epithelial cells
Ans. D

Explanation:

Surfactant is  synthesized by  Type  II  pneumocytes that  line  the  alveolar  sacs.  They  begin  to differentiate after the 26th week of Gestation. These cells can be recognized on electron microscopy by the presence of lamellar bodies. Surfactant production increases greatly after 35 weeks’ gestation. Other structures in the lung do not synthesize the phosphatidylcholine and phosphatidylglycerol compounds that are important in reducing alveolar surface tension.


Q. 2

The mechanism of action of surfactant is

 A

Lubricates the flow of CO2 diffusion

 B

Binds oxygen

 C

Makes the capillary surface hydrophilic

 D

Breaks the structure of water in the alveoli

Q. 2

The mechanism of action of surfactant is

 A

Lubricates the flow of CO2 diffusion

 B

Binds oxygen

 C

Makes the capillary surface hydrophilic

 D

Breaks the structure of water in the alveoli

Ans. D

Explanation:

Breaks the structure of water in the alveoli (Ref: Guyton 11/e, p 474; Ganong 22/e, p 654]

  • The water molecules on the surface of water (at air-water interphase) have an especially strong attraction for one another. This results in the water surface to contract producing surface tension.
  • Thus the water molecules lining the alveoli produce an elastic contractile force due to surface tension which causes the alveoli to collapse.
  • This surface tension force of water molecules is broken by surfactant (a mixture of phospholipids, proteins and ions, most important phospholipid of which is dipalmitoyl phosphatidyl-choline-DPCC). It is secreted by type II alveolar epithelial cells and lines the alveoli.
  • The phospholipid molecules have a hydrophilic head and two parallel hydrophobic ‘tails’. Thus only head part of the molecule dissolves in the fluid lining the alveolar surface and the hydrophobic tails face the alveolar lumen. This new surface thus formed of the surfactant has a significantly reduced surface tension than the water molecules.
  • By reducing the surface tension, surfactant serves two purposes:

1.   Prevents the alveoli, from collapsing

2.   Prevents pulmonary edema.


Q. 3 Surfactant is secreted by:
 A Pneumocyte-I
 B Pneumocyte-II
 C Both (A and B)
 D Macrophages 
Q. 3 Surfactant is secreted by:
 A Pneumocyte-I
 B Pneumocyte-II
 C Both (A and B)
 D Macrophages 
Ans. B

Explanation:

Pneumocyte-II


Q. 4

Which of the following components of the pulmonary surfactant mainly contribute to its physiological function?

 A

Sugar & salt

 B

Soap & water

 C

Lipid and protein

 D

Base and lipid

Q. 4

Which of the following components of the pulmonary surfactant mainly contribute to its physiological function?

 A

Sugar & salt

 B

Soap & water

 C

Lipid and protein

 D

Base and lipid

Ans. C

Explanation:

Pulmonary surfactant, composed of lecithin and myelin, is secreted on a continuous basis by type II alveolar cells and Clara cells beginning at approximately week 20 of gestation.

Pulmonary surfactant has both a lipid (approximately 90% of total) and a protein (approximately 10% of total) component.

About half of the lipids are dipalmitoylphosphatidylcholine. The remaining lipids include phosphatidylglycerol, which modulates the fluidity of the surfactant as well as cholesterol and other lipids. 

About half of the proteins are apoproteins, which have both hydrophobic and hydrophilic portions; the other half is composed of proteins normally found in blood plasma. These lipids and proteins all have the capability of interacting with hydrophilic or hydrophobic environments and it is this quality that leads to their specialized function in the lung. 
Ref: Huang H.J. (2012). Chapter 17. The Respiratory System. In L.W. Janson, M.E. Tischler (Eds), The Big Picture: Medical Biochemistry.

Q. 5

The mechanism of action of surfactant is?

 A

Lubricates the flow of C02 diffusion

 B

Binds oxygen

 C

Makes the capillary surface hydrophilic

 D

Breaks the structure of water in the alveoli

Q. 5

The mechanism of action of surfactant is?

 A

Lubricates the flow of C02 diffusion

 B

Binds oxygen

 C

Makes the capillary surface hydrophilic

 D

Breaks the structure of water in the alveoli

Ans. D

Explanation:

The low surface tension when the alveoli are small is due to the presence of surfactant in the fluid lining the alveoli. It is secreted by type II alveolar epithelial cells and lines the alveoli. Surfactant is a mixture of dipalmitoylphosphatidylcholine (DPPC), other lipids, and proteins. If the surface tension is not kept low when the alveoli become smaller during expiration, they collapse in accordance with the law of Laplace.

The water molecules lining the alveoli produce an elastic contractile force due to surface tension which causes the alveoli to collapse. This surface tension force of water molecules is broken by surfactant. In spherical structures like an alveolus, the distending pressure equals two times the tension divided by the radius (P = 2T/r); if T is not reduced as r is reduced, the tension overcomes the distending pressure. Surfactant also helps to prevent pulmonary edema.

Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 34. Introduction to Pulmonary Structure and Mechanics. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong’s Review of Medical Physiology, 24e.


Q. 6

Surfactant production in lungs starts at :

 A

28 weeks

 B

28 weeks

 C

34 wks

 D

36 wks

Q. 6

Surfactant production in lungs starts at :

 A

28 weeks

 B

28 weeks

 C

34 wks

 D

36 wks

Ans. A

Explanation:

A i.e. 28 Weeks


Q. 7

Surfactant appears in amniotic fluid at the gesta‑tional age of :

 A

20 weeks

 B

32 weeks

 C

36 weeks

 D

26 weeks

Q. 7

Surfactant appears in amniotic fluid at the gesta‑tional age of :

 A

20 weeks

 B

32 weeks

 C

36 weeks

 D

26 weeks

Ans. B

Explanation:

Ans. is b i.e. 32 weeks

Friends, I had to search a lot for this answer but all in vain.

Only Nelson Paediatric throws some light on this issue :

“Pulmonary surfactant is a heterogenous mixture of phospholipids and proteins secreted into the sac­cular or alveolar sub-phase by the type ll pneumocytes. Its presence is first recognised in characteristic secretory organelles known as lamellar bodies as early as the 24th week of gestation. However, surfac­tant lipids of which the most abundant is phosphatidyicholine, are not detectable in the amniotic fluid until the 30th week of gestation, suggesting that there is a chronologic gap between surfactant synthesis and secretion. Labour probably shortens this gap because phospholipids are consistently found in the air spaces of infants born before the 30th week of gestation.”

From the above lines it is clear :

Surfactant synthesis occurs in 24th week of gestation, it appear in amniotic fluid in 30th week of gestation. Since 30 weeks is not given in options, I would personally go with 32 weeks, when surfactant will absolutely be detected in amniotic fluid, whereas at 28 weeks it can detected only if preterm labour occurs.



Q. 8

Deficiency of surfactant is seen in –

 A

BPD

 B

Neonatal asphyxia

 C

HMD

 D

Anencephaly

Q. 8

Deficiency of surfactant is seen in –

 A

BPD

 B

Neonatal asphyxia

 C

HMD

 D

Anencephaly

Ans. C

Explanation:

Ans. is ‘c’ i.e., HMD

Etiopathogenesis of HMD

o Surfactant deficiency (decreased production and secretion) is the primary cause of RDS.

o Normally surfactant helps to reduce surface tension in the alveoli —> In the absence of surfactant, surface tension increases and alveoli collapse during expiration.

Why HMD is more common in preterm baby ?

o With advancing gestational age increasing amounts of surfactant is produced.

o Mature levels of pulmonary surfactant are usually present after 35 weeks.

o So, chance of HMD is inversely related to gestational age -f more the gestational age (more mature the neonate), less the chance of HMD.

Also know

o Surfactant starts appearing in fetal lung by 20th week of gestation.

o It appears in amniotic fluid between 28-32 weeks of gestation.


Q. 9

Surfactant acts on:       

March 2013

 A

Bronchi

 B

Alveoli

 C

Bronchioles

 D

Trachea

Q. 9

Surfactant acts on:       

March 2013

 A

Bronchi

 B

Alveoli

 C

Bronchioles

 D

Trachea

Ans. B

Explanation:

Ans. B i.e. Alveoli

Pulmonary surfactant

  • It is a surface-active lipoprotein complex (phospholipoprotein) formed by type II alveolar cells.
  • The proteins and lipids that comprise the surfactant have both a hydrophilic region and a hydrophobic region.
  • By adsorbing to the air-water interface of alveoli with the hydrophilic head groups in the water and the hydrophobic tails facing towards the air
  • The main lipid component of surfactant, dipalmitoyl phosphatidyl choline (DPPC), reduces surface tension.

Q. 10

Surfactant is produced by:          

March 2010

 A

Alveolar macrophages

 B

Lymphocytes in the alveoli

 C

Type I alveolar cells

 D

Type II alveolar cells

Q. 10

Surfactant is produced by:          

March 2010

 A

Alveolar macrophages

 B

Lymphocytes in the alveoli

 C

Type I alveolar cells

 D

Type II alveolar cells

Ans. D

Explanation:

Ans. D: Type II alveolar cells

The low surface tension when the alveoli are small is due to the presence in the fluid lining the alveoli of surfactant, a lipid surface-tension-lowering agent.

Surfactant is a mixture of dipalmitoylphosphatidylcholine (DPPC), other lipids, and proteins. If the surface tension is not kept low when the alveoli become smaller during expiration, they collapse in accordance with the law of Laplace. Surfactant is produced by type II alveolar epithelial cells. Typical lamellar bodies, membrane-bound organelles containing whorls of phospholipid, are formed in these cells and secreted into the alveolar lumen by exocytosis. Tubes of lipid called tubular myelin form from the extruded bodies, and the tubular myelin in turn forms the phospholipid film. Some of the protein-lipid complexes in surfactant are taken up by endocytosis in type II alveolar cells and recycled. Surfactant is important at birth. The fetus makes respiratory movements in utero, but the lungs remain collapsed until birth. After birth, the infant makes several strong inspiratory movements and the lungs expand. Surfactant keeps them from collapsing again. Surfactant deficiency is an important cause of infant respiratory distress syndrome (IRDS, also known as hyaline membrane disease), the serious pulmonary disease that develops in infants born before their surfactant system is functional.

Maturation of surfactant in the lungs is accelerated by glucocorticoid hormones. Fetal and maternal cortisol increase near term, and the lungs are rich in glucocorticoid receptors.

Patchy atelectasis is also associated with surfactant deficiency in patients who have undergone cardiac surgery involving use of a pump oxygenator and interruption of the pulmonary circulation. In addition, surfactant deficiency may play a role in some of the abnormalities that develop following occlusion of a main bronchus, occlusion of one pulmonary artery, or long-term inhalation of 100% 0,. Cigarette smoking also decreases lung surfactant.


Q. 11

Main part of surfactant is formed by ‑

 A

Dipalmitoyl phosphatidyl choline

 B

Phosphotidyl-inositol

 C

Dipalmitoyl cephaline

 D

Dipalmitoyl serine

Q. 11

Main part of surfactant is formed by ‑

 A

Dipalmitoyl phosphatidyl choline

 B

Phosphotidyl-inositol

 C

Dipalmitoyl cephaline

 D

Dipalmitoyl serine

Ans. A

Explanation:

Ans. is ‘a’ i.e., Dipalmitoyl phosphatidyl choline

Important glycerophospholipids are :‑

  • Phosphatidylcholine (lecithin):- These glycerophospholipids contain choline. These are most abundant phospholipids in cell membrane. Dipalmitoyl lecithin (dipalmitoyl phosphatidylcholine) is an important phosphatidylcholine found in lungs, secreted by type II pneumocytes. It acts as a lung surfactant and is  necessary for normal lung compliance. It reduces surface tension in alveoli and prevents alveolar collapse. Acute pulmonary respiratory distress syndrome (RDS) in premature infants is due to inefficient production of surfactant dipalmitoyl lecithin (dipalmitoyl phosphatidylcholine).
  • Phosphatidylethanolamine (cephaline) :- They have ethanolamine instead of choline. Thromboplastin (coagulation factor 111) is composed of cephaline. Like lecithin, Phosphatidylethanolamine is also a component of cell membrane.
  • Phosphatidylserine:- They have serine nitrogenous base. Phosphatidylserine is also a componant of cell membrane. It plays a role in apoptosis.
  • Phosphatidylinositol:- They have inositol as the stereoisomers, myoinositol. Phosphatidylinositol4,5‑bisphosphate is an important constituent of cell membrane phospholipids, upon stimulation by a suitable hormone agonist, it is cleaved into diacylglycerol and inositol triphosphate both of which act as second messengers.
  • Plasmalogens:- Plasmalogens are similar to other glycerophospholipids but fatty acid at Cl of glycerol is linked through an ether, rather than ester bond as occurs in other glycerophospholipids. So, plasmalogen is called ether glycerolipid. Plasmalogens are found in brain (myelin) and cardiac muscles. Plasmalogen is a platelet activating factor (PAF) and involved in platelet aggregation and degranulation.
  • Lysophospholipids:- Lysophospholipids are formed when one of the two fatty acid is removed from glycerophospholipids (i.e. Lysophospholipids=glycerophospholipid-1 fatty acid or lysophospholipids=glyceral + phosphoric acid + 1 fatty acid + nitrogenous base). Most common of these are lysophosphatidykholine (lysolecithine) and lysophosphatidylethanolamine (lysocephalin).
  • Cardiolipin (Diphosphatidylglycerol):- Cardiolipin’ is composed of two molecules of phosphatidic acid connected by a molecule of glycerol, i.e. cardiolipin phosphatidic acid + glycerol + phosphatidic acid. There is no nitrogenous base. Cardiolipin is a major lipid of inner mitochondrial membrane and is necessary for optimum functioning of ETC.


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