True regarding physiology of surfactant is

Answer: c) SP-A plays a major role in innate host defense of the lungSURFACTANT* Produced by type-ll alveolar cells(granular pneumocytes)* Synthesis and storage of surfactant begins at about 16 weeks of gestation, and lung homogenate: have high concentrations of surfactant by 20 weeks.* However, surfactant is not secreted until later, appearing in amniotic fluid between 28 and 38 weeks of gestation.* Starts at 28 weeks of gestation & peaks at 35weeks* The lamellar inclusion bodies in type-ll cells store surfactant.* Lamellar inclusion bodies are continuously discharged into the alveolar surface by constitutive exocytosis.* Lamellar inclusion body materials are converted to tubular myelin, which forms the surfactant layer.* Composition:o Pulmonary surfactant is a complex mixture of phospholipids, neutral lipids, fatty acids, and proteins.o 85% to 90% lipids, predominantly phospholipids:# Major phospholipid is phosphatidylcholine, approximately 75% of which is present as dipalmitoyl phosphatidylcholine (DPPC)# Second most abundant phospholipid is phosphatidylglycerol (PG)o 10% to 15% proteins.* Functions: -o Reduces alveolar surface tension (surface tension is inversely proportional to the surfactant concentration per unit area)o Prevent alveolar collapseo Prevents pulmonary edema* Surfactant proteins are (SP)-A, SP-B, SP-C, and SP-D.* SP-A: large glycoprotein regulates the feedback uptake of surfactant by the type II alveolar cells.* SP-B and SP-C: hydrophobic, facilitates formation of the monomolecular film of phospholipid. Mutation of SP-C -familial interstitial lung disease.* SP-A and SP-D: members of collectin family, involved in innate immunity in the conducting airway as well as in the alveoli.* Synthesis of surfactant is facilitated by:o Glucocorticoidso Thyroxineo Insulino Stretching of lungs in infancy (yawning & sighing)o Beta agonistso Calciumo Regular exercise* Deficient conditions: occlusion of main bronchus/pulmonary artery, smoking, long term inhalation of 100% oxygen* Deficiency causes: IRDS/ Hyaline membrane disease (In 1959, Avery and Mead discovered it), patchy atelectasis & ARDS* Over production or dysregulation of surfactant synthesis lead to respiratory distress & is the cause for pulmonary alveolar proteinosis * Surface tension of:o Pure water, 72 dynes/cm;o Normal fluids lining the alveoli without surfactant: 50 dynes/cm;o Normal fluids lining the alveoli with normal amounts of surfactant: between 5 and 30 dynes/cm.