QS Study

Surfactant is a mixture of dipalmitoyl phosphatidyl-choline, other lipids and protein, and ions which decrease the surface tension of the fluid lining alveoli, thus preventing the collapse of lungs. The main function of surfactant is to lower the surface tension at the air/liquid interface within the alveoli of the lung.

Site of secretion

A surfactant is produced by type II alveolar epithelial cells. (granular pneumocyte) and Clara cells of terminal bronchioles.

Beginning at secretion

Surfactant does not normally begin to be secreted into the alveoli until between the sixth and seventh months of gestation, & in some cases even later than that.

Composition

Approximate composition of surfactant –

 

Function

  1. It lowers alveolar surface tension and thereby prevents the collapse of alveoli.
  2. The surfactant fluid regulates the alveolar size.
  3. It helps in maintaining negative intra plural pressure by importing the recoil of the lungs.
  4. It acts as a detergant.
  5. It also helps to prevent pul. edema.

Surfactant helps to prevent pul. edema: It has been calculated that if it were not present the unopposed surface tension in the alveoli would produce 20 mm Hg force favoring transudation of fluid from the blood into the alveoli.

Production of Surfactant

  • The surfactant is produced by type II alveolar epithelial cells.
  • These type II cells form lamellar bodies which are membrane-bound organelles containing whorles of phospholipid. They are secreted into the alveolar lumen by exocytosis.
  • Tubular myelin (tubes of lipids) is formed from the extruded bodies. The tubular myelin, in turn, forms the phospholipid film.
  • Some of the protein lipids complexes in surfactant are taken up by endocytosis in type II alveolar cells are recycled.
  • Formation of surfactant in the lungs is accelerated by thyroid and glucocorticoid hormone.