VPB 121: Veterinary Physiology - II (2+1)

SURFACE TENSION OF ALVEOLAR FLUID AIDS IN EXPIRATION

• Alveolar fluid coats the inside surface of the alveoli. The surface tension produces an inwardly directed force causing the alveoli to assume the smallest possible diameter.
• In order to expand the lungs, this surface tension must be exceeded. This surface tension accounts for approximately two thirds of the lung's elastic recoil.
• Surfactant, produced by type II alveolar cells, is a complex of lipids and proteins that reduces the surface tension in much the same way the soap allows lipids to dissolve in aqueous solutions.
• Surfactant decreases surface tension which increases pulmonary compliance (reducing the effort needed to expand the lungs) reduces tendency for alveoli to collapse.
• Since surfactant is one of the last compounds produced during embry­onic development, premature animals often have respiratory distress as a result of the underdeveloped respiratory system.
• In the case of sheep, surfactant is released into the alveolar spaces near the beginning of the fourth month of gestation. Its release correlates with a rise in plasma cortisol levels.