Surfactant

Surface Tension

• The force (eg. in dynes) acting across an imaginary line 1cm long in the surface of a liquid
• Measured with a surface balance - tray of saline which is expanded and compressed by a moveable barrier while force exerted on a platinum strip is measured
• Occurs due to attraction between adjacent liquid molecules being much stronger than attraction between liquid and gas, therefore liquid surface area becomes as small as possible
• Seen in a soap bubble forming at the end of a tube - makes smallest surface area possible for a volume (a sphere), and therefore generate pressure within the bubble
• Pressure inside a bubble predicted by Laplace's law: pressure = (4 x surface tension)/radius (4 becomes 2 if only the inner surface involved - eg. in an alveolus)
• Smaller bubbles have higher pressures, therefore tend to blow up larger bubbles
• Researchers found that surface tension might contribute to pressure-volume behavior of lung when they found that lungs distend more easily with saline than with air

Surfactant

• A phospholipid - dipalmitoyl phosphatidylcholine (DPPC)
• Produced by type II alveolar epithelial cells. Lamellated bodies extrude into the alveoli and then transform into surfactant.
• Synthesized in the lung from fatty acids extracted from blood or made in lungs
• Rapid synthesis and turnover, therefore if a portion of lung loses blood supply eg. in an embolus, surfactant is depleted.
• Formed late in fetal life, and babies without adequate amounts may develop respiratory distress and die
• Pure saline gives a surface tension of 70 dynes/cm, while lung extract with surfactant gives variable low surface tension
• How surfactant works:

• DPPC molecules are hydrophobic at one end and hydrophilic at the other.
• They align on the surface, and intermolecular repulsive forces oppose the normal attracting forces between the liquid surface molecules.
• Reduction in surface tension is greater when the film is compressed because the DPPC molecules are then crowded closer and repel more

• Advantages of surfactant:

• Low surface tension in alveoli increases lung compliance and reduces the work of expanding it with each breath
• Stability of lung is promoted. Normally there is a tendency for small bubbles to collapse and blow up larger ones. When surfactant is present, surface tension is proportional to area, and tendency for this to occur is reduced.
• Helps to keep alveoli dry. Surface tension forces collapse alveoli and suck fluid out of capillaries. By reducing these forces, surfactant prevents transudation of fluid.
• Therefore if surfactant is absent, the result is poorly compliant, atelectatic lungs filled with transudate - seen in infant ARDS. Surfactant can be instilled at birth to prevent this.

Interdependence is another factor preventing lung collapse - the large expanding forces developed on alveoli due to the expansion of surrounding parenchyma.