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Pulmonary Surfactant Function and Ventilation

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  • 0:42 Water and Surface Tension
  • 1:44 Surface Tension and…
  • 2:23 Pulmonary Surfactant
  • 3:11 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

Our lungs are lined with a thin layer of water. The water creates surface tension, which makes it difficult for the lungs to expand and allow for gas exchange. Pulmonary surfactant is made by our lungs and decreases the surface tension so we can breathe.

Gas Exchange Requires a Wet Surface

Lungs are kept wet inside the body so that the gas exchange needed for respiration can occur
Gas exchange

Have you ever wondered why lungs are on the inside of land-dwelling animals, while gills are on the outside of fish? In order for oxygen and carbon dioxide to get into and out of the blood, these gases have to cross a wet surface. Since fish live in the water, their gills can hang out and stay wet. As we live on land, our lungs are best kept wet on the inside of the body. A thin layer of water lines the inside of our respiratory system and provides that wet surface for gas exchange.

Water and Surface Tension

As it turns out, the thin layer of water lining the respiratory system presents a challenge. While the water is necessary for gas exchange, it creates surface tension that makes ventilation difficult. Have you ever performed a belly flop off a diving board into a swimming pool? If you have, then you have experienced the effect of surface tension, an attractive force created by chemical bonds between water molecules at a water's surface. This property is the reason why water can resist external force. Surface tension allows insects to walk on water and holds water together in drops dripping from a leaking faucet. For that matter, have you ever played the backyard game Red Rover? Think of the people holding hands as being like water molecules creating surface tension. In order to break through the line, the runner has to force the people's hands apart.

Surface Tension and Respiration

Surface tension of the water lining the lungs keeps the lungs from expanding
Surface tension

Okay, so what does surface tension have to do with the respiratory system and gas exchange? In short, surface tension created by water lining the respiratory system prevents the lungs from expanding. Not only do the water molecules hold onto each other, they hold onto the cells lining the respiratory system as well. Here, the surface tension creates an inward directed force, and that prevents the lungs from expanding. This is a problem as breathing is not possible unless the lungs can expand. If we can't breathe in fresh air, gas exchange cannot occur.

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