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
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
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.
Fortunately, specialized cells within the lungs produce pulmonary surfactant, which decreases the surface tension by breaking the bond between water molecules. Therefore, pulmonary surfactant allows the lungs to expand so air can enter and gas exchange can occur. Respiratory distress syndrome, or (RDS), occurs when a person cannot breathe due to lack of sufficient surfactant. RDS is a breathing disorder that affects newborns. While it rarely occurs in full-term infants, it is most common in premature infants. For example, nearly all infants born before 28 weeks gestation will not have enough surfactant to be able to breathe without assistance.
Pulmonary surfactant decreases surface tension by breaking the bond between water molecules
In summary, gas exchange must occur across a wet surface. The lungs are lined with a thin layer of water to provide a wet surface for gas exchange. That water creates surface tension in which water molecules are attracted to each other. That surface tension prevents the lungs from expanding, which we need for ventilation. Pulmonary surfactant is produced by cells within the lungs and decreases surface tension by breaking bonds between water molecules. Therefore, pulmonary surfactant allows the lungs to expand so we can breathe. Respiratory distress syndrome occurs when a person cannot breathe due to not having enough pulmonary surfactant.
After viewing this lesson you should be able to explain the importance of water to breathing, identify pulmonary surfactant, and the cause of respiratory distress syndrome.