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Blood and the respiratory system
Blood and the respiratory system

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2.4 Surface tension

In the previous section, you saw how partial pressure gradients drive the exchange of O2 and CO2 between the blood and the alveoli. Diffusion of the gases at this air–liquid interface is facilitated by a thin layer of water that coats the inner surface of the alveoli. Condensation of the water vapour that is exhaled when you breathe out is the reason why you ‘see’ your breath in cold weather.

Individual molecules of water (H2O) bind together because hydrogen and oxygen atoms are strongly attracted to each other. This is why your hair sticks together when it’s wet. This force is called hydrogen bonding.

Because hydrogen bonds are quite strong, when water molecules come into contact with each other, they will be held together tightly. This tight packing creates a surface tension in the water that forces it to adopt the smallest shape possible (e.g. a droplet) (Figure 6).

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Figure 6 Surface tension of water holds it in a droplet form.

However, because the alveoli are round in shape, the surface tension that holds the water molecules together also puts an inward pressure on the inside of the alveolus (Figure 7).

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Figure 7 Surface tension (T) created by hydrogen bonding of water puts pressure on the inside of the alveoli, which have a small diameter or radius (R).

As you have just learnt, if the pressure in the alveolus is higher than the atmospheric pressure, air from the atmosphere will not enter and the alveolus will collapse (a medical condition called atelectasis). How does the lung combat the surface tension of water to ensure that the alveoli can expand with each breath? Cells within the alveoli secrete surfactant, a substance that attaches to the water molecules and prevents them from interacting with each other. This reduces the surface tension in the alveolus to near-zero levels. This effect is nicely demonstrated in Video 9. Why not try this experiment yourself at home?

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Video 9 Surface tension broken by surfactant.
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Surfactant also serves to prevent the collapse of the alveoli of newborn babies when they take their first breaths. Premature babies born before their surfactant production system is fully functional suffer from respiratory distress syndrome (RDS). Surface tension in the lungs of these babies is high and many alveoli fail to expand. Failure to produce enough surfactant may also be a problem in adult life; for example, surfactant production in the lungs of smokers is greatly reduced, increasing the likelihood of breathing difficulties compared to non-smokers.