Definition of Diffusion

In the activity below you need to type in the correct answers from the words and phrases provided here: 'higher,' 'net movement,' 'lower,' 'substance.'

Diffusion is the of a from a region of concentration to a region of concentration.

Molecules move across the capillary walls even when there the concentration is the same on both sides of the capillary this is known as capillary exchange but for efficient respiration there needs to be a difference in the concentration of oxygen and carbon dioxide in the alveoli compared to the capillaries surrounding them to enable gas exchange to occur by diffusion


The alveoli provide a huge surface area for efficient gas exchange. The alveoli have to stay open and not collapse to prevent their sides sticking together. If the alveoli do collapse, as can happen in some conditions, it is very difficult to open them up again.

To stop this happening each alveoli is lined with a layer of fluid that acts like a “bubble” holding them open. The lining of alveoli contain substances which help strengthen the curved surface of fluid, these substances are called surfactants. These naturally occurring surfactants are similar to substances added to washing up liquid to strengthen bubbles and make them “longer lasting”.

Surfactants are phosopholipids secreted by epithelial cells lining the alveoli. Premature babies born before they are producing surfactant have a lower chance of survival than those born later who do produce surfactant. They will need intensive care and there is now artificial surfactant available.

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