We all know that we can't live without oxygen, so how is it possible to breathe for another person by doing Expired Air Resuscitation or Mouth-to-Mouth Resuscitation?

It is only possible because we breathe out almost as much oxygen as we breathe in. And the reason we can do that is because we have the most amazing body. Lungs are the first part of this incredible design for oxygen delivery: Oxygen gets into our bloodstream by passing through the very thin walls that of our alveoli, or air sacs, that are found at the end of the airways, and into the capillaries that wrap around the alveoli, which also have walls that are only one cell thick.

The surface area of all the alveoli is approximately the same size as a tennis court, and so this is why we can get lots of oxygen into our blood stream very quickly and efficiently.

Once the oxygen gets into the blood stream, it joins up with haemoglobin, which is the second part of the wonderful design, to make "oxyhaemoglobin". Think of chemistry classes at high school now, when you put together sticks and balls to demonstrate what molecules of certain things looked like. For instance, you combined two oxygen balls with one hydrogen ball to show what water looks like and it is this kind of combination that happens with haemoglobin and oxygen.

The haemoglobin carries oxygen around the bloodstream until it gets into the capillaries that wrap around tissue cells. Once the blood is passing through the capillaries, haemoglobin starts to release some of the oxygen and because the walls of the capillaries and the tissue cells are only one cell thick, oxygen can get through the walls to combine with glucose inside the tissue cells to keep each one alive. Each molecule of haemoglobin can carry 4 molecules of oxygen, which makes it an efficient oxygen carrier. If we didn't have haemoglobin to carry around the oxygen in our blood, we would need to inhale miles more oxygen and we probably wouldn't be able to mouth-to-mouth resuscitation.

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