Discribe the diffusion of oxygen and carbondyoxied between blood and air in the lungs?

Question

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Answer 1

It is important to understand first of all what diffusion is.  Molecules that are free to move around in their medium (like molecules in a gas or liquid phase) will tend to migrate from areas where the concentration (number of molecules per unit volume) is high to areas where the concentration is low.  This is consistent with the concept of random motion of the these molecules.

With respect to blood, the concentration of oxygen in the bloodstream is always lower than it would be in blood outside of the body in prolonged contact with air.  This is because in the body respiratory uptake of oxygen is occurring in body tissues.  Hence, in the lungs oxygen will continually move from where its concentration is higher (in the air of the alveoli) to where it is lower (in the bloodstream).  The argument for carbon dioxide is just the reverse of this.  Due to respiratory activity in body tissues the concentration of this gas in the bloodstream is higher that it would be in a vessel containing blood outside the body is free contact with normal air.  Thus, carbon dioxide tends to move (diffuse) out of the bloodstream into the alveoli and into the air therein.