Re: why does oxygen diffuse into cells from the blood stream

Diffusion of different gases from the blood into tissues or from tissues
into blood depends on the difference in partial pressures of a gas on the
two sides. e.g. the partial pressure of oxygen in the arterial blood is 95
mmHg and in the tissue spaces is 40 mmHg and therefore there is a net
pressure of 55 mmHg that pushes oxygen from blood into the tissues. You may
ask here why the oxygen is low in the tissues. This is because the tissues
continuously consume oxygen so the oxygen partial pressure tends to fall
whereas the blood in arteries has much oxygen as it is continuously
oxygenated in the lungs.

Similarly the carbon dioxide tends to diffuse out of the tissues into the
blood because it is high in the tissues due to continuous production i.e. 
45 mmHg and in the arterial blood it is 40 mmHg. Thus net pressure of 5 
mmHg pushes the carbon dioxide out of the tisseus into the blood.

I hope its clear; if there is any confusion feel free to ask me.