Diffusion
O2 and CO2 diffuse across the boundary between the alveolus and the capillary down their respective concentration gradients.
Room air has approximately 21% oxygen.
The majority of O2 in the blood is bound to haemoglobin, only a very small percentage is dissolved in plasma.
CO2 is highly soluble in blood, carried mostly as bicarbonate and dissolved CO2.
Only 10 - 20% of CO2 is bound to haemoglobin, so the carriage is approximately proportionate to it's partial pressure.
Oxygen demand and delivery increases during times of physical stress, like exercise, and decreases at rest, e.g. while sleeping.
! We can use the Oxygen Saturation Level of haemoglobin to assess how much oxygen is in the blood stream, since the amount of haemoglobin is fixed.
