Fick's Law
(the gas diffusion law) states that gas diffuses from an area of high
concentration (the 'gas' side of the alveoli) to an area of low concentration
(the 'liquid' side of the alveoli) where the de-oxygenated blood is waiting for
it.
Since the oxygen is attached to the haemoglobin rather than carried in
solution, Henry’s Law (amount of gas dissolved in a liquid is proportional to
the pressure of the gas over the liquid) does not apply.