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.