Movement of O2 from alveoli to blood and blood to tissue occurs by simple passive diffusion (Fig 1).
Gases move down a pressure gradient from an area of high to low pressure.
Blood flow and ventilation maintain the pressure gradients, thus sustaining gas exchange.
The solubility of O2 in plasma is very low (0.23 mL/L/kPa), and wholly inadequate to meet tissues needs.
O2 carriage is increased hugely to meet tissue requirements by the specialized carriage system haemoglobin.
Fig 1 Movement of oxygen from alveoli to blood and blood to tissue