Unloading and loading in lungs with partial pressure of oxygen

- O2 enters the blood capillaries at the alveoli - Alveoli have high pO2 - So O2 loads onto haemoglobin, forming oxyhaemoglobin - When cells respire, use up O2 - Lowering pO2 - Red blood cells deliver oxyhaemoglobin to tissues - Unloads O2 - Haemoglobin returns to lungs, picks up more O2