Human respiratory gas system

1.Contaminated blood comes to capillaries by Pulmonary artery 

2.High concentration of oxygen in Alveoli 

3.Here,gas exchange occur by diffusion process where oxygen moves from alveoli to capillaries 

4.carbon dioxide moves from blood vessel to alveoli due to the concentration process 

Blood become oxygenated or purified through this process than transfer From lungs to heart through pulmonary vein