Schematic  diagram to show gas exchange between alveolus, blood, and tissue
A    Diffusion of carbon dioxide
B    Diffusion of oxygen
C    Direction of blood flow
1    Alveolus
2    Pulmonary artery
3    Capillary
4    Vena cava
5    Pulmonary vein
6    Aorta
7    Tissue fluid
8    Tissue cells