When we breathe in air with oxygen will flow into our lungs each lung contains a network of very small tubes each tube has an air sac at the end of it/ Ait goes along the tubes into the air sas. air contains oxygen and our blood contains haemoglobin which has a strong chemical attraction for oxygen. oxygen combines with haemoglobin to form oxxyhaemoglobin. When bloog reaches tissues which have a low conventration of oxygen, the oxyhaemoglobin breaks down and oxygen is released. Oxygen passes through the walls o f the air sacs into small blood vessels by diffusion.

Blood travels from the lungs to every part of our body and the ozygen diffuses from the blogg into every cell in our body. in the cells the oxygen is used up and carbon dioxide and water is produced. The carbon dioxide goes into th eblood system by diffusion The structure of the alveoli is well adapted for this process.

the alveoli increase th surface area for diffusion. this is beacasue there are thousands of very small alveoli. the walls of the alveoli are very thing the lining of the alveoli is always moist the alveoli are surrounded by a dense network of tiny bloo dvessels