The lungs are part of the respiratory system and a nice metaphor for the lungs is an inverted tree. With a tree you have a trunk that splits into smaller and smaller branches and ends in leaves. The lungs are similar in that they consist of a large tube, which carries air and then the tube divides into the two lungs. The lungs consist of small tubes -- think of the branches in our tree metaphor -- that then connect to smaller “twigs” until finally we see the “leaves” at the end of the twigs.
The leaves in the tree example correspond to what are called “alveoli”. Alveoli are the air sacs through which air comes in and out of the body. More specifically, they are the structures through which we exchange air in and out of our blood. The tiniest air tubes that connect to the alveoli are called “bronchioles”. The entire respiratory tree is what enables us to receive oxygen in and remove carbon dioxide from our bloodstream.