Lungs are organs of providing oxygen to the blood from the breathing air, and as well as removing carbon dioxide from the blood. So this can be described as cleaning the blood. What are lungs of the world? They are great natural forests like Amazon and Congo. Why they are called as lungs of the world? They can clean the atmosphere. Plants can absorb carbon dioxide of the air when the photosynthesis is taken place. So they can reduce the amount of carbon dioxide and increase the level of oxygen in the atmosphere. The natural rain forests like Amazon and Congo can greatly do this.
And this is more like the function of lungs in the human body. That is why they are called as lungs of the world.