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The lungs are massive vessel-like organs in the chest cavity. They are the largest and most essential part of the respiration system. The air we breathe enters and exits our lungs through two passages; the bronchioles and the bronchi. Lungs' main responsibilty is to take oxygen from the atmosphere and transport it into the bloodstreem. It then excretes carbon dioxide from blood into the atmoshpere. This gas exchange happens through specialized cells that form many millions of thin-walled air sacs. Some non-respiratory functions are also performed by the lungs.

In single celled bacteria and other small organisms, the respiration process of gas exhange can happen through simple diffusion. In large organisms, however, this is impossible, because a very small amount of cells in large organisms are located close enough to the surface for diffusion to take place. Two major changes made it possible for all organisms to attain multicellularity. One being an efficient circulatory system to transport gases to and from all tissues in the body. The other is a large respiratory system. The respiratory system centralizes the responsibilty of collecting oxygen from the atmosphere and brings it into the body, where it can be rapidly sent to tissues through the circulatory system.

Lungs also have several jobs that do not include the respiratory function. A main job for the lungs is to protect the heart. They surround the heart and act as cushions to soften any blow that the heart may have to endure. Without this padding, simply bumping into someone could be fatal. Another responsibilty of this organ is to influence the concentration of biologically active drugs and other substances which are used in medicine. Filtering out small blood clots formed in the systemic veins is another task performed by the lungs.