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The heart is absolutely essential, because it provides oxygen to the other organs, and the brain is necessary for conscious life (versus instinctual life). The third most important organ, not counting the skin (which keeps everything contained and protected) are the lungs, which are the primary method of the body for absorbing and transferring oxygen. Since oxygen is an important fuel source, there needs to be a way of easily transferring mass quantities of it into the bloodstream; the skin absorbs a small amount of oxygen through osmosis, but not enough to keep all the organs properly oxygenated, and nowhere nearly enough for the brain to stay healthy. By extracting oxygen from air (which is mostly inert nitrogen) the lungs allow the blood to absorb oxygen from the alveoli, where it travels through the heart and to other parts of the body. After releasing its oxygen, the blood passes through the heart and returns to the lungs to collect more oxygen. Humans can live with only one lung, since it provides sufficient oxygen to keep the body operating, but at reduced respiratory function.
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