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An organ is a discrete group of specialized cells that are separated from the rest of the body. It excludes tissue that is found throughout the body. The major organs of the body are the brain, heart, lungs, liver, kidneys, stomach, intestines and the sex organs (ovaries and testicles). In most cases, failure of an organ can be life threatening. However, many organs can suffer damage that can be repaired without adverse effect on a patient. For example, a person can live well with only one kidney or half their lung capacity, and can easily give up a few feet of the small intestine.

Organ at Wikipedia