Most cells are microscopic. A group of similar cells gathered together is called a tissue . Tissues, which may be visible to the naked eye, include bone, muscle , fat , and skin. Different kinds of tissue working together in the same place may form an organ. Organs, like the heart, lungs , eyes, and brain, perform specific tasks necessary for the body's survival.
Several organs can work together in a system .
The organs of a system may be close together, or spread across the body. Some examples are the digestive , reproductive, respiratory, excretory , and nervous systems. Some kinds of tissue may be found in more than one system. Muscle tissue , for example, is part of the muscular, respiratory, and circulatory systems. The Learning Network Inc. All rights reserved.