It is dependent on smaller, simpler units that each serve specific purposes. Cells themselves are extremely complex, come in many different shapes and sizes, and serve countless different functions.
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 .