Every living organism must maintain its boundaries so that its internal environment (inside) remains distinct from the external environment surrounding it (outside). In single-celled organisms, the external boundary is a limiting membrane that encloses its contents and admits needed substances while restricting entry of potentially damaging or unnecessary substances. Similarly, all the cells of our body are surrounded by a selectively permeable membrane. Additionally, the body as a whole is enclosed and protected by the integumentary system, or skin, which protects our internal organs from drying out (a fatal change), bacteria, and the damaging effects of heat, sunlight, and an unbelievable number of chemicals in the external environment.
