As strange as it may seem, defining what is outside and what is inside the body is not always easy because the body has many surfaces. The skin, which is actually an organ system, is obviously on the outside of the body. It forms a barrier that prevents many harmful substances from entering the body. The digestive system is a long tube that begins at the mouth, winds through the body, and exits at the anus. Is food as it passes through this tube inside or outside of the body? Nutrients and fluid are not really inside the body until they are absorbed into the bloodstream.

Air passes through the nose and throat into the windpipe (trachea), then into the extensive, branching airways of the lungs (bronchi). At what point is this passageway inside the body? Oxygen in the lungs (see Overview of the Respiratory System) is not useful to the body until it enters the bloodstream. To enter the bloodstream, oxygen must cross through a thin layer of cells lining the lungs. This layer acts as a barrier to viruses and bacteria, such as those that cause tuberculosis, which may be carried into the lungs with air. Unless these organisms penetrate the cells or enter the bloodstream, they generally do not cause disease. Because the lungs have many protective mechanisms, such as antibodies to fight infection and cilia to sweep debris out of the airways, most airborne infectious organisms never cause disease.

Body surfaces not only separate the outside from the inside but also keep structures and substances in their proper place so that they can function properly. For example, internal organs do not float in a pool of blood because blood is normally confined to blood vessels. If blood leaks out of the vessels into other parts of the body (hemorrhage), it not only fails to bring oxygen and nutrients to tissues but also can cause severe harm. For example, a small amount of blood leaking into the brain can destroy brain tissue because there is no room for expansion in the skull. On the other hand, a similar amount of blood leaking into the abdomen does not destroy tissue because the abdomen has room for expansion.