Merck Manual

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Barriers on the Outside and the Inside


Alexandra Villa-Forte

, MD, MPH, Cleveland Clinic

Reviewed/Revised Apr 2022 | Modified Sep 2022

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 Structure and Function of the Skin The skin is the body’s largest organ. It serves many important functions, including Protecting the body against trauma Regulating body temperature Maintaining water and electrolyte balance Sensing... read more Structure and Function of 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 Overview of the Digestive System The digestive system, which extends from the mouth to the anus, is responsible for receiving food, breaking it down into nutrients (a process called digestion), absorbing the nutrients into... read more 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 Overview of the Respiratory System To sustain life, the body must produce sufficient energy. Energy is produced by burning molecules in food, which is done by the process of oxidation (whereby food molecules are combined with... read more ) 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 tiny hairs called 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.

Saliva, so important in the mouth, can cause severe damage if inhaled into the lungs because saliva carries bacteria that can cause an abscess to form in the lung. The hydrochloric acid produced by the stomach rarely causes harm there. However, the acid can burn and damage the esophagus if it flows backward and can damage other organs if it leaks through the stomach wall. Stool, the undigested part of food expelled through the anus, can cause life-threatening infections if it leaks into the abdominal cavity, which can happen if a hole develops in the intestinal wall.

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