View transcript
Transcript:
The Body's Relationship to the Outside World
One of the four main types of tissues in the human body is epithelial tissue, which protects the body from physical damage and controls which substances enter and leave the body.
The surface of the skin, represented here by the outermost border around the rectangle, is an example of an epithelium that faces the outside world. The underlying layer of skin faces inside the body.
This simple rectangle can help you see how epithelia separate outside from inside and how different body systems control the flow of substances in and out of the body. The digestive system takes food in from the outside world, through the mouth, passes needed nutrients from the food into the bloodstream, so that the blood can carry those nutrients to all of the body's cells. Wastes from these processes are expelled to the outside world by both the digestive system and the urinary system. The respiratory system brings oxygen into the body and carries waste, in the form of carbon dioxide, outside the body.
In all of these situations epithelia separate inside from outside.