What is it?

The diagram shows the major topological relationships in the body.

The linings of all

• exocrine glands, including digestive glands,
• nasal passages, trachea, and lungs,
• kidney tubules, collecting ducts, and bladder,
• reproductive structures like the vagina, uterus, and fallopian tubes

are all continuous with the surface of the body. Anything placed within their lumen (the cavity inside a tubular structure such as a blood vessel) is, strictly speaking, outside the body. This includes

• the secretions of all exocrine glands (in contrast to the secretions of endocrine glands, which are deposited in the blood).
• Any indigestible material placed in the mouth which will appear, in due course, at the other end.