Your body is shaped like a very complicated donut

The digestive tract is a passage, not a pocket

Food travels through a tube whose entrance is the mouth and whose exit is the anus. Although much of that tube lies inside the body, its inner passage is continuous with the outside world: food in the gut has not crossed into the body’s tissues. It is inside the digestive tract, but still separated from the blood and organs by the gut wall.

That distinction is what makes the “donut” comparison useful. In topology—the study of shapes and connections that remain unchanged when something is stretched or bent—a donut has a tunnel through it. The digestive tract is not literally a donut-shaped organ, but its continuous passage gives the body a similar kind of through-route.

A long route packed into a small space

The tube is folded and coiled to fit within the abdomen. Its walls do more than contain food: muscular contractions push it forward, while the lining absorbs useful molecules into the body. Material that is not absorbed keeps moving toward the exit.

So the route’s boundary matters. The gut wall separates what is passing through from the body’s internal environment. Nutrients become part of that environment only after crossing the lining; waste can pass all the way through without entering the tissues at all. The surprising point is that a passage can run through the middle of the body while remaining, in this sense, connected to the outside.