Your small intestine is folded to the size of a room

The small intestine is a tube only a few centimeters wide, yet its inner absorbing surface is enormous — estimated at around 175 square meters, vastly more than the gut’s plain length would suggest. It pulls this off with three nested levels of folding.

First, the lining is thrown into large circular folds that roughly triple the surface. Those folds are carpeted in millions of finger-like villi, each projecting into the channel and adding about a tenfold increase. Finally, every cell on a villus bristles with hundreds of microscopic microvilli, multiplying the area perhaps 20 times more.

Stack those factors and a modest pipe becomes one of the largest surfaces in the body.

The payoff is absorption. Nutrients from digested food must cross this lining to reach the blood, and the more surface available, the more efficiently it happens — which is why diseases that flatten the villi cause severe malnutrition even when a person eats normally.