In humans, the small intestine is generally more than 20 feet long, making it a much more prominent digestive structure than the small intestines of most nonhuman primates. This notable difference is reflective of the significant divergence in diet of omnivorous humans from the great apes, which primarily are vegetarians that consume large amounts of low-quality plants with a high proportion of roughage.

The surface area of the lining of the human small intestine is approximately 300 square meters, an area roughly equivalent to that of a standard tennis court. An average of three to six hours is required for foodstuff to be transported all the way through the length of small intestine, which is typically considered to be divided into three regions: the duodenum, jejunum, and ileum.

The small intestine employs small finger-like appendages termed villi for absorbing nutrients from material being digested. Villi are comprised of large, circular folds of the small intestine, a capillary bed, a lymphatic capillary, smooth muscle, and a brush border (microvilli). The appendages are important for the digestive process because they greatly increase the surface area available for absorption of nutrients and for the secretion of digestive enzymes. Thousands of villi cover each square inch of the small intestine, but are most numerous near the beginning of the organ. The collective movements of villi are thought to enhance blood and lymph flow through the small intestine.

The sample of human small intestine tissue presented in the digital image above was stained with Texas Red-X conjugated to wheat germ agglutinin (WGA), one of the most commonly used lectins in microbiology. WGA selectively binds to N-acetylglucosamine and N-acetylneuraminic (sialic acid) residues. In addition, the specimen was labeled with Alexa Fluor 488 conjugated to phalloidin, a phallotoxin that targets filamentous actin. DAPI, which emits blue fluorescence when it binds to AT regions of DNA, was employed as a counterstain. Images were recorded in grayscale with a 12-bit digital camera coupled to either a Nikon E-600 or Eclipse 80i microscope equipped with bandpass emission fluorescence filter optical blocks. During the processing stage, individual image channels were pseudocolored with RGB values corresponding to each of the fluorophore emission spectral profiles.