The small intestine is the organ where the digestion of protein, fats, and carbohydrates is completed. It is mainly responsible for mixing food with digestive juices and bringing it into contact with the intestine lining for absorption. The small intestine absorbs 90% of nutrients and water. It also moves food further down the gastrointestinal tract.

The small intestine is a long tube-like organ. Its inside is covered with the intestinal epithelium that increases the surface area of the intestine and increase absorption efficiency of the nutrients. The small intestine is surrounded by two layers of smooth muscles that contract in a wavelike pattern. This peristaltic movement mixes the content of the small intestine and slowly moves it towards the large intestine. Absorbed nutrients in the blood are carried into the hepatic portal vein, which leads to the liver.

Three panels. Left most shows a view of the small intestine from outside the tube. Lines pointing to the side of the small intestine are labelled muscle layers. Blood vessels are seen attaching to the outside of the small intestine. One side of the lumen opens and villi are seen elevated from the internal wall of the small intestine. A square is seen around a protrusion and leads to panel two. 
Panel two shows a cross section of two protrusions. At the base, blue veins and red arteries are seen leading into each protrusion along with a yellow tube labelled lymphatic vessel which come from below a layer of cells at the bottom of the image. Each protrusion is lined by a layer of white cells labelled absorptive enterocyte cells, interspersed with a few orange cells. 
A third panel shows one white cell at full zoom showing internal organelles. At the top of the enterocyte are a bunch of tiny protrusions labelled microvilli.

Figure 1: Components of the small intestine: (1) Muscle layers, (2) villi, (3) blood vessels, (4) lumen, (5) lymphatic vessel, (6) absorptive enterocyte cells, (7) capillary, (8) artery, (9) vein, (10) microvilli

The human small intestine is over 6 meters long and is divided into three parts: the duodenum, the jejunum, and the ileum.