Small intestine
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.
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.