Embryologically the small intestine develops from the midgut region connected to the yolk sac by the yoke stalk (a part of the umbilical cord) and connects to the hindgut region ending at the cloacal membrane, which is formed by the ectoderm. The developmental stage of the small intestine follows a process of elongation and oration. The elongation growth in length herniates the midgut at the umbilicus and external to the abdomen. Rotationally the intestine rotates 90 degrees in a counterclockwise direction from midsagittal to transverse. As the intestine is drawn back into the abdominal, the small intestine folds and forms a wavy coiled structure. During the absorption of the insertion back into the abdomen, the cecum, which is the connection between the small intestine and the large intestine, rotates 180 degrees in a counterclockwise direction to establish the position of the small intestine and large intestine in the peritoneal space.
The small intestine develops into a long narrow, coiled tube that extends from the stomach to the large intestine. Within the small intestine, most of the bodies digestion and absorption of food takes place. It is between 6.7 and 7.6 meters long. It is sophisticatedly intricate and contained in the central lower abdominal cavity. The lining of the internal wall of the small intestine is a mucous membrane that transversally folds into what is called plicae circulates. In the superior areas, minute fingerlike projections known as villi project into the cavity, greatly increasing the recreating and absorbing surface area. The small intestine is supported exteriorly by a thin membranous material called the mesentery that suspends the intestine. The mesentery contains fat that helps retain heat in the small intestine and an extensive web of blood vessels.
Even though there is no distinctly marked separation, there are three areas of the small intestine which form the regions of the continuous tube:
- The first area, the duodenum, is not supported by the mesentery. It is the smallest area of the small intestine between 23 and 28 cm long. It is positioned posteriorly to the liver and adjacently right of the stomach. It is roughly horseshoe-shaped, curling inferiorly to the stomach back into the direction of the midline, following the curvature of the inferior side of the stomach up and to the left. It has the widest diameter of the small intestine, and ducts from the liver, gallbladder and pancreas enter the duodenum to provide juices such as bicarbonate to neutralise the acid in the gastric secretions from the pylorus of the stomach, pancreatic enzymes to further digestion, and bile salts to emulsify fat. The mucous lining of the duodenum begins the absorption of nutrients, particularly iron and calcium, before the food contents enter the next area of the small intestine.
- The second part is the jejunum, supported by a low-fat content mesentery, interior to the duodenum. It is the central part of the small intestine located in the abdomen's central section. It is around 2.5 meters long and is a deep red colour because of its extensive blood supply. Its peristaltic movements are rapid and vigorous, moving the food content through and absorbing water and nutrients. 40 per cent of digestion takes place in the Jejunum.
- The third and most inferior section of the small intestine is the ilium, located in the lower abdomen and is around 3.5 meters long. The wall of the ilium is narrower and thinner than the rest of the small intestine. The peristaltic movements are slower, and the supporting mesentery has a high-fat content. The ilium absorbs any final nutrients left, focusing on absorbing vitamin B12 and bile acid. The ilium ends at the cecum.