The ileum joins the cecum, the first portion of the large intestine, at the ileocecal sphincter (or valve). It is thicker, more vascular, and has more developed mucosal folds than the jejunum. The ileum is the longest part of the small intestine, measuring about 1.8 meters (6 feet) in length. No clear demarcation exists between the jejunum and the final segment of the small intestine, the ileum. Jejunum means “empty” in Latin and supposedly was so named by the ancient Greeks who noticed it was always empty at death. The jejunum is about 0.9 meters (3 feet) long (in life) and runs from the duodenum to the ileum. The hepatopancreatic sphincter (sphincter of Oddi) regulates the flow of both bile and pancreatic juice from the ampulla into the duodenum.įigure 23.18 Small Intestine The three regions of the small intestine are the duodenum, jejunum, and ileum. This ampulla opens into the duodenum at a tiny volcano-shaped structure called the major duodenal papilla. Located in the duodenal wall, the ampulla marks the transition from the anterior portion of the alimentary canal to the mid-region, and is where the bile duct (through which bile passes from the liver) and the main pancreatic duct (through which pancreatic juice passes from the pancreas) join. Of particular interest is the hepatopancreatic ampulla (ampulla of Vater). The duodenum can therefore be subdivided into four segments: the superior, descending, horizontal, and ascending duodenum. Just past the pyloric sphincter, it bends posteriorly behind the peritoneum, becoming retroperitoneal, and then makes a C-shaped curve around the head of the pancreas before ascending anteriorly again to return to the peritoneal cavity and join the jejunum. The shortest region is the 25.4-cm (10-in) duodenum, which begins at the pyloric sphincter. From proximal (at the stomach) to distal, these are the duodenum, jejunum, and ileum ( Figure 23.18). The coiled tube of the small intestine is subdivided into three regions. This large surface area is necessary for complex processes of digestion and absorption that occur within it. As we’ll see shortly, in addition to its length, the folds and projections of the lining of the small intestine work to give it an enormous surface area, which is approximately 200 m 2, more than 100 times the surface area of your skin. Since this makes it about five times longer than the large intestine, you might wonder why it is called “small.” In fact, its name derives from its relatively smaller diameter of only about 2.54 cm (1 in), compared with 7.62 cm (3 in) for the large intestine. The longest part of the alimentary canal, the small intestine is about 3.05 meters (10 feet) long in a living person (but about twice as long in a cadaver due to the loss of muscle tone). Not only is this where most digestion occurs, it is also where practically all absorption occurs. The Small IntestineĬhyme released from the stomach enters the small intestine, which is the primary digestive organ in the body. In addition, called the small and large bowel, or colloquially the “guts,” they constitute the greatest mass and length of the alimentary canal and, with the exception of ingestion, perform all digestive system functions. The word intestine is derived from a Latin root meaning “internal,” and indeed, the two organs together nearly fill the interior of the abdominal cavity. Trace the pathway of food waste from its point of entry into the large intestine through its exit from the body as feces.Identify the beneficial roles of the bacterial flora in digestive system functioning.List three features unique to the wall of the large intestine and identify their contributions to its function.
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