The duodenum of the small intestine and related organs

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Small Intestine

The small intestine extends from the pyloric sphincter to the ileocecal valve, where it empties into the large intestine. The small intestine finishes the process of digestion, absorbs the nutrients, and passes the residue on to the large intestine. The liver, gallbladder, and pancreas are accessory organs of the digestive system that are closely associated with the small intestine.

The small intestine is divided into the duodenum, jejunum, and ileum. The small intestine follows the general structure of the digestive tract in that the wall has a mucosa with simple columnar epithelium, submucosa, smooth muscle with inner circular and outer longitudinal layers, and serosa. The absorptive surface area of the small intestine is increased by plicae circulares, villi, and microvilli.

Exocrine cells in the mucosa of the small intestine secrete mucus, peptidase, sucrase, maltase, lactase, lipase, and enterokinase. Endocrine cells secrete cholecystokinin and secretin.

The most important factor for regulating secretions in the small intestine is the presence of chyme. This is largely a local reflex action in response to chemical and mechanical irritation from the chyme and in response to distention of the intestinal wall. This is a direct reflex action, thus the greater the amount of chyme, the greater the secretion.

Large Intestine

The large intestine is larger in diameter than the small intestine. It begins at the ileocecal junction, where the ileum enters the large intestine, and ends at the anus. The large intestine consists of the colon, rectum, and anal canal.

The wall of the large intestine has the same types of tissue that are found in other parts of the digestive tract but there are some distinguishing characteristics. The mucosa has a large number of goblet cells but does not have any villi. The longitudinal muscle layer, although present, is incomplete. The longitudinal muscle is limited to three distinct bands, called teniae coli, that run the entire length of the colon. Contraction of the teniae coli exerts pressure on the wall and creates a series of pouches, called haustra, along the colon. Epiploic appendages, pieces of fat-filled connective tissue, are attached to the outer surface of the colon.

Unlike the small intestine, the large intestine produces no digestive enzymes. Chemical digestion is completed in the small intestine before the chyme reaches the large intestine. Functions of the large intestine include the absorption of water and electrolytes and the elimination of feces.

Rectum and Anus

The rectum continues from the sigmoid colon to the anal canal and has a thick muscular layer. It follows the curvature of the sacrum and is firmly attached to it by connective tissue. The rectum ends about 5 cm below the tip of the coccyx, at the beginning of the anal canal.

The last 2 to 3 cm of the digestive tract is the anal canal, which continues from the rectum and opens to the outside at the anus. The mucosa of the rectum is folded to form longitudinal anal columns. The smooth muscle layer is thick and forms the internal anal sphincter at the superior end of the anal canal. This sphincter is under involuntary control. There is an external anal sphincter at the inferior end of the anal canal. This sphincter is composed of skeletal muscle and is under voluntary control.

What Are the Intestines?

The intestine is a muscular tube which extends from the lower end of your stomach to your anus, the lower opening of the digestive tract. It is also called the bowel or bowels. Food and the products of digestion pass through the intestine, which is divided into two sections called the small intestine and the large intestine.

What Is the Small Intestine?

The small intestine is made up of three segments, which form a passage from your stomach (the opening between your stomach and small intestine is called the pylorus) to your large intestine:

• Duodenum: This short section is the part of the small intestine that takes in semi-digested food from your stomach through the pylorus, and continues the digestion process. The duodenum also uses bile from your gallbladder, liver, and pancreas to help digest food.
• Jejunum: The middle section of the small intestine carries food through rapidly, with wave-like muscle contractions, towards the ileum.
• Ileum: This last section is the longest part of your small intestine. The ileum is where most of the nutrients from your food are absorbed before emptying into the large intestine.

By the time food reaches your small intestine, it has already been broken up and mashed into liquid by your stomach. Each day, your small intestine receives between one and three gallons (or six to twelve liters) of this liquid. The small intestine carries out most of the digestive process, absorbing almost all of the nutrients you get from foods into your bloodstream. The walls of the small intestine make digestive juices, or enzymes, that work together with enzymes from the liver and pancreas to do this.

How can the small intestine digest so much?

Looking at the small intestine as a pipe, it seems hard to believe that an organ so narrow could do such a big job. However, looks can be deceiving. The absorptive surface area of the small intestine is actually about 250 square meters (almost 2,700 square feet) – the size of a tennis court! How is this possible? The small intestine has three features which allow it to have such a huge absorptive surface area packed into a relatively small space:

• Mucosal folds: The inner surface of the small intestine is not flat, but thrown into circular folds. This not only increases the surface area, but helps regulate the flow of digested food through your intestine.
• Villi: The folds form numerous tiny projections which stick out into the open space inside your small intestine (or lumen), and are covered with cells that help absorb nutrients from the food that passes through.
• Microvilli: The cells on the villi are packed full of tiny hairlike structures called microvilli. This helps increase the surface of each individual cell, meaning that each cell can absorb more nutrients.

Although the small intestine is narrower than the large intestine, it is actually the longest section of your digestive tube, measuring about 22 feet (or seven meters) on average, or three-and-a-half times the length of your body.

What Is the Large Intestine?

Your large intestine is about five feet (or 1.5 meters) long. The large intestine is much broader than the small intestine and takes a much straighter path through your belly, or abdomen. The purpose of the large intestine is to absorb water and salts from the material that has not been digested as food, and get rid of any waste products left over. By the time food mixed with digestive juices reaches your large intestine, most digestion and absorption has already taken place.

What's left is mainly fiber (plant matter which takes a long time to digest), dead cells shed from the lining of your intestines, salt, bile pigments (which give this digested matter its color), and water. In the large intestine, bacteria feed on this mixture. These helpful bacteria produce valuable vitamins that are absorbed into your blood, and they also help digest fiber. The large intestine is made up of the following parts:

• Cecum: This first section of your large intestine looks like a pouch, about two inches long. It takes in digested liquid from the ileum and passes it on to the colon.
• Colon: This is the major section of the large intestine; you may have heard people talk about the colon on its own. The colon is also the principal place for water reabsorption, and absorbs salts when needed. The colon consists of four parts:
  • Ascending colon: Using muscle contractions, this part of the colon pushes any undigested debris up from the cecum to a location just under the right lower end of the liver.
  • Transverse colon: Food moves through this second portion of the colon, across your front (or anterior) abdominal wall, traveling from left to right just under your stomach.
  • Descending colon: The third portion of colon pushes its contents from just near the spleen, down to the lower left side of your abdomen.
  • Sigmoid colon: The final S-shaped length of the colon, curves inward among the coils of your small intestine, then empties into the rectum.
  • Rectum: The final section of digestive tract measures from 1 to 1.6 inches (or 2.5 to 4 cm). Leftover waste collects there, expanding the rectum, until you go to the bathroom. At that time, it is ready to be emptied through your anus.

Learn more about Intestine Transplant Disease States.

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To make a referral to the Hillman Center for Pediatric Transplantation at UPMC Children’s Hospital of Pittsburgh for an intestine transplant, contact our referral transplant coordinators:

Jan Blice
Phone: 412-692-8184
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Renee Brown-Bakewell
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What 3 organs are connected to the duodenum?

What is the duodenum of the small intestine?

What organs are connected to the small intestine?

What 3 structures enter the small intestine at the duodenum?