12.5: Anatomy of the Small Intestine

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The small intestine is a long, folded tube designed to finish digestion and efficiently absorb nutrients.

Master this section and you'll be able to:

• Describe the overall structure of the small intestine and explain how its regions and surface adaptations support digestion and absorption.
• Compare circular folds, villi, and microvilli and summarize how each increases the absorptive surface area of the small intestine.
• Differentiate between intraperitoneal and retroperitoneal organs and explain the role of the peritoneum and mesentery in supporting the intestines.

The word intestine is derived from a Latin root meaning “internal”, and indeed, the intestinal tract nearly fills the whole interior of the abdominal cavity. Also known as 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 small intestine is the primary site of digestion and absorption in the human body. After food leaves the stomach, it enters this long, highly folded tube where nutrients are broken down into their simplest forms and moved into the bloodstream. Although called “small,” it is actually the longest part of the digestive tract, and its internal surface is vastly expanded by circular folds, villi, and microvilli that maximize contact with chyme. As it moves through the duodenum, jejunum, and ileum, chyme encounters enzymes, bile, and bicarbonate that complete digestion and create ideal conditions for absorption. The small intestine’s structure and function work together to ensure that nearly all usable nutrients from a meal are efficiently extracted before the remaining material passes into the large intestine.

Anatomy of the Small Intestine

Chyme released from the stomach enters the small intestine, which is the primary digestive organ in the body. Not only is this where most digestion occurs, it is also where practically all absorption occurs. The longest part of the alimentary canal, the small intestine is about 3 meters (10 feet) long in a living person (but about twice as long in a cadaver due to the loss of muscle tone). 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 2.5 cm (1 in), compared with 7.5 cm (3 in) for the large intestine. As we will 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 m², more than 100 times the surface area of your skin. This large surface area is necessary for complex processes of digestion and absorption that occur within it.

The coiled tube of the small intestine is subdivided into three regions. From proximal (at the stomach) to distal, these are the duodenum, jejunum, and ileum (see the figure below).

Figure \(\PageIndex{1}\): Regions of the Small Intestine. The three regions of the small intestine, from proximal to distal, are the duodenum, jejunum, and ileum. (Image credit: "Regions of the Small Intestine" by Jennifer Lange is licensed under CC BY-NC-SA, modification of original “Drawing of Abdominal Organs”  by KnowledgeWorks Global Ltd. and Open Learning Initiative.)

Duodenum

The duodenum is the first and shortest segment of the small intestine, measuring about 25 centimeters (10 inches). It begins immediately at the pyloric sphincter. Just past this point, the duodenum curves backward behind the peritoneum, becoming retroperitoneal, and then forms a C-shaped loop around the head of the pancreas. After this curve, it travels upward and forward to reenter the peritoneal cavity and connect with the jejunum.

Within the wall of the duodenum is the hepatopancreatic ampulla, the site where the bile duct — carrying bile from the liver (via the gallbladder) — and the main pancreatic duct join. This shared duct system opens into the duodenum at the major duodenal papilla, allowing bile and pancreatic juice to enter and assist in digestion.

Jejunum

The jejunum is about 0.9 meters (3 feet) long (in life) and runs from the duodenum to the ileum. Jejunum means “empty” in Latin and supposedly was so named by the ancient Greeks who noticed it was always empty at death. No clear demarcation exists between the jejunum and the final segment of the small intestine, the ileum.

Ileum

Measuring about 1.8 meters (6 feet), the ileum is the longest portion of the small intestine. Compared with the jejunum, it is slightly thicker and more highly vascularized, and it contains more prominent mucosal folds. At its distal end, the ileum meets the cecum, the first part of the large intestine, at the ileocecal valve (or sphincter), which helps regulate the flow of intestinal contents. Both the jejunum and ileum are suspended from the posterior abdominal wall by the mesentery, and together these regions are framed on all sides by the large intestine.

The small intestine receives dual autonomic innervation, with parasympathetic fibers supplied by the vagus nerve and sympathetic fibers arriving through the thoracic splanchnic nerves. Its primary blood supply comes from the superior mesenteric artery, while veins that run alongside these arteries drain into the superior mesenteric vein. From there, nutrient-rich blood travels directly to the liver through the hepatic portal vein for processing and storage.
 

Histology of the Small Intestine

The wall of the small intestine is composed of the same four layers typically present in the digestive tract wall. However, three features of the mucosa and submucosa are unique. These features, which increase the absorptive surface area of the small intestine more than 600 times, include circular folds (plicae circulares), villi, and microvilli (see figure below). These adaptations are most abundant in the proximal two-thirds of the small intestine, where the majority of absorption occurs.

Figure \(\PageIndex{2}\): Structural Features That Increase Absorptive Surface Area in the Small Intestine. The small intestine maximizes nutrient absorption through multiple levels of folding. Circular folds create large ridges in the mucosa and submucosa, each covered with numerous fingerlike villi that contain blood vessels and a central lacteal for transporting absorbed nutrients. At an even smaller scale, the apical surfaces of the epithelial cells form countless microvilli, together called the brush border, which further increase surface area and hold digestive enzymes.l lustration" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original by OpenStax; slides b-d provided by the Regents of the University of Michigan Medical School © 2022.)
 

Circular Folds

Also called a plica circulare, a circular fold is a tall ridge of the mucosa and submucosa (see image above). These transverse folds can be up to 1 cm high and are easily visible to the naked eye. They begin near the proximal duodenum and extend to about the midpoint of the ileum, increasing the surface area available for absorption. Their curved shape also causes chyme to spiral rather than move straight through the lumen, slowing its flow and allowing more time for nutrients to be absorbed.

Villi

Within the circular folds are small (0.5–1 mm long) hairlike, vascularized projections of the epithelium and lamina propria called villi (singular: villus). Each square millimeter of mucosa contains roughly 20 to 40 villi, greatly expanding the surface area for absorption. The villi are covered by simple columnar epithelium, made mostly of absorptive cells.

Each villus contains connective tissue, a small amount of smooth muscle, a capillary bed (one arteriole and one venule), and a lymphatic capillary called a lacteal. Sugars and amino acids, produced from the digestion of carbohydrates and proteins, enter the bloodstream through these capillaries. In contrast, lipid breakdown products are absorbed into the lacteals and reach the bloodstream later via the lymphatic system (see image above).

Microvilli

As their name implies, microvilli (singular: microvillus) are much smaller than villi, measuring only about 1 micrometer in length. These tiny cylindrical projections extend from the apical (luminal) surface of the epithelial cells lining the mucosa. Each microvillus is supported internally by microfilaments, which help maintain its shape.

Although individual microvilli are too small to be seen clearly with a light microscope, the many thousands on each absorptive cell create a fuzzy, bristle-like appearance known as the brush border. Embedded in the membranes of the microvilli are enzymes that complete the digestion of carbohydrates and proteins, as well as transporter proteins that assist with nutrient uptake.

The small intestine contains an estimated 200 million microvilli per square millimeter. This enormous number dramatically increases the surface area of the epithelial membrane, greatly improving the efficiency of absorption.

Figure \(\PageIndex{3}\): Structural Adaptations That Increase Absorptive Surface Area in the Small Intestine. Shown are the three levels of surface area amplification in the small intestine. (b) Circular folds are large ridges of mucosa and submucosa that slow the movement of chyme and increase contact time with the intestinal wall. (c) Villi are fingerlike projections of the mucosa that further expand the absorptive surface and contain blood vessels and a lacteal for nutrient uptake. (d) Microvilli form tiny membrane extensions on each epithelial cell, creating the brush border where many final digestive enzymes are located.

Intestinal Glands

In addition to the three specialized absorptive features just discussed, the mucosa between the villi is dotted with deep crevices that each lead into a tubular intestinal gland (crypt of Lieberkühn), which is formed by cells that line the crevices. These produce intestinal juice, a slightly alkaline (pH 7.4 to 7.8) mixture of water and mucus. Each day, about 1 to 1.9 liters (1 to 2 quarts) are secreted in response to the distention of the small intestine or the irritating effects of chyme on the intestinal mucosa. In addition to the exocrine glands making intestinal juice, there are enteroendocrine cells to secrete many regulatory hormones that communicate with other digestive organs including the pancreas and gallbladder.

The submucosa of the duodenum is the only site of the complex mucus-secreting duodenal glands (Brunner’s glands), which produce a bicarbonate-rich alkaline mucus that buffers the acidic chyme as it enters from the stomach.

Lymphatic Nodules

The layer of loose connective tissue that lies just under the epithelium of the mucosa in the small intestine contains clusters of immune cells called mucosa-associated lymphoid tissue (MALT). In the ileum, these clusters form larger groups known as Peyer’s patches, which help prevent bacteria in the intestine from getting into the bloodstream. Peyer’s patches are easiest to see in younger people and become less noticeable with age, similar to the overall activity of the immune system.

Figure \(\PageIndex{4}\): Histology of the Ileum with Peyer’s Patches. This section of the ileum shows several key features of the small intestine. Fingerlike villi project into the lumen to increase the surface area for absorption, while the intestinal crypts sit between them as sites of cell renewal. Beneath the mucosa lies the submucosa, where large clusters of lymphoid tissue called Peyer’s patches help monitor and protect against bacteria entering from the gut. Below this region, the circular and longitudinal layers of the muscularis work together to mix and move intestinal contents.

 

Visualizing the Intestine: Colonoscopy

Learn more about colonoscopy by watching the video below. A colonoscope is the special tool used to perform a colonoscopy. It is a thin, flexible, tubular ‘telescope’ with a light and video camera that your doctor carefully guides through your colon in order to see and determine the health of your colon. Watch this animation to learn about the features of the colonoscope, how the colonoscopy procedure is performed and how polyps are removed, and the follow-up care you and your doctor should talk about after your procedure.