The small intestine is a crucial component of the digestive system that allows for the breakdown and absorption of important nutrients that permit the body to function at its peak performance. The small intestine accomplishes this via a complex network of blood vessels, nerves, and muscles that work together to achieve this task. It is a massive organ that has an average length of 3 to 5 meters. It divides into the duodenum, jejunum, and ileum.[rx][rx][rx]
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The duodenum is the shortest section, on average measuring from 20 cm to 25 cm in length. Its proximal end is connected to the antrum of the stomach, separated by the pylorus, and the distal end blends into the beginning of the jejunum. The duodenum surrounds the pancreas, in the shape of a “C” and receives chyme from the stomach, pancreatic enzymes, and bile from the liver; this is the only part of the small intestines where Brunner’s glands are present on histology.
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The jejunum is roughly 2.5 meters in length, contains plicae circulares (muscular flaps), and villi to absorb the products of digestion.
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The ileum is the final portion of the small intestine, measuring around 3 meters, and ends at the cecum. It absorbs any final nutrients, with major absorptive products being vitamin B12 and bile acids.
Layers of the Small Intestine
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Serosa: The serosa is the outside layer of the small intestine and consists of mesothelium and epithelium, which encircles the jejunum and ileum, and the anterior surface of the duodenum since the posterior side is retroperitoneal. The epithelial cells in the small intestine have a rapid renewal rate, with cells lasting for only 3 to 5 days.
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Muscularis: The muscular consists of two smooth muscle layers, a thin outer longitudinal layer that shortens and elongates the gut, and a thicker inner circular layer of smooth muscle, which causes constriction. Nerves lie between these two layers and allow these to muscle layers to work together to propagate food in a proximal to distal direction.
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Submucosa: The submucosa consists of a layer of connective tissue that contains the blood vessels, nerves, and lymphatics.
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Mucosa: The mucosa is the innermost layer and is designed for maximal absorption by being covered with villi protruding into the lumen that increases the surface area. The crypt layer of the small bowel is the area of continual cell renewal and proliferation. Cells move from the crypts to the villi and change into either enterocytes, goblet cells, Paneth cells, or enteroendocrine cells.
Of importance is the mesentery, which is a double fold of the peritoneum that not only anchors the small intestines to the back of the abdominal wall, but also contains the blood vessels, nerves, and lymphatic vessels that supply the small intestine.[rx][rx]
Structure-Function and Anatomy of the Small Intestine
The principal function of the small intestine is to break down food, absorb nutrients needed for the body, and get rid of unnecessary components. It also plays a role in the immune system, acting as a barrier to a multitude of flora that inhabits the gut and making sure no harmful bacteria enter the body.
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The duodenum is the initial portion of the small intestine and is where absorption actually begins. It is often described as being split into four parts: superior, descending, horizontal, and ascending. The superior portion is the only section that is peritoneal; the rest is retroperitoneal. Pancreatic enzymes enter the descending duodenum via the hepatopancreatic ampulla and break down chyme, a mix of stomach acid and food, from the stomach. Bicarbonate is also secreted into the duodenum to neutralize stomach acid before reaching the jejunum. Lastly, the liver introduces bile into the duodenum, which allows for the breakdown and absorption of lipids from food products. A significant landmark for the duodenum is the ligament of Trietz, a ligament made of skeletal muscle that tethers the duodenal-jejunal flexure to the posterior wall.
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The primary function of the jejunum is to absorb sugars, amino acids, and fatty acids. Both the jejunum and ileum are peritoneal.
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The ileum absorbs any remaining nutrients that did not get absorbed by the duodenum or jejunum, in particular vitamin B12, as well as bile acids that will go on to be recycled.
The small intestine is the part of the gastrointestinal tract where much of the digestion and absorption of food takes place.
Key Points
The small intestine is the part of the gastrointestinal tract that follows the stomach, which is in turn followed by the large intestine.
The average length of the small intestine in an adult human male is 6.9 m (22 feet, 6 inches), and in the adult female 7.1 m (23 feet, 4 inches).
The small intestine is divided into the duodenum, jejunum, and ileum.
Much of the small intestine is covered in projections called villi that increase the surface area of the tissue available to absorb nutrients from the gut contents.
Key Terms
- duodenum: The first part of the small intestine that starts at the lower end of the stomach and extending to the jejunum.
- ileum: The last, and usually the longest, division of the small intestine; the part between the jejunum and large intestine.
- small intestine: The upper part of the intestine, between the stomach and the large intestine, that is divided into the duodenum, the jejunum, and the ileum.
- jejunum: The center of the three divisions of the small intestine that lies between the duodenum and the ileum.
The Small Intestine
The small intestine is the part of the gastrointestinal tract that follows the stomach, which is in turn followed by the large intestine. The small intestine is the site where almost all of the digestion and absorption of nutrients and minerals from food takes place.
Small intestine: An illustration of the small intestine with the duodenum, jejunum, and ileum labeled.
The average length of the small intestine in an adult human male is 6.9 m (22 feet, 6 inches), and in the adult female 7.1 m (23 feet, 4 inches). It can vary greatly, from as short as 4.6 m (15 feet) to as long as 9.8 m (32 feet). The small intestine is approximately 2.5–3 cm in diameter, and is divided into three sections:
- The duodenum is the first section of the small intestine and is the shortest part of the small intestine. It is where most chemical digestion using enzymes takes place.
- The jejunum is the middle section of the small intestine. It has a lining that is designed to absorb carbohydrates and proteins. The inner surface of the jejunum, its mucous membrane, is covered in projections called villi, which increase the surface area of tissue available to absorb nutrients from the gut contents. The epithelial cells which line these villi possess even larger numbers of microvilli. The transport of nutrients across epithelial cells through the jejunum includes the passive transport of some carbohydrates and the active transport of amino acids, small peptides, vitamins, and most glucose. The villi in the jejunum are much longer than in the duodenum or ileum.
- The ileum is the final section of the small intestine. The function of the ileum is mainly to absorb vitamin B12, bile salts, and any products of digestion that were not absorbed by the jejunum. The wall itself is made up of folds, each of which has many tiny finger-like projections known as villi on its surface. The ileum has an extremely large surface area both for the adsorption of enzyme molecules and for the absorption of products of digestion.
The Villi
The villi contain large numbers of capillaries that take the amino acids and glucose produced by digestion to the hepatic portal vein and the liver. Lacteals are the small lymph vessels that are present in the villi. They absorb fatty acids and glycerol, the products of fat digestion, into direct circulation.
Layers of circular and longitudinal smooth muscle enable the digested food to be pushed along the ileum by waves of muscle contractions called peristalsis. The undigested food (waste and water) is sent to the colon.
Histology of the Small Intestine
The small intestine wall has four layers: the outermost serosa, muscularis, submucosa, and innermost mucosa.
Key Points
The outermost layer of the intestine, the serosa, is a smooth membrane consisting of a thin layer of cells that secrete serous fluid, and a thin layer of connective tissue.
The muscular is a region of muscle adjacent to the submucosa membrane. It is responsible for gut movement (also called peristalsis ). It usually has two distinct layers of smooth muscle: circular and longitudinal.
The submucosa is the layer of dense irregular connective tissue or loose connective tissue that supports the mucosa; it also joins the mucosa to the bulk of underlying smooth muscle.
The mucosa is the innermost tissue layer of the small intestines and is a mucous membrane that secretes digestive enzymes and hormones. The intestinal villi are part of the mucosa.
The three sections of the small intestine look similar to each other at a microscopic level, but there are some important differences. The jejunum and ileum do not have Brunner’s glands in the submucosa, while the ileum has Peyer’s patches in the mucosa, but the duodenum and jejunum do not.
Key Terms
- Brunner’s glands: Compound, tubular, submucosal glands found in that portion of the duodenum that is above the hepatopancreatic sphincter (sphincter of Oddi).
- Peyer’s patches: Patches of lymphoid tissue or lymphoid nodules on the walls of the ileum in the small intestine.
- intestinal wall: The wall of the small intestine is composed of four layers, from the outside to the inside: serosa, muscularis, submucosa, and mucosa.
The Small Intestine’s Layers
Section of duodenum: This image shows the layers of the duodenum: the serosa, muscularis, submucosa, and mucosa.
The small intestine has four tissue layers:
- The serosa is the outermost layer of the intestine. The serosa is a smooth membrane consisting of a thin layer of cells that secrete serous fluid and a thin layer of connective tissue. Serous fluid is a lubricating fluid that reduces friction from the movement of the muscular.
- The muscular is a region of muscle adjacent to the submucosa membrane. It is responsible for gut movement, or peristalsis. It usually has two distinct layers of smooth muscle: circular and longitudinal.
- The submucosa is the layer of dense, irregular connective tissue or loose connective tissue that supports the mucosa, as well as joins the mucosa to the bulk of underlying smooth muscle.
- The mucosa is the innermost tissue layer of the small intestines and is a mucous membrane that secretes digestive enzymes and hormones. The intestinal villi are part of the mucosa.
The three sections of the small intestine look similar to each other at a microscopic level, but there are some important differences. The jejunum and ileum do not have Brunner’s glands in the submucosa, while the ileum has Peyer’s patches in the mucosa, but the duodenum and jejunum do not.
Brunner’s Glands
Brunner’s glands (or duodenal glands) are compound tubular submucosal glands found in the duodenum. The main function of these glands is to produce a mucus-rich, alkaline secretion (containing bicarbonate) in order to neutralize the acidic content of chyme that is introduced into the duodenum from the stomach, and to provide an alkaline condition for optimal intestinal enzyme activity, thus enabling absorption to take place and lubricate the intestinal walls.
Peyer’s Patches
Peyer’s patches are organized lymph nodules. They are aggregations of lymphoid tissue that are found in the lowest portion of the small intestine, which differentiate the ileum from the duodenum and jejunum.
Because the lumen of the gastrointestinal tract is exposed to the external environment, much of it is populated with potentially pathogenic microorganisms. Peyer’s patches function as the immune surveillance system of the intestinal lumen and facilitate the generation of the immune response within the mucosa.
Intestinal Villi
Micrograph of the small intestine: A low-magnification micrograph of small intestinal mucosa that shows villi.
Intestinal villi (singular: villus) are tiny, finger-like projections that protrude from the epithelial lining of the mucosa. Each villus is approximately 0.5–1.6 mm in length and has many microvilli (singular: microvillus), each of which are much smaller than a single villus.
Villi increase the internal surface area of the intestinal walls. This increased surface area allows for more intestinal wall area to be available for absorption. An increased absorptive area is useful because digested nutrients (including sugars and amino acids) pass into the villi, which is semi-permeable, through diffusion, which is effective only at short distances.
In other words, the increased surface area (in contact with the fluid in the lumen) decreases the average distance traveled by the nutrient molecules, so the effectiveness of diffusion increases.
The villi are connected to blood vessels that carry the nutrients away in the circulating blood.
Blood Supply and Lymphatics
The arterial blood supply for the small intestine first comes from the celiac trunk and the superior mesenteric artery (SMA).
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The superior pancreaticoduodenal artery is fed from the gastroduodenal artery, which branches from the proper hepatic artery, which is traceable back to the celiac trunk. It anastomoses with the inferior pancreaticoduodenal artery, which comes from the SMA, to supply blood to the duodenum.
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The jejunum and ileum receive their blood supply from a rich network of arteries that travel through the mesentery and originate from the SMA. The multitude of arterial branches that split from the SMA is known as the arterial arcades, and they give rise to the vasa recta that deliver the blood to the jejunum and ileum.
The venous blood mimics that of the arterial supply, which coalesces into the superior mesenteric vein (SMV), which then joins with the splenic vein to form the portal vein.
Lymphatic drainage starts at the mucosa of the small intestine, into nodes next to the small intestine in the mesentery, to nodes near the arterial arcades, then to nodes near the SMA/SMV. Lymph then flows into the cisterna chyli and then up the thoracic ducts, and then empties into the venous system left internal jugular, and subclavian veins meet. The lymphatic drainage of the small intestine is a major transport system for absorbed lipids, the immune defense system, and the spread of cancer cells coming from the small intestine, explaining Virchow’s node enlargement from the small intestine cancers.
Nerves
The nervous system of the small intestine is made up of the parasympathetic and sympathetic divisions of the autonomic nervous system. The parasympathetic fibers originate from the Vagus nerve and control secretions and motility. The sympathetic fibers come from three sets of splanchnic nerve ganglion cells located around the SMA. Motor impulses from these nerves control blood vessels, along with gut secretions and motility. Painful stimuli from the small intestine travel through the sympathetic fibers as well.
Muscles
Two layers of smooth muscle form the small intestine. The outermost layer is the thin, longitudinal muscle that contracts, relaxes, shortens, and lengthens the gut allowing food to move in one direction. The innermost layer is a thicker, circular muscle. This layer enables the gut to contract and break apart larger food particles. It also stops food from moving in the wrong direction by blocking the more proximal end. The two muscle layers work together to propagate food from the proximal end to the distal end.
Digestive Processes of the Small Intestine
The small intestine uses different enzymes and processes to digest proteins, lipids, and carbohydrates.
Key Points
The small intestine is where most chemical digestion in the human body takes place.
Most of the digestive enzymes in the small intestine are secreted by the pancreas and enter the small intestine via the pancreatic duct.
The three major classes of nutrients that undergo digestion are proteins, lipids (fats), and carbohydrates.
Key Terms
- Digest enzymes: Enzymes that break down polymeric macromolecules into their smaller building blocks to facilitate their absorption by the body.
Chemical Digestion in the Small Intestine
The small intestine is where most chemical digestion takes place. Most of the digestive enzymes in the small intestine are secreted by the pancreas and enter the small intestine via the pancreatic duct.
These enzymes enter the small intestine in response to the hormone cholecystokinin, which is produced in response to the presence of nutrients. The hormone secretin also causes bicarbonate to be released into the small intestine from the pancreas to neutralize the potentially harmful acid coming from the stomach.
The three major classes of nutrients that undergo digestion are proteins, lipids (fats), and carbohydrates.
Proteins
Proteins are degraded into small peptides and amino acids before absorption. Their chemical breakdown begins in the stomach and continues through the large intestine.
Proteolytic enzymes, including trypsin and chymotrypsin, are secreted by the pancreas and cleave proteins into smaller peptides. Carboxypeptidase, a pancreatic brush border enzyme, splits one amino acid at a time. Aminopeptidase and dipeptidase free the end amino acid products.
Lipids
Lipids (fats) are degraded into fatty acids and glycerol. Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides. Pancreatic lipase works with the help of the salts from bile secreted by the liver and the gallbladder.
Bile salts attach to triglycerides and help to emulsify them; this aids access by pancreatic lipase because the lipase is water-soluble, but the fatty triglycerides are hydrophobic and tend to orient toward each other and away from the watery intestinal surroundings.
The bile salts act to hold the triglycerides in their watery surroundings until the lipase can break them into the smaller components that are able to enter the villi for absorption.
Carbohydrates
Some carbohydrates are degraded into simple sugars, or monosaccharides (e.g., glucose, galactose) and are absorbed by the small intestine. Pancreatic amylase breaks down some carbohydrates (notably starch) into oligosaccharides. Other carbohydrates pass undigested into the large intestine, where they are digested by intestinal bacteria.
Brush border enzymes take over from there. The most important brush border enzymes are dextrinase and glucoamylase, which further break down oligosaccharides. Other brush border enzymes are maltase, sucrase, and lactase.
Lactase is absent in most adult humans and for them, lactose, like most poly-saccharides, is not digested in the small intestine. Some carbohydrates, such as cellulose, are not digested at all, despite being made of multiple glucose units. This is because the cellulose is made out of beta-glucose that makes the inter-monosaccharide bindings different from the ones present in starch, which consists of alpha-glucose. Humans lack the enzyme for splitting the beta-glucose-bonds—that is reserved for herbivores and bacteria in the large intestine.
| Major Digestive Enzymes | |||
|---|---|---|---|
| Enzyme | Produced In | Site of Release | pH Level |
| Carbohydrate Digestion | |||
| Salivary amylase | Salivary glands | Mount | Neutral |
| Pancreatic amylase | Pancreas | Small intestine | Basic |
| Maltase | Small intestine | Small intestine | Basic |
| Protein Digestion | |||
| Pepsin | Gastric glands | Stomach | Acidic |
| Trypsin | Pancreas | Small intestine | Basic |
| Peptidases | Small intestine | Small intestine | Basic |
| Nucleic Acid Digestion | |||
| Nuclease | Pancreas | Small intestine | Basic |
| Nucleosidases | Pancreas | Small intestine | Basic |
| Fat Digestion | |||
| Lipase | Pancreas | Small intestine | Basic |
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