The small intestine is the part of the gastrointestinal tract following the stomach and followed by
the large intestine, and is where much of the digestion and absorption of food takes
place. The small intestine is composed of a duodenum, jejunum, and ileum. It receives
bile juice and pancreatic juice through the hepatopancreatic duct, controlled by the sphincter
of Oddi. In invertebrates such as worms, the terms "gastrointestinal tract" and "large
intestine" are often used to describe the entire intestine. This article is primarily
about the human gut, though the information about its processes is directly applicable
to most placental mammals. The primary function of the small intestine is the absorption of
nutrients and minerals found in food.
Structure The average length of the small intestine
in an adult human male is 6.9 m, and in an adult female 7.1 m. It can vary greatly,
from as short as 4.6 m to as long as 9.8 m. It is approximately 2.5–3 cm in diameter.
The surface area of the human small intestinal mucosa averages 30 square meter
The small intestine is divided into three structural parts:
Duodenum Jejunum
Ileum Histology
The three sections of the small intestine look similar to each other at a microscopic
level, but there are some important differences. The parts of the intestine are as follows:
Function Food from the stomach is allowed into the
duodenum through the pylorus by a muscle called the pyloric sphincter.
Digestion The small intestine is where most chemical
digestion takes place. Most of the digestive enzymes that act in the small intestine are
secreted by the pancreas and enter the small intestine via the pancreatic duct. Enzymes
enter the small intestine in response to the hormone cholecystokinin, which is produced
in the small intestine in response to the presence of nutrients. The hormone secretin
also causes bicarbonate to be released into the small intestine from the pancreas in order
to neutralize the potentially harmful acid coming from the stomach.
The three major classes of nutrients that undergo digestion are proteins, lipids and
carbohydrates: Proteins are degraded into small peptides
and amino acids before absorption. Chemical breakdown begins in the stomach and continues
in the small intestine. Proteolytic enzymes, including trypsin and chymotrypsin, are secreted
by the pancreas and cleave proteins into smaller peptides. Carboxypeptidase, which is a pancreatic
brush border enzyme, splits one amino acid at a time. Aminopeptidase and dipeptidase
free the end amino acid products. Lipids 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 the bile secreted by the liver
and the gall bladder. Bile salts attach to triglycerides to help emulsify them, which
aids access by pancreatic lipase. This occurs because the lipase is water-soluble but the
fatty triglycerides are hydrophobic and tend to orient towards each other and away from
the watery intestinal surroundings. The bile salts emulsify the triglycerides in the watery
surroundings until the lipase can break them into the smaller components that are able
to enter the villi for absorption. Some carbohydrates are degraded into simple
sugars, or monosaccharides. Pancreatic amylase breaks down some carbohydrates into oligosaccharides.
Other carbohydrates pass undigested into the large intestine and further handling 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, are 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, making the inter-monosaccharidal
bindings different from the ones present in starch, which consists of alpha-glucose. Humans
lack the enzyme for splitting the beta-glucose-bonds, something reserved for herbivores and bacteria
from the large intestine. Absorption
Digested food is now able to pass into the blood vessels in the wall of the intestine
through either diffusion or active transport. The small intestine is the site where most
of the nutrients from ingested food are absorbed. The inner wall, or mucosa, of the small intestine
is lined with simple columnar epithelial tissue. Structurally, the mucosa is covered in wrinkles
or folds called plicae circulares, which are considered permanent features in the wall
of the organ. They are distinct from rugae which are considered non-permanent or temporary
allowing for distention and contraction. From the plicae circulares project microscopic
finger-like pieces of tissue called villi. The individual epithelial cells also have
finger-like projections known as microvilli. The functions of the plicae circulares, the
villi, and the microvilli are to increase the amount of surface area available for the
absorption of nutrients, and to limit the loss of said nutrients to intestinal fauna.
Each villus has a network of capillaries and fine lymphatic vessels called lacteals close
to its surface. The epithelial cells of the villi transport nutrients from the lumen of
the intestine into these capillaries and lacteals. The absorbed substances are transported via
the blood vessels to different organs of the body where they are used to build complex
substances such as the proteins required by our body. The material that remains undigested
and unabsorbed passes into the large intestine. Absorption of the majority of nutrients takes
place in the jejunum, with the following notable exceptions:
Iron is absorbed in the duodenum. Vitamin B12 and bile salts are absorbed in
the terminal ileum. Water and lipids are absorbed by passive diffusion
throughout the small intestine. Sodium bicarbonate is absorbed by active transport
and glucose and amino acid co-transport. Fructose is absorbed by facilitated diffusion.
Immunological The small intestine supports the body's immune
system. The presence of probiotic flora appear to contribute positively to the host's immune
system. Clinical relevance
The small intestine is a complex organ, and as such, there are a very large number of
possible conditions that may affect the function of the small bowel. A few of them are listed
below, some of which are common, with up to 10% of people being affected at some time
in their lives, while others are vanishingly rare.
In other animals The small intestine is found in all tetrapods
and also in teleosts, although its form and length vary enormously between species. In
teleosts, it is relatively short, typically around one and a half times the length of
the fish's body. It commonly has a number of pyloric caeca, small pouch-like structures
along its length that help to increase the overall surface area of the organ for digesting
food. There is no ileocaecal valve in teleosts, with the boundary between the small intestine
and the rectum being marked only by the end of the digestive epithelium.
In tetrapods, the ileocaecal valve is always present, opening into the colon. The length
of the small intestine is typically longer in tetrapods than in teleosts, but is especially
so in herbivores, as well as in mammals and birds, which have a higher metabolic rate
than amphibians or reptiles. The lining of the small intestine includes microscopic folds
to increase its surface area in all vertebrates, but only in mammals do these develop into
true villi. The boundaries between the duodenum, jejunum,
and ileum are somewhat vague even in humans, and such distinctions are either ignored when
discussing the anatomy of other animals, or are essentially arbitrary.
There is no small intestine as such in non-teleost fish, such as sharks, sturgeons, and lungfish.
Instead, the digestive part of the gut forms a spiral intestine, connecting the stomach
to the rectum. In this type of gut, the intestine itself is relatively straight, but has a long
fold running along the inner surface in a spiral fashion, sometimes for dozens of turns.
This valve greatly increases both the surface area and the effective length of the intestine.
The lining of the spiral intestine is similar to that of the small intestine in teleosts
and non-mammalian tetrapods. In lampreys, the spiral valve is extremely
small, possibly because their diet requires little digestion. Hagfish have no spiral valve
at all, with digestion occurring for almost the entire length of the intestine, which
is not subdivided into different regions. Additional images
See also Large intestine
Stomach Notes and references
Bibliography Sherwood, Lauralee. Fundamentals of physiology:
a human perspective. Florence, KY: Cengage Learning. p. 768. ISBN 0-534-46697-4.
Solomon et al. Biology Sixth Edition, Brooks-Cole/Thomson Learning ISBN 0-03-033503-5
Townsend et al. Sabiston Textbook of Surgery, Elsevier ISBN 0-7216-0409-9
Thomson A, Drozdowski L, Iordache C, Thomson B, Vermeire S, Clandinin M, Wild G. "Small
bowel review: Normal physiology, part 1.". Dig Dis Sci 48: 1546–64. doi:10.1023/A:1024719925058.
PMID 12924651. Thomson A, Drozdowski L, Iordache C, Thomson
B, Vermeire S, Clandinin M, Wild G. "Small bowel review: Normal physiology, part 2.".
Dig Dis Sci 48: 1565–81. doi:10.1023/A:1024724109128. PMID 12924652.