Small Intestine

The small intestine (or small bowel) 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. It receives bile juice and pancreatic juice through heptopancreatic duct, controlled by Spincter of oddi. Ininvertebrates 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. (A major exception to this is cows; for information about digestion in cows and other similar mammals, see ruminants.)

Size and divisions

The average length of the small intestine in an adult human male is 6.9 m (22 feet 6 inches), and in an 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). It is approximately 2.5–3 cm in diameter.

The small intestine is divided into three structural parts:

• Duodenum • Jejunum • Ileum
Histology

[pic]
Micrograph of the small intestinemucosa showing the intestinal villi andcrypts of Lieberkühn.

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:

|Layer |Duodenum |Jejunum |Ileum |
|serosa |1st part serosa, 2nd - 4th adventitia |normal |normal |
|muscularis externa |longitudinal and circular layers, with Auerbach's |same as duodenum |same as duodenum |
| |(myenteric) plexus in between | | |
|submucosa |Brunner's glands and Meissner's (submucosal) plexus |no BG |no BG |
|mucosa:muscularis mucosae |normal |normal |normal |
|mucosa: lamina propria |no PP |no PP |Peyer's patches |
|mucosa: intestinal epithelium |simple columnar. Contains goblet cells, Paneth cells|Similar to duodenum. |Similar to duodenum. Villi|
| | |Villi very long. |very short. |

Digestion and absorption

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 pancreasand 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 (fats) 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 (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 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 (e.g., glucose). Pancreatic amylase breaks down some carbohydrates (notably starch) 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.

Absorptions

Digested food is now able to pass into the blood vessels in the wall of the intestine through the process of diffusion. 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 (Latin for "shaggy hair"). The individual epithelial cells also have finger-like projections known asmicrovilli. The function of the plicae circulares, the villi and the microvilli is to increase the amount of surface area available for the absorption of nutrients.

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 (amino acids and carbohydrates) and lacteals (lipids). 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 food 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 (Extrinsic factor) 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.