Small intestine

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Small Intestine
Diagram showing the small intestine
Latin	intestinum tenue
Gray's	subject #248 1168
Nerve	celiac ganglia, vagus [1]
MeSH	Small+intestine
Dorlands/Elsevier	Small intestine

In vertebrates, including mammals, reptiles, birds, and bony fish, the small intestine is the part of the gastrointestinal tract (gut) following the stomach and followed by the large intestine, and is where the vast majority of digestion and absorption of food takes place. 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 are directly applicable to most mammals.^[2] (A major exception to this are cows; for information about digestion in cows and other similar mammals, see ruminants.) Cows regurgitate their food.

Contents 1 Size and divisions 2 Histology 3 Digestion and absorption 3.1 Peristalsis 3.2 Digestion 3.3 Absorption 4 Small intestine disorders 5 References 6 Notes 7 Additional images

[edit] Size and divisions

The small intestine in an adult human measures about 7 meters in length and is approximately 2.5-3 cm in diameter. Although the small intestine is much longer than the large intestine (typically 4-5 times longer), it gets its name from its comparatively smaller diameter. Although a simple tube the length and diameter of the small intestine would have a surface area of only about 0.5m², the surface complexity of the inner lining of the small intestine increase its surface area by a factor of 500 to approximately 250m², or roughly the size of a tennis court.

The small intestine is divided into three structural parts:

• Duodenum 26 cm (9.8 in) in length
• Jejunum 2.5 m (8.2 ft)
• Ileum 3.5 m (11.5 ft)

[edit] Histology

The three sections of the small intestine look similar to each other at a macroscopic level, but there are some important differences.

The parts of the intestine are as follows:

Layer	Duodenum	Jejunum	Ileum
serosa	normal	normal	normal
muscularis externa	longitudinal and circular layers, with Auerbach's (myenteric) plexus in between	same as duodenum	same as duodenum
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. Villi very long.	Similar to duodenum. Villi very short.

[edit] Digestion and absorption

[edit] Peristalsis

Food from the stomach is allowed into the duodenum by a muscle called the pylorus, or pyloric sphincter, and is then pushed through the small intestine by a process of muscular-wavelike contractions called peristalsis.

[edit] 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. The 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 and peptides are degraded into amino acids. 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 are the "middle man" that holds 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.
• Carbohydrates are degraded into simple sugars, or monosaccharides (e.g., glucose). Pancreatic amylase breaks down carbohydrates into oligosaccharides. 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.

[edit] Absorption

The digested food can now pass into the blood vessels in the wall of the intestine through the process known as absorption. 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 as microvilli. 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. This is called assimilation. 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 and bile salts are absorbed in the terminal ileum.
• Water and lipids are absorbed by passive diffusion throughout the small intestine.
• Sodium is absorbed by active transport and glucose and amino acid co-transport.
• Fructose is absorbed by facilitated diffusion.

[edit] Small intestine disorders

[edit] References

• Sherwood, Lauralee (2005). Fundamentals of physiology: a human perspective (Third ed.). Florence, KY: Cengage Learning. pp. 768. ISBN 0-53-446697-4. http://books.google.com/books?id=GoMD0tpYgBkC. 
• Solomon et al. (2002) Biology Sixth Edition, Brooks-Cole/Thomson Learning ISBN 0-03-033503-5
• Townsend et al. (2004) Sabiston Textbook of Surgery, Elsevier ISBN 0-7216-0409-9
• Thomson A, Drozdowski L, Iordache C, Thomson B, Vermeire S, Clandinin M, Wild G (2003). "Small bowel review: Normal physiology, part 1.". Dig Dis Sci 48 (8): 1546–64. doi:10.1023/A:1024719925058. PMID 12924651. 
• Thomson A, Drozdowski L, Iordache C, Thomson B, Vermeire S, Clandinin M, Wild G (2003). "Small bowel review: Normal physiology, part 2.". Dig Dis Sci 48 (8): 1565–81. doi:10.1023/A:1024724109128. PMID 12924652. 

[edit] Notes

1. ^ Physiology at MCG 6/6ch2/s6ch2_30
2. ^ http://www.britannica.com/EBchecked/topic/275485/human-body

[edit] Additional images

Lower half of right sympathetic cord.

Topography of thoracic and abdominal viscera.

v • d • e Anatomy of torso, digestive system: Gastrointestinal tract Upper GI To stomach Mouth • Pharynx (nasopharynx, oropharynx, hypopharynx) • Esophagus (UES, LES)  • Crop Stomach rugae · gastric pits · cardia/gland · fundus/gland · pylorus/gland · pyloric antrum · pyloric canal · greater curvature · lesser curvature · angular incisure Lower GI Intestine: small Duodenum (Suspensory muscle, Major duodenal papilla, Minor duodenal papilla) • Duodenojejunal flexure • Jejunum • Ileum • Ileocecal valve continuous (intestinal villus, crypts of Lieberkühn, circular folds) Intestine: large Vermiform appendix • Cecum • Colon (ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, sigmoid colon) continuous (taenia coli, haustra, epiploic appendix) Termination Rectum: Houston valve • rectal ampulla • pectinate line Anal canal: anal valves • anal sinuses • anal columns • Hilton's white line Anus: Sphincter ani internus muscle • Sphincter ani externus muscle Lymph GALT: Peyer's patches (M cells)