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 <ref>Physiology at MCG 6/6ch2/s6ch2_30</ref>
MeSH	Small+intestine
Dorlands/Elsevier	i_11/12456563

In biology the small intestine is the part of the gastrointestinal tract (gut) between the stomach and the large intestine.

1 Size and sections
2 Peristalsis
3 Extensions into lumen
4 Absorption
5 Digestion
6 Histology
7 Small Intestine Disorders
8 References
9 See also
10 Additional images
11 External links

[edit] Size and sections

In humans over 5 years old it is 270 to 290 cm long.

It is divided into three structural parts:

duodenum (20 cm in length)
jejunum (100-110 cm)
ileum (150-160 cm)

Although the small intestine is much longer than the large intestine (typically 4-5 times longer), it is referred to as such due to its comparatively smaller diameter. On average, the diameter of the large intestine of an adult human measures approximately 3 times the diameter of the small intestine.

[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] Extensions into lumen

The small intestine is the site where most of the nutrients from ingested food are absorbed. It is covered in wrinkles which are called plicae circulara. These are considered permanent folds in the wall of the organ. They are distinct from rugae which are considered non-permanent or temporary fold which allow for distention and contraction.

From the Plicae circulara project microscopic finger-like pieces of tissue called villi.

[edit] Absorption

The purpose of these wrinkles and projections is to increase surface area for absorption of nutrients. Each villus is covered in microvilli, which increase the surface area manyfold. Each villus contains a lacteal and capillaries. The lacteal absorbs the digested fat into the lymphatic system which will eventually drain into the circulatory system. The capillaries absorb all other digested nutrients.

The surface of the cells on the microvilli are covered with a brush border of proteins which helps to catch a molecule-thin layer of water within itself. This layer, called the "unstirred water layer," has a number of functions in absorption of nutrients.

[edit] Digestion

The digestion of proteins into peptides and amino acids principally occurs in the stomach but some also occurs in the small intestine. The small intestine is where the most chemical digestion takes place:

peptides are degraded into amino acids
lipids (fats) are degraded into fatty acids and glycerol
carbohydrates are degraded into simple sugars (e.g., glucose)

[edit] Histology

The three sections of the small intestine look similar to each other at a microscopic level, but there are some important differences The layers of the intestine are as follows (from the inside, moving outwards):

Layer	Duodenum	Jejunum	Ileum
epithelium	simple columnar. Contains goblet cells, Paneth cells	Similar to duodenum. Villi very long.	Similar to duodenum. Villi very short.
lamina propria	no PP	no PP	Peyer's patches
Meissner's plexus and submucosa	Brunner's glands	no BG	no BG
muscularis externa	two layers	two layers	two layers
Auerbach's plexus and serosa	normal	normal	normal

[edit] Small Intestine Disorders

Image:Celiac 3.jpg

(see also gastroenterology)

Small intestine cancer
Small intestine obstruction ("high" mechanic ileus)
- Obstruction from external pressure
- Obstruction by masses in the lumen (foreign bodies, bezoar, gallstones)
Paralytic ileus
Maropthisis
Crohn's disease
Celiac disease
Carcinoid
Meckel's Diverticulum
watery skeletosis
regurgitatis outof mouthus
costipationus maximus
Gastric dumping syndrome
Infectious diseases
Mesenteric ischemia
Short bowel syndrome
Inguinal hernia

[edit] References

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 0721604099
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. 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. PMID 12924652.