Digestive system

"Gut" redirects here. For the physiological acronym 'GUT', see genitourinary tract. For the Physicsterm 'GUT', please refer to Grand unification theory

The gastrointestinal tract or digestive tract, also referred to as the GI tract or the alimentary canal or the gut, is the system of organs within multicellular animalswhich takes in food, digestsit to extract energy and nutrients, and expels the remaining waste.

The GI tract differs substantially from animal to animal. For instance, some animals have multi-chambered stomachs.

Basic anatomy

The gut

Image:Stomach colon rectum diagram.gif In a normal humanadult male, the GI tract is approximately 7 and a half metreslong (25 feet) and consists of the following components:

Upper gastrointestinal tract

• Mouth(buccal cavity; includes salivary glands, mucosa, teethand tongue)
• Pharynx
• Esophagusand cardia
• Stomach, which includes the antrumand pylorusand pyloric sphincter

Lower gastrointestinal tract

• Bowel or intestine:
  • small intestine, which has three parts:
    • duodenum
    • jejunum
    • ileum
  • large intestine, which has three parts:
    • cecum(the vermiform appendixis attached to the cecum).
    • colon(ascending colon, transverse colon, descending colonand sigmoid flexure)
    • rectum
• anus

Related organs

The liversecretes bileinto the small intestine via the biliary system, employing the gallbladderas a reservoir. The pancreassecretes an isosmotic fluid containing bicarbonateand several enzymes, including trypsin, chymotrypsin, lipase, and pancreatic amylase, as well as nucleolytic enzymes, into the small intestine. Both these secretoryorgans aid in digestion.

Physiology

Digestion and excretion

Main article: Digestion

Food, after being mostly mechanically broken down in the mouthby the teeth and tongue, and slightly chemically broken down by the saliva, passes through the esophagusby means of peristalsisto the stomach, where the process of breakdown continues, mostly mechanical, as relatively large parts of food (now called "bolus") are minimized into smaller portions, and slight amounts of chemical processing takes place, especially on protein, by the enzymes present in the stomach. It then passes to the small intestinewhere further breakdown occurs, by enzymes and with the aid of bacteria, and the useful particles are absorbed into the bloodstream. The remaining particles pass through the large intestineand are ultimately expelled as feces.

Digestion is regulated both hormonallyand by the autonomic nervous system:

• The major hormonesthat control the functions of the digestive system are produced and released by cells in the mucosa of the stomach and small intestine. These hormones, such as secretin, gastrinand cholecystokinin, are released into the blood by the digestive tract and stimulate digestive juices and cause organ movement.
• The two arms of the autonomic nervous system both influence the digestive process; parasympathetic nervesstimulate secretions and peristalsis while the sympatheticinfluence is more inhibitory.

Specialization of organs

Four organs are subject to specialization in the kingdom Animalia.

• The first organ is the tongue which is only present in the phylum Chordata.
• The second organ is the esophagus. The crop is an enlargement of the esophagus in birds, insects and other invertebrates that is used to store food temporarily.
• The third organ is the stomach. In addition to a glandular stomach (proventriculus), birds have a muscular "stomach" called the ventriculus or "gizzard." The gizzard is used to mechanically grind up food.
• The fourth organ is the large intestine. An outpouching of the large intestine called the cecum is present in non-ruminant herbivores such as rabbits. It aids in digestion of plant material such as cellulose.

Immune function

The gastrointestinal tract is also a prominent part of the immune system(Coico et al 2003). The low pH(ranging from 1 to 4) of the stomach is fatalfor many microorganismsthat enter it. Similarly, mucus(containing IgAantibodies) neutralize many of these microorganisms. Other factors in the GI tract help with immune function as well, including enzymein the salivaand bile. Health enhancing intestinal bacteriaserve to prevent the overgrowth of potentially harmful bacteriain the gut.

Uses of gut

• The use of animal gut strings by musicians can be traced back to the third dynasty of Egypt. In the recent past, strings were made out of lambgut. With the advent of the modern era, musicians tend to use synthetic strings made of nylon, silk or steel, however some orchestral violinists still use gut strings. Contrary to belief, catswere never used as a source for gut strings
• Sheep gut was the original source for natural gut string used in racquets, such as for tennis. Today, synthetic strings are much more common, but the best strings are now made out of cowgut. Gut cord has also been used to provide the snares which provide the snare drum's characteristic buzzing timbre; although the snare drum almost always uses metal wire for this purpose in the modern era, the North Africanbendirframe drum still uses gut for this purpose.
• "Natural" sausage hulls are made of animal gut, especially pig and cow
• Animal gut was used to make the cord lines in grandfather clocks, but may be replaced by wire

References

• National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health.
• Coico, R., Sunshine, G., and Benjamini, E. (2003) "Immunology: A short Course 5th ed." Pgs 11-12.

External links

• Anatomy atlas of the Digestive System

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This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Digestive+system Wikipedia article Digestive system.