Gastrointestinal tract: Encyclopedia - Gastrointestinal tract

Gastrointestinal tract

The gastrointestinal or digestive tract, also referred to as the GI tract or the alimentary canal or the gut, is the system of organs within multicellular animals which takes in food, digests it 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.

Gastrointestinal tract - Basic anatomy

Gastrointestinal tract - The gut

In a normal human adult male, the GI tract is approximately 7 and a half metres long (25 feet) and consists of the following components:

• Mouth (buccal cavity; includes salivary glands, mucosa, teeth and tongue)
• Pharynx
• Esophagus and cardia
• Stomach, which includes the antrum and pylorus and pyloric sphincter

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

Gastrointestinal tract - Related organs

The liver secretes bile into the small intestine via the biliary system, employing the gallbladder as a reservoir. The pancreas secretes an isosmotic fluid containing bicarbonate and several enzymes, including trypsin, chymotrypsin, lipase, and pancreatic amylase, as well as nucleolytic enzymes, into the small intestine. Both these secretory organs aid in digestion.

Gastrointestinal tract - Physiology

Gastrointestinal tract - Digestion and excretion

Food, after being mostly mechanically broken down in the mouth by the teeth and tongue, and slightly chemically broken down by the saliva, passes through the esophagus by means of peristalsis to 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 intestine where 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 intestine and are ultimately expelled as feces.

Digestion is regulated both hormonally and by the autonomic nervous system:

• The major hormones that 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, gastrin and 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 nerves stimulate secretions and peristalsis while the sympathetic influence is more inhibitory.

Gastrointestinal tract - 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.

Gastrointestinal tract - 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 fatal for many microorganisms that enter it. Similarly, mucus (containing IgA antibodies) neutralize many of these microorganisms. Other factors in the GI tract help with immune function as well, including enzyme in the saliva and bile. Health enhancing intestinal bacteria serve to prevent the overgrowth of potentially harmful bacteria in the gut.

Gastrointestinal tract - 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 lamb gut. 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, cats were 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 cow gut. 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 African bendir frame 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

Adapted from the Wikipedia article "Gastrointestinal tract", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki