Gastrointestinal tract
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- "Gut" redirects here. For the physiological acronym 'GUT', see genitourinary tract. For the Physics term '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 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.
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Basic anatomy
The gut
Image:Stomach colon rectum diagram.gif 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:
Upper gastrointestinal tract
- Mouth (buccal cavity; includes salivary glands, mucosa, teeth and tongue)
- Pharynx
- Oesophagus or Esophagus (gullet) and cardia
- Stomach, which includes the antrum and pylorus and pyloric sphincter.
Lower gastrointestinal tract
- Bowel or intestine:
- small intestine, which has three parts:
- large intestine, which has three parts:
- caecum (the vermiform appendix is attached to the cecum).
- colon (ascending colon, transverse colon, descending colon and sigmoid flexure)
- rectum
- anus
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.
Physiology
Digestion and excretion
In the mouth, food is broken down mechanically by the teeth and tongue, and also broken down chemically to some extent by saliva. It then passes through the esophagus by means of peristalsis to the stomach. Here, the process of breakdown continues. This is still mostly mechanical, as relatively large parts of food (now called "bolus") are reduced to smaller portions. However, slight amounts of chemical processing also take place at this point, especially on protein, by the action of enzymes present in the stomach. The food 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.
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 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.
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. Although such strings were commonly referred to as "catgut" strings, 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 produce strings for 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 longcase 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
| Digestive system - edit |
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| Mouth | Pharynx | Esophagus | Stomach | Pancreas | Gallbladder | Liver | Small intestine (duodenum, jejunum, ileum) | Colon | Cecum | Rectum | Anus |
| Human organ systems - edit |
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| Cardiovascular system | Digestive system | Endocrine system | Immune system | Integumentary system | Lymphatic system | Muscular system | Nervous system | Skeletal system | Reproductive system | Respiratory system | Urinary system |
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