Gastrointestinal tract
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<imagemap> Image:Digestive system diagram en.svg|340px|thumb|Upper and Lower gastrointestinal tract
rect 31 8 171 33 salivary glands rect 31 33 172 54 parotid gland rect 31 52 173 72 submandibular gland rect 29 73 194 97 sublingual gland rect 421 116 590 144 pharynx rect 421 145 590 168 tongue rect 421 172 589 197 esophagus rect 427 310 591 339 pancreas rect 427 340 591 361 pancreatic duct rect 428 362 591 386 stomach rect 462 656 588 724 ileum rect 302 803 385 838 anus rect 165 781 271 818 rectum rect 4 737 152 772 Vermiform appendix rect 1 707 158 736 cecum rect 1 662 168 692 descending colon rect 2 633 167 660 ascending colon rect 3 607 166 631 transverse colon rect 3 575 165 604 [[colon (anatomy)|]] rect 5 492 154 552 bile duct rect 4 450 153 491 duodenum rect 3 408 153 448 gallbladder rect 4 361 150 406 liver rect 31 148 202 194 oral cavity
</imagemap> The digestive tract is the system of organs within multicellular animals that takes in food, digests it to extract energy and nutrients, and expels the remaining waste. The major functions of the GI tract are ingestion, digestion, absorption, and defecation.
The GI tract differs substantially from animal to animal. Some animals have multi-chambered stomachs, while some animals' stomachs contain a single chamber. In a normal human adult male, the GI tract is approximately 6.5 meters (20 feet) long and consists of the upper and lower GI tracts. The tract may also be divided into foregut, midgut, and hindgut, reflecting the embryological origin of each segment of the tract.
Contents |
[edit] Upper gastrointestinal tract
The upper GI tract consists of the mouth, pharynx, esophagus, and stomach.
- The mouth contains the buccal mucosa, which contains the openings of the salivary glands; the tongue; and the teeth.
- Behind the mouth lies the pharynx, which leads to a hollow muscular tube, the esophagus.
- Peristalsis takes place, which is the contraction of muscles to propel the food down the esophagus which extends through the chest and pierces the diaphragm to reach the stomach.
[edit] Lower gastrointestinal tract
The lower GI tract comprises the intestines and anus.
- Bowel or intestine
- Small intestine, which has three parts:
- 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
[edit] Accessory organs
Accessory organs to the alimentary canal include the liver, gallbladder, and pancreas. The liver secretes bile into the small intestine via the biliary system, employing the gallbladder as a reservoir. Apart from storing and concentrating bile, the gallbladder has no other specific function. The pancreas secretes an isosmotic fluid containing bicarbonate and several enzymes, including trypsin, chymotrypsin, lipase, and pancreatic amylase, as well as nucleolytic enzymes (deoxyribonuclease and ribonuclease), into the small intestine. Both of these secretory organs aid in digestion.
[edit] Embryology
The gut is an endoderm-derived structure. At approximately the 16th day of human development, the embryo begins to fold ventrally (with the embryo's ventral surface becoming concave) in two directions: the sides of the embryo fold in on each other and the head and tail fold towards one another. The result is that a piece of the yolk sac, an endoderm-lined structure in contact with the ventral aspect of the embryo, begins to be pinched off to become the primitive gut. The yolk sac remains connected to the gut tube via the vitelline duct. Usually this structure regresses during development; in cases where it does not, it is known as Meckel's diverticulum.
During fetal life, the primitive gut can be divided into three segments: foregut, midgut, and hindgut. Although these terms are often used in reference to segments of the primitive gut, they are nevertheless used regularly to describe components of the definitive gut as well.
Each segment of the primitive gut gives rise to specific gut and gut-related structures in the adult. Components derived from the gut proper, including the stomach and colon, develop as swellings or dilatations of the primitive gut. In contrast, gut-related derivatives—that is, those structures that derive from the primitive gut but are not part of the gut proper—in general develop as outpouchings of the primitive gut. The blood vessels supplying these structures remain constant throughout development.[1]
| Part | Range in adult | Gives rise to | Arterial supply |
| foregut | the pharynx, to the upper duodenum | pharynx, esophagus, stomach, upper duodenum, respiratory tract (including the lungs), liver, gallbladder, and pancreas | branches of the celiac artery |
| midgut | lower duodenum, to the first half of the transverse colon | lower duodenum, jejunum, ileum, cecum, appendix, ascending colon, and first half of the transverse colon | branches of the superior mesenteric artery |
| hindgut | second half of the transverse colon, to the upper part of the anal canal | remaining half of the transverse colon, descending colon, rectum, and upper part of the anal canal | branches of the inferior mesenteric artery |
[edit] Physiology
[edit] 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 oesophagus. The crop is an enlargement of the oesophagus 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
[edit] Immune function
The gastrointestinal tract is also a prominent part of the immune system.[2] The low pH (ranging from 1 to 4) of the stomach is fatal for many microorganisms that enter it. Similarly, mucus (containing IgA antibodies) neutralizes many of these microorganisms. Other factors in the GI tract help with immune function as well, including enzymes in the saliva and bile. Enzymes such as Cyp3A4, along with the antiporter activities, are also instrumental in the intestine's role of detoxification of antigens and xenobiotics, such as drugs, involved in first pass metabolism. Health-enhancing intestinal bacteria serve to prevent the overgrowth of potentially harmful bacteria in the gut. Microorganisms are also kept at bay by an extensive immune system comprising the gut-associated lymphoid tissue (GALT).
[edit] Histology
The gastrointestinal tract has a uniform general histology with some differences which reflect the specialization in functional anatomy.[3] The GI tract can be divided into 4 concentric layers:
- Mucosa
- Submucosa
- Muscularis externa (the external muscle layer)
- Adventitia or serosa
[edit] Mucosa
The mucosa is the innermost layer of the gastrointestinal tract, surrounding the lumen, or space within the tube. This layer comes in direct contact with the food (or bolus), and is responsible for absorption and secretion, important processes in digestion.
The mucosa can be divided into:
The mucosae are highly specialized in each organ of the gastrointestinal tract, facing a low pH in the stomach, absorbing a multitude of different substances in the small intestine, and also absorbing specific quantities of water in the large intestine. Reflecting the varying needs of these organs, the structure of the mucosa can consist of invaginations of secretory glands (e.g., gastric pits), or it can be folded in order to increase surface area (examples include villi and plicae circulares).
[edit] Submucosa
The submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics and nerves branching into the mucosa and muscularis. It contains Meissner's plexus, an enteric nervous plexus, situated on the inner surface of the muscularis externa.
[edit] Muscularis externa
The muscularis externa consists of a circular inner muscular layer and a longitudinal outer muscular layer. The circular muscle layer prevents the food from going backwards and the longitudinal layer shortens the tract. The coordinated contractions of these layers is called peristalsis and propels the bolus, or balled-up food, through the GI tract. Between the two muscle layers are the myenteric or Auerbach's plexus.
[edit] Adventitia
The adventitia consists of several layers of epithelia. When the adventitia is facing the mesentery or peritoneal fold, the adventitia is covered by a mesothelium supported by a thin connective tissue layer, together forming a serosa, or serous membrane.
[edit] Uses of animal gut by humans
- The stomachs of calves have commonly been used as a source of rennet for making cheese.
- 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 have tended to use strings made of silk, or synthetic materials such as nylon or steel. Some instrumentalists, however, still use gut strings in order to evoke the older tone quality. 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. While the snare drum currently almost always uses metal wire rather than gut cord, the North African bendir frame drum still uses gut for this purpose.
- "Natural" sausage hulls (or casings) are made of animal gut, especially hog, beef, and lamb.
- Animal gut was used to make the cord lines in longcase clocks and for fusee movements in bracket clocks, but may be replaced by metal wire.
- The oldest known condoms, from 1640, were made from animal intestine.
[edit] See also
- Dysbiosis
- Gastrointestinal hormone
- Dorland's Illustrated Medical Dictionary
- Major systems of the human body
[edit] Notes
- ^ Bruce M. Carlson (2004). Human Embryology and Developmental Biology, 3rd edition, Saint Louis: Mosby. ISBN 0-323-03649-X.
- ^ Richard Coico, Geoffrey Sunshine, Eli Benjamini (2003). Immunology: a short course. New York: Wiley-Liss. ISBN 0-471-22689-0.
- ^ Abraham L. Kierszenbaum (2002). Histology and cell biology: an introduction to pathology. St. Louis: Mosby. ISBN 0-323-01639-1.
[edit] References
- National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health.
[edit] External links
Anatomy of torso, digestive system: Digestive glands | |
|---|---|
| Liver | by region: Left lobe (Caudate lobe, Quadrate lobe) - Right lobe - Transverse fissure of liver
by function (Fibrous capsule of Glisson, Hepatocyte, Space of Disse, Space of Mall, Kupffer cell, Liver sinusoid, Ito cell, Hepatic lobule) bile ducts: (Bile canaliculus, Canals of Hering, Interlobular bile ducts, Intrahepatic bile ducts, Left and Right hepatic ducts, Common hepatic duct) |
| Gallbladder | by region (Body, Fundus, Neck) ducts: Cystic duct |
| Pancreas | by region (Tail, Body, Head, Uncinate process)
by function (Islets of Langerhans, Exocrine pancreas) ducts: Pancreatic duct • Accessory pancreatic duct |
| Common | Common bile duct - Hepatopancreatic ampulla - Sphincter of Oddi |
Digestive system, physiology: gastrointestinal physiology | |
|---|---|
| Enteric nervous system | Meissner's plexus - Auerbach's plexus |
| Exocrine | Chief cells (Pepsinogen) - Parietal cells (Gastric acid, Intrinsic factor) - Goblet cells (Mucus) |
| Endocrine/paracrine | G cells (gastrin), D cells (somatostatin) - ECL cells (Histamine) - enterogastrone: I cells (CCK), K cells (GIP), S cells (secretin) |
| Border | Brunner's glands - Paneth cells - Enterocytes |
| Fluids | Saliva - Bile - Intestinal juice - Gastric juice - Pancreatic juice |
| Processes | Swallowing - Vomiting - Peristalsis (Interstitial cell of Cajal) - Migrating motor complex - Borborygmus - Gastrocolic reflex - Segmentation contractions - Defecation |
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