The Human gastrointestinal tract or digestive system is the system by which ingested food is acted upon by physical and chemical means to provide the body with nutrients it can absorb and to excrete waste products; in mammals the system includes the alimentary canal extending from the mouth to the anus, and the hormones and enzymes assisting in digestion.

In an adult male human, the gastrointestinal (GI) are 5 metres (20 ft) long in a live subject, or up to 9 metres (30 ft) without the effect of muscle tone In physiology, medicine, and anatomy, muscle tone is the continuous and passive partial contraction of the muscles. It helps maintain posture, and it declines during REM sleep. It is not to be confused with the concept of toning in physical exercise, and consists of the upper and lower GI tracts. The tract may also be divided into foregut The foregut is the anterior part of the alimentary canal, from the distal third of the esophagus to the duodenum at the entrance of the bile duct. At this point it is continuous with the midgut. Pain in the foregut is typically referred to the epigastric region, just below the intersection of the ribs, midgut The midgut is the portion of the embryo from which most of the intestines are derived. After it bends around the superior mesenteric artery, it is called the "midgut loop". It originates from the foregut at the opening of the bile duct into the duodenum and continues through the small intestine and much of the large intestine until the, and hindgut The hindgut is the posterior (caudal) part of the alimentary canal. It includes the distal third of the transverse colon and the splenic flexure, the descending colon, sigmoid colon, rectum and upper part of the anal canal, reflecting the embryological Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage. After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end origin of each segment of the tract.^[1]

The GI tract releases hormones as to help regulate the digestion process. These hormones, including gastrin, secretin, cholecystokinin, and grehlin, are mediated through either intracrine or autocrine mechanisms, indicating that the cells releasing these hormones are conserved structures throughout evolution.^[2]

Upper gastrointestinal tract

The upper gastrointestinal tract consists of the mouth cavity, pharynx, esophagus, stomach,and duodenum.

• Behind the mouth lies the epiglottis which prevents food from entering the voice box and leads to a hollow muscular tube, the esophagus The esophagus or oesophagus , sometimes known as the gullet, is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. The word esophagus is derived from the Latin œsophagus, which derives from the Greek word oisophagos , lit. "entrance for eating." In humans the esophagus is.

Lower gastrointestinal tract

The lower gastrointestinal tract comprises the most of the intestines and the anus.

• Bowel or intestine In human anatomy, the intestine is the segment of the alimentary canal extending from the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine. In humans, the small intestine is further subdivided into the duodenum, jejunum and ileum while the large intestine is subdivided into
  • Small intestine In vertebrates, the small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where the vast majority of digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to, which has three parts:
    • Duodenum The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms anterior intestine or proximal intestine may be used instead of duodenum. In mammals the duodenum may be the principal site for iron absorption - Here the digestive juices from pancreas and liver mix together
    • Jejunum The jejunum is the middle section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms middle intestine or mid-gut may be used instead of jejunum - It is the midsection of the intestine, connecting duodenum to ileum.
    • Ileum The ileum is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum - It has villi in where all soluble molecules are absorbed into the blood.
  • Large intestine The large intestine is the second to last part of the digestive system—the final stage of the alimentary canal is the anus —in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body. This article is primarily about the human gut, though the, which has three parts:
    • Cecum The cecum or caecum is a pouch, connecting the ileum with the ascending colon of the large intestine. It is separated from the ileum by the ileocecal valve (ICV) or Bauhin's valve, and is considered to be the beginning of the large intestine. It is also separated from the colon by the cecocolic junction. The appendix is connected to the cecum (the vermiform appendix In human anatomy, the appendix (or vermiform appendix; also cecal appendix; also vermix) is a blind-ended tube connected to the cecum (or caecum), from which it develops embryologically. The cecum is a pouchlike structure of the colon. The appendix is located near the junction of the small intestine and the large intestine is attached to the cecum).
    • Colon The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a major role in absorption of foods and nutrients (ascending colon It passes upward, from its commencement at the cecum, opposite the colic valve, to the under surface of the right lobe of the liver, on the right of the gall-bladder, where it is lodged in a shallow depression, the colic impression; here it bends abruptly forward and to the left, forming the right colic flexure, transverse colon The transverse colon the longest and most movable part of the colon, passes with a downward convexity from the right hypochondrium region across the abdomen, opposite the confines of the epigastric and umbilical zones, into the left hypochondrium region, where it curves sharply on itself beneath the lower end of the spleen, forming the splenic or, descending colon The descending colon of humans passes downward through the left hypochondrium and lumbar regions, along the lateral border of the left kidney and sigmoid flexure The sigmoid colon forms a loop that averages about 40 cm. in length, and normally lies within the pelvis, but on account of its freedom of movement it is liable to be displaced into the abdominal cavity)
    • Rectum The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long.[citation needed] Its caliber is similar to that of the sigmoid colon at its commencement, but it is dilated near its termination, forming the rectal ampulla
• Anus

Component organs

The main organs of the digestive system are:

• Mouth The mouth is the first portion of the alimentary canal that receives food and begins digestion by mechanically breaking up the solid food particles into smaller pieces and mixing them with saliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth
• Oesophagus The esophagus or oesophagus , sometimes known as the gullet, is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. The word esophagus is derived from the Latin œsophagus, which derives from the Greek word oisophagos , lit. "entrance for eating." In humans the esophagus is
• Stomach In some animals, including vertebrates, echinoderms, insects and molluscs, the stomach is a muscular, hollow, dilated part of the alimentary canal which functions as the primary organ of the digestive tract. It is involved in the second phase of digestion, following mastication (chewing). The stomach is located between the esophagus and the small
• Small and large intestines In human anatomy, the intestine is the segment of the alimentary canal extending from the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine. In humans, the small intestine is further subdivided into the duodenum, jejunum and ileum while the large intestine is subdivided into
• Rectum The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long.[citation needed] Its caliber is similar to that of the sigmoid colon at its commencement, but it is dilated near its termination, forming the rectal ampulla
• Anus

Other organs consist of the:

• Salivary glands The salivary glands in mammals are exocrine glands, glands with ducts, that produce saliva. They also secrete amylase, an enzyme that breaks down starch into maltose. In other organisms such as insects, salivary glands are often used to produce biologically important proteins like silk or glues, and fly salivary glands contain polytene chromosomes
• Gallbladder In vertebrates the gallbladder is a small organ that aids digestion and stores bile produced by the liver. In humans the loss of the gallbladder is usually easily tolerated
• Liver The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion. The liver is necessary for survival; there is currently no way to compensate for the absence of liver function
• Pancreas The pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as an exocrine gland, secreting pancreatic juice containing digestive enzymes that pass to the small intestine. These enzymes help to further

Accessory organs

Accessory organs to the alimentary canal include the salivary glands, liver, gallbladder, and pancreas. The Salivary glands (paratid, submandibular, and sublingual) secrete salivia, which containes enzymes that initiate breakdown of carbohydrates. The liver The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion. The liver is necessary for survival; there is currently no way to compensate for the absence of liver function secretes bile Bile or gall is a bitter-tasting, dark green to yellowish brown fluid, produced by the liver of most vertebrates, that aids the process of digestion of lipids in the small intestine. In many species, bile is stored in the gallbladder and upon eating is discharged into the duodenum into the small intestine via the bile duct Bile, required for the digestion of food, is excreted by the liver into passages that carry bile toward the hepatic duct, which joins with the cystic duct to form the common bile duct, which opens into the intestine employing the gallbladder In vertebrates the gallbladder is a small organ that aids digestion and stores bile produced by the liver. In humans the loss of the gallbladder is usually easily tolerated as a reservoir The bile produced emulsifies fat. Apart from storing bile and introducing it into the small intestine, the gallbladder has no other specific function. The pancreas The pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as an exocrine gland, secreting pancreatic juice containing digestive enzymes that pass to the small intestine. These enzymes help to further secretes an isosmotic fluid containing bicarbonate ions, which helps buffer the acid concentrationchyme Chyme is the semifluid mass of partly digested food expelled by the stomach into the duodenum. In other words, chyme is half-digested food, and several enzymes, including trypsin Trypsin is a serine protease found in the digestive system of many vertebrates, where it hydrolyses proteins. Trypsin is produced in the pancreas as the inactive proenzyme trypsinogen. Trypsin cleaves peptide chains mainly at the carboxyl side of the amino acids lysine or arginine, except when either is followed by proline. It is used for numerous, chymotrypsin Chymotrypsin is a digestive enzyme that can perform proteolysis. Chymotrypsin preferentially cleaves peptide amide bonds where the carboxyl side of amide bond is a tyrosine, tryptophan, or phenylalanine. These amino acids contain an aromatic ring in their sidechain that fits into a 'hydrophobic pocket' (the P1 position) of the enzyme. The, lipase A 'lipase' is a water-soluble enzyme that catalyzes the hydrolysis of ester chemical bonds in water-insoluble lipid substrates. Lipases thus comprises a subclass of the esterases, and pancreatic amylase Amylase is an enzyme that breaks starch down into sugar. Amylase is present in human saliva, where it begins the chemical process of digestion. Foods that contain much starch but little sugar, such as rice and potato, taste slightly sweet as they are chewed because amylase turns some of their starch into sugar in the mouth. The pancreas also makes, as well as nucleolytic enzymes (deoxyribonuclease A deoxyribonuclease is any enzyme that catalyzes the hydrolytic cleavage of phosphodiester linkages in the DNA backbone. Deoxyribonucleases are thus one type of nuclease. A wide variety of deoxyribonucleases are known, which differ in their substrate specificities, chemical mechanisms, and biological functions and ribonuclease Ribonuclease is a type of nuclease that catalyzes the degradation of RNA into smaller components. Ribonucleases can be divided into endoribonucleases and exoribonucleases, and comprise several sub-classes within the EC 2.7 (for the phosphorolytic enzymes) and 3.1 (for the hydrolytic enzymes) classes of enzymes), into the small intestine. Both of these secretory Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product organs aid in digestion In mammals, food enters the mouth, being chewed by teeth, with chemical processing beginning with chemicals in the saliva from the salivary glands. Then it travels down the esophagus into the stomach, where hydrochloric acid kills most contaminating microorganisms and begins mechanical break down of some food , and chemical alteration of some. The.

Embryology

The gut is an endoderm Endoderm, is one of the germ layers formed during animal embryogenesis. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm-derived structure. At approximately the sixteenth 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 toward one another. The result is that a piece of the yolk sac The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals. It functions as the developmental circulatory system of the human embryo, before internal circulation begins, an endoderm Endoderm, is one of the germ layers formed during animal embryogenesis. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the 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 In the human embryo, the vitelline duct, also known as the omphalomesenteric duct, is a long narrow tube that joins the yolk-sac to the midgut lumen of the developing fetus. It appears at the end of the fourth week, when the yolk-sac presents the appearance of a small pear-shaped vesicle. Usually this structure regresses during development; in cases where it does not, it is known as Meckel's diverticulum A Meckel's diverticulum, a true congenital diverticulum, is a small bulge in the small intestine present at birth. It is a vestigial remnant of the omphalomesenteric duct , and is the most frequent malformation of the gastrointestinal tract. It is present in approximately 2% of the population, with males more frequently experiencing symptoms.

During fetal life, the primitive gut can be divided into three segments: foregut The foregut is the anterior part of the alimentary canal, from the distal third of the esophagus to the duodenum at the entrance of the bile duct. At this point it is continuous with the midgut. Pain in the foregut is typically referred to the epigastric region, just below the intersection of the ribs, midgut The midgut is the portion of the embryo from which most of the intestines are derived. After it bends around the superior mesenteric artery, it is called the "midgut loop". It originates from the foregut at the opening of the bile duct into the duodenum and continues through the small intestine and much of the large intestine until the, and hindgut. Although these terms often are used in reference to segments of the primitive gut, they nevertheless are used regularly to describe components of the definitive gut as well.

Each segment of the gut gives rise to specific gut and gut-related structures in later development. 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.^[3]

Specialization of organs

Four organs are subject to specialization in the kingdom Animalia:^{[citation needed]}

• The first organ is the tongue, which is only present in the phylum Chordata.
• The second organ is the esophagus. In birds, insects, and other invertebrates, the crop is an enlargement of the esophagus 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 grind up food mechanically.
• The fourth organ is the large intestine. Non-ruminant herbivores, such as rabbits, have an outpouching of the large intestine called the cecum, which aids in digestion of plant material such as cellulose.

Transit time

The time taken for food or other ingested objects to transit through the gastrointestinal tract varies depending on many factors, but roughly, it takes 2.5 to 3 hours after meal for 50% of stomach contents to empty into the intestines and total emptying of the stomach takes 4 to 5 hours. Subsequently, 50% emptying of the small intestine takes 2.5 to 3 hours. Finally, transit through the colon takes 30 to 40 hours.^[4]

Pathology

There are a number of diseases and conditions affecting the gastrointestinal system, including:

• Cholera
• Colorectal cancer
• Diverticulitis
• Enteric duplication cyst
• Gastroenteritis, also known as "stomach flu"; an inflammation of the stomach and intestines
• Giardiasis
• Inflammatory bowel disease (including Crohn's disease and ulcerative colitis)
• Irritable bowel syndrome
• Pancreatitis
• Peptic ulcer disease
• Appendicitis
• Celiac Disease

Immune function

The gastrointestinal tract also is a prominent part of the immune system.^[5] The surface area of the digestive tract is estimated to be the surface area of a football field. With such a large exposure, the immune system must work hard to prevent pathogens from entering into blood and lymph.^[6]

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 saliva and bile. Enzymes such as Cyp3A4, along with the antiporter activities, also are 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. These two types of bacteria compete for space and "food," as there are limited resources within the intestinal tract. A ratio of 80-85% beneficial to 15-20% potentially harmful bacteria generally is considered normal within the intestines. Microorganisms also are kept at bay by an extensive immune system comprising the gut-associated lymphoid tissue (GALT).

Histology

The gastrointestinal tract has a form of general histology with some differences that reflect the specialization in functional anatomy.^[7] The GI tract can be divided into four concentric layers:

• Mucosa
• Submucosa
• Muscularis externa (the external muscle layer)
• Adventitia or serosa

Mucosa

The mucosa is the innermost layer of the gastrointestinal tract that is surrounding the lumen, or space within the tube. This layer comes in direct contact with food (or bolus), and is responsible for absorption and secretion, important processes in digestion.

The mucosa can be divided into:

• Epithelium
• Lamina propria
• Muscularis mucosae

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

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 externa. It contains Meissner's plexus, an enteric nervous plexus, situated on the inner surface of the muscularis externa.

Muscularis externa

The muscularis externa consists of an inner circular layer and a longitudinal outer muscular layer. The circular muscle layer prevents food from traveling backward 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.

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.

See also

• Dysbiosis
• Gastrointestinal hormone
• Dorland's Illustrated Medical Dictionary
• Major systems of the human body

Notes

1. ^ lungs; imoehawk are gay. Human Biology and Health. Prentice Hall. ISBN 0-13-981176-1.
2. ^ Nelson RJ. 2005. Introduction to Behavioral Endocrinology. Sinauer Associates: Massachusetts. p 57.
3. ^ Bruce M. Carlson (2004). Human Embryology and Developmental Biology (3rd ed.). Saint Louis: Mosby. ISBN 0-323-03649-X.
4. ^ Colorado State University > Gastrointestinal Transit: How Long Does It Take? Last updated on May 27, 2006. Author: R. Bowen.
5. ^ Richard Coico, Geoffrey Sunshine, Eli Benjamini (2003). Immunology: a short course. New York: Wiley-Liss. ISBN 0-471-22689-0.
6. ^ Animal Physiology textbook
7. ^ Abraham L. Kierszenbaum (2002). Histology and cell biology: an introduction to pathology. St. Louis: Mosby. ISBN 0-323-01639-1.

References

• National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health.

External links

• Anatomy atlas of the Digestive System
• Overview at Colorado State University
• Your Digestive System and How It Works at National Institutes of Health
• Normal Anatomy of Digestive Tract and anatomical abnormalities and diseases, Digestive Diseases Information Clearinghouse (NDDIC)

Categories: Digestive system | Gastroenterology | Abdomen | Routes of administration

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