Human gastrointestinal tract

Human gastrointestinal tract (Digestive System)
Stomach colon rectum diagram

The human gastrointestinal tract refers to the stomach and intestine,^[1] and sometimes to all the structures from the mouth to the anus.^[2] (The "digestive system" is a broader term that includes other structures, including the accessory organs of digestion).^[3]
In an adult male human, the gastrointestinal (GI) tract is 5 metres (20 ft) long in a live subject, or up to 9 metres (30 ft) without the effect of muscle tone, 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.
The GI tract always releases hormones 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.^[4]

Upper gastrointestinal tract

The upper gastrointestinal tract consists of the esophagus, stomach, and duodenum.^[5] The exact demarcation between "upper" and "lower" can vary. Upon gross dissection, the duodenum may appear to be a unified organ, but it is often divided into two parts based upon function, arterial supply, or embryology.

Lower gastrointestinal tract

The Lower Gastrointestinal Tract includes most of the Small Intestine and all of the Large Intestine.^[6] According to some sources, it also includes the Anus.^{[citation needed]}

• Bowel or Intestine
  • Small Intestine: Has three parts:
    • Duodenum: Here the digestive juices from the Pancreas (digestive enzymes) and the Gallbladder (Bile) mix together. The Digestive Enzymes break down proteins and bile and emulsify fats into micelles. The Duodenum contains Brunner's glands which produce bicarbonate, and pancreatic juice which contains bicarbonate to neutralize Hydrochloric Acid of the Stomach
    • Jejunum: This is the midsection of the Intestine, connecting the Duodenum to the Ileum. It contains the plicae circulares, and villi to increase the surface area of that part of the GI Tract.
    • Ileum: Has villi, where all soluble molecules are absorbed into the blood (capillaries and lacteals).
  • Large Intestine: Has three parts:
    • Cecum: The Vermiform appendix is attached to the Cecum).
    • Colon: Includes the Ascending Colon, Transverse Colon, Descending Colon and Sigmoid Flexure): The main function of the Colon is to absorb water, but it also contains bacteria that produce beneficial vitamins like Vitamin K.
    • Rectum
• Anus

The Ligament of Treitz is sometimes used to divide the upper and lower GI tracts.^[7]

Embryology

The gut is an 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, 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 also 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 out-pouchings of the primitive gut. The blood vessels supplying these structures remain constant throughout development.^[8]

Part	Part in adult	Gives rise to	Arterial supply
Foregut	Esophagus to first 2 sections of the duodenum	Esophagus, Stomach, Duodenum (1st and 2nd parts), Liver, Gallbladder, Pancreas, Superior portion of pancreas (Note that though the Spleen is supplied by the celiac trunk, it is derived from dorsal mesentery and therefore not a foregut derivative)	celiac trunk
Midgut	lower duodenum, to the first two-thirds of the transverse colon	lower duodenum, jejunum, ileum, cecum, appendix, ascending colon, and first two-thirds of the transverse colon	branches of the superior mesenteric artery
Hindgut	last third of the transverse colon, to the upper part of the anal canal	last third of the transverse colon, descending colon, rectum, and upper part of the anal canal	branches of the inferior mesenteric artery

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 a 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.^[9]

Pathology

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

• Appendicitis
• Cancer
• Celiac Disease
• Cholera
• Colorectal cancer
• Diarrhoea
• 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
• Ulcerative colitis
• Yellow Fever

Immune function

The gastrointestinal tract is also a prominent part of the immune system.^[10] 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.^[11]
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.^[12] The GI tract can be divided into four concentric layers:

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

Mucosa

The mucosa is the innermost layer of the gastrointestinal tract. that is surrounding the lumen, or open space within the tube. This layer comes in direct contact with digested food (chyme),
The mucosa is made up of three layers:

• Epithelium - innermost layer. Responsible for most digestive, absorptive and secretory processes.
• Lamina propria - a layer of connective tissue. Unusually cellular compared to most connective tissue
• Muscularis mucosae - a thin layer of smooth muscle. Function is still under debate

The mucosae are highly specialized in each organ of the gastrointestinal tract to deal with the different conditions. The most variation is seen in the epithelium.
In the oesophagus, the epithelium is stratified, squamous and non-keratinising, for protective purposes.
In the stomach it is simple columnar, and is organised into gastric pits and glands to deal with secretion. The gastro-oesophageal junction is extremely abrupt.
The small intestine epithelium (particularly the ileum) is specialised for absorption; it is organised into plicae circulares and villi, and the enterocytes have microvilli. This creates a brush border which greatly increases the surface area for absoption. The epithelium is simple columnar with microvilli. In the ileum there are occasionally Peyer's patches in the lamina propria.
The colon has simple columnar epithelium with no villi. There are goblet cells.
The appendix has a mucosa resembling the colon but is heavily infiltrated with lymphocytes.
The ano-rectal junction (at the pectinate line) is again very abrupt; there is a transition from simple columnar to stratified squamous non-keratinising epithelium (as in the oesophagus) for protective purposes.

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. Actually the layers are not truly longitudinal or circular, rather the layers of muscle are helical with different pitches. The inner circular is helical with a steep pitch and the outer longitudinal is helical with a much shallower pitch.
The coordinated contractions of these layers is called peristalsis and propels the food, through the tract. Food in the GI tract is called a bolus (ball of food) from the mouth down to the stomach. After the stomach the food is partially digested and semi-liquid, and is referred to as chyme. In the large intestine the remaining semi-solid substance is referred to as faeces.
Between the two muscle layers are the myenteric or Auerbach's plexus. This controls peristalsis. Activity is initiated by the pacemaker cells (interstitial cells of Cajal). The gut has intrinsic peristaltic activity (basal electrical rhythm) due to its self-contained enteric nervous system. The rate can of course be modulated by the rest of the autonomic nervous system.
The thickness of muscularis externa varies in each part of the tract. In the colon, for example, the muscularis externa is much thicker because the faeces are large and heavy, and require more force to push along. The outer longitudinal layer of the colon thins out into 3 discontinuous longitudinal bands, known as tiniae coli (bands of the colon). This is one of the 3 features helping to distinguish between the large and small intestine.
Occasionally in the large intestine (2-3 times a day) there will be mass contraction of certain segments, moving a lot of faeces along. This is generally when one gets the urge to defacate.
The pylorus of the stomach has a thickened portion of the inner circular layer: the pyloric sphincter. Alone among the GI tract, the stomach has a third layer of muscularis externa. This is the inner oblique layer, and helps churn the chyme in the stomach.

Adventitia/serosa

The outermost layer of the GI tract consists of several layers of connective tissue.
Intraperitoneal parts of the GI tract are covered with serosa. These include most of the the stomach, first part of the duodenum, all of the small intestine, caecum and appendix, transverse colon, sigmoid colon and rectum. In these sections of the gut there is clear boundary between the gut and the surrounding tissue. These parts of the tract have a mesentery.
Retroperitoneal parts are covered with adventitia. They blend in to the surrounding tissue and are fixed in position. For example, the retroperitoneal section of the duodenum usually passes through the transpyloric plane. These include the oesophagus, pylorus of the stomach, distal duodenum, ascending colon, descending colon and anal canal. In addition, the oral cavity has adventitia.