Gastrointestinal tract/ja

消化管
平均的な人間の胃、腸、直腸の図
Details
System	消化器系
Identifiers
Latin	消化管（口～肛門）、 消化管（食道～大腸）、 消化管（胃～大腸）
Anatomical terminology

ほとんどの動物は「貫通腸」または完全な消化管を持っている。例外はより原始的なものである： 海綿動物は消化のための小さな孔（オスチウム）を全身に持ち、排泄のための大きな背側の孔（排水孔）を持ち、有櫛動物は腹側の口と背側の肛門孔の両方を持ち、刺胞動物とアコヤガイは消化と排泄の両方のための単一の孔を持つ。

ヒトの消化管は食道、胃、腸からなり、上部消化管と下部消化管に分けられる。消化管は、口から肛門までのすべての構造を含み、各区分の胚学的起源を反映して、前腸、中腸、後腸に分けられることもある。ヒトの消化管全体の長さは剖検で約9メートル（30フィート）である。腸は平滑筋組織の管であり、中途半端に緊張した状態では一定の筋緊張を維持するが、局所的な膨張と蠕動を可能にするために部分的に弛緩することができるため、生体内ではかなり短い。

消化管には、腸内細菌叢が存在し、約1,000種類の異なる細菌株が含まれ、免疫健康状態と代謝の維持において様々な役割を果たしており、その他にも多くの微生物が存在する。 消化管の細胞はホルモンを放出し、消化プロセスの調節を助ける。これらの消化ホルモンは、ガストリン、セクレチン、コレシストキニン、グレリンを含み、イントラクリンまたはオートクリン機構を介して介在しており、これらのホルモンを放出する細胞は進化を通じて保存された構造であることを示している。

ヒトの消化管

構造

Upper and lower human gastrointestinal tract

構造と機能は肉眼解剖学としても顕微鏡解剖学または組織学としても説明できる。消化管自体は上部管と下部管に分けられ、腸は小腸部分と大腸部分に分けられる。

上部消化管

詳細は「Mouth/ja」、「Pharynx/ja」、「Esophagus/ja」、「Stomach/ja」、および「duodenum/ja」を参照

上部消化管は口、咽頭、食道、胃、十二指腸からなる。 上下の管の正確な境界は十二指腸の懸垂筋である。これは前腸と中腸の胚の境界を区別するものであり、また臨床医が消化管出血を「上部」由来か「下部」由来かのどちらかであると表現するためによく用いる区分でもある。解剖すると、十二指腸は一体化した臓器のように見えるが、機能、位置、内部解剖学に基づいて4つのセグメントに分かれている。十二指腸の4つの区分は以下の通りである（胃から始まり空腸に向かう）： 球部、下行、水平、上行である。懸垂筋は上行十二指腸の上縁を横隔膜に付着している。

懸垂筋は、小腸の第一部分である十二指腸と第二部分である空腸の正式な区分けを示す解剖学的に重要な目印である。これは胚の中胚葉に由来する細い筋肉である。

下部消化管

下部消化管は小腸の大部分と大腸のすべてを含む。人体解剖学では、腸（bowel、またはgut、ギリシア語: éntera）は胃の幽門括約筋から肛門まで伸びる消化管の区分であり、他の哺乳類と同様に小腸と大腸の2つの区分からなる。ヒトの場合、小腸はさらに十二指腸、空腸、回腸に細分され、大腸は盲腸、上行結腸、横行結腸、下行結腸、S状結腸結腸、直腸、肛門管に細分される。

Small intestine

Illustration of the small intestine

Main articles: Small intestine, Duodenum, Jejunum, and Ileum

The small intestine begins at the duodenum and is a tubular structure, usually between 6 and 7 m long. Its mucosal area in an adult human is about 30 m² (320 sq ft). The combination of the circular folds, the villi, and the microvilli increases the absorptive area of the mucosa about 600-fold, making a total area of about 250 m² (2,700 sq ft) for the entire small intestine. Its main function is to absorb the products of digestion (including carbohydrates, proteins, lipids, and vitamins) into the bloodstream. There are three major divisions:

1. Duodenum: A short structure (about 20–25 cm long) that receives chyme from the stomach, together with pancreatic juice containing digestive enzymes and bile from the gall bladder. The digestive enzymes break down proteins, and bile emulsifies fats into micelles. The duodenum contains Brunner's glands which produce a mucus-rich alkaline secretion containing bicarbonate. These secretions, in combination with bicarbonate from the pancreas, neutralize the stomach acids contained in the chyme.
2. Jejunum: This is the midsection of the small intestine, connecting the duodenum to the ileum. It is about 2.5 m (8.2 ft) long and contains the circular folds also known as plicae circulares and villi that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here.
3. Ileum: The final section of the small intestine. It is about 3 m long, and contains villi similar to the jejunum. It absorbs mainly vitamin B12 and bile acids, as well as any other remaining nutrients.

Large intestine

Main article: Large intestine

The large intestine, also called the colon, forms an arch starting at the cecum and ending at the rectum and anal canal. It also includes the appendix, which is attached to the cecum. Its length is about 1.5 m, and the area of the mucosa in an adult human is about 2 m² (22 sq ft). Its main function is to absorb water and salts. The colon is further divided into:

1. Cecum (first portion of the colon) and appendix
2. Ascending colon (ascending in the back wall of the abdomen)
3. Right colic flexure (flexed portion of the ascending and transverse colon apparent to the liver)
4. Transverse colon (passing below the diaphragm)
5. Left colic flexure (flexed portion of the transverse and descending colon apparent to the spleen)
6. Descending colon (descending down the left side of the abdomen)
7. Sigmoid colon (a loop of the colon closest to the rectum)
8. Rectum
9. Anal canal

Development

Main article: Development of the digestive system

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 is gradually patterned 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 regions of the definitive gut as well.

Each segment of the gut is further specified and 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 in the cells 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.

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 (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

Histology

Main article: Gastrointestinal wall

General structure of the gut wall

• 1: Mucosa: Epithelium
• 2: Mucosa: Lamina propria
• 3: Mucosa: Muscularis mucosae
• 4: Lumen
• 5: Lymphatic tissue
• 6: Duct of gland outside tract
• 7: Gland in mucosa
• 8: Submucosa
• 9: Glands in submucosa
• 10: Meissner's submucosal plexus
• 11: Vein
• 12: Muscularis: Circular muscle
• 13: Muscularis: Longitudinal muscle
• 14: Serosa: Areolar connective tissue
• 15: Serosa: Epithelium
• 16: Auerbach's myenteric plexus
• 17: Nerve
• 18: Artery
• 19: Mesentery

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

• Mucosa
• Submucosa
• Muscular layer
• Adventitia or serosa

Mucosa

See also: Oral mucosa and Gastric mucosa

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

• 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 that aids the passing of material and enhances the interaction between the epithelial layer and the contents of the lumen by agitation and peristalsis

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.

Submucosa

Main article: 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 the submucosal plexus, an enteric nervous plexus, situated on the inner surface of the muscularis externa.

Muscular layer

The muscular layer consists of an inner circular layer and a longitudinal outer layer. The circular layer prevents food from traveling backward and the longitudinal layer shortens the tract. 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. Whilst the muscularis externa is similar throughout the entire gastrointestinal tract, an exception is the stomach which has an additional inner oblique muscular layer to aid with grinding and mixing of food. The muscularis externa of the stomach is composed of the inner oblique layer, middle circular layer, and outer longitudinal layer.

Between the circular and longitudinal muscle layers is the myenteric plexus. This controls peristalsis. Activity is initiated by the pacemaker cells, (myenteric interstitial cells of Cajal). The gut has intrinsic peristaltic activity (basal electrical rhythm) due to its self-contained enteric nervous system. The rate can be modulated by the rest of the autonomic nervous system.

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.

Adventitia and serosa

Main articles: Serous membrane and Adventitia

The outermost layer of the gastrointestinal tract consists of several layers of connective tissue.

Intraperitoneal parts of the GI tract are covered with serosa. These include most of 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 a 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 into 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 esophagus, pylorus of the stomach, distal duodenum, ascending colon, descending colon and anal canal. In addition, the oral cavity has adventitia.

Gene and protein expression

Approximately 20,000 protein coding genes are expressed in human cells and 75% of these genes are expressed in at least one of the different parts of the digestive organ system. Over 600 of these genes are more specifically expressed in one or more parts of the GI tract and the corresponding proteins have functions related to digestion of food and uptake of nutrients. Examples of specific proteins with such functions are pepsinogen PGC and the lipase LIPF, expressed in chief cells, and gastric ATPase ATP4A and gastric intrinsic factor GIF, expressed in parietal cells of the stomach mucosa. Specific proteins expressed in the stomach and duodenum involved in defence include mucin proteins, such as mucin 6 and intelectin-1.

Transit time

The time taken for food to transit through the gastrointestinal tract varies on multiple factors, including age, ethnicity, and gender. Several techniques have been used to measure transit time, including radiography following a barium-labeled meal, breath hydrogen analysis, scintigraphic analysis following a radiolabeled meal, and simple ingestion and spotting of corn kernels. It takes 2.5 to 3 hours for 50% of the contents to leave the stomach. The rate of digestion is also dependent of the material being digested, as food composition from the same meal may leave the stomach at different rates. Total emptying of the stomach takes around 4–5 hours, and transit through the colon takes 30 to 50 hours.

Immune function

The gastrointestinal tract forms an important part of the immune system.

Immune barrier

The surface area of the digestive tract is estimated to be about 32 square meters, or about half a badminton court. With such a large exposure (more than three times larger than the exposed surface of the skin), these immune components function to prevent pathogens from entering the blood and lymph circulatory systems. Fundamental components of this protection are provided by the intestinal mucosal barrier, which is composed of physical, biochemical, and immune elements elaborated by the intestinal mucosa. Microorganisms also are kept at bay by an extensive immune system comprising the gut-associated lymphoid tissue (GALT)

There are additional factors contributing to protection from pathogen invasion. For example, 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 pathogenic microorganisms. Other factors in the GI tract contribution to immune function include enzymes secreted in the saliva and bile.

Immune system homeostasis

Beneficial bacteria also can contribute to the homeostasis of the gastrointestinal immune system. For example, Clostridia, one of the most predominant bacterial groups in the GI tract, play an important role in influencing the dynamics of the gut's immune system. It has been demonstrated that the intake of a high fiber diet could be responsible for the induction of T-regulatory cells (Tregs). This is due to the production of short-chain fatty acids during the fermentation of plant-derived nutrients such as butyrate and propionate. Basically, the butyrate induces the differentiation of Treg cells by enhancing histone H3 acetylation in the promoter and conserved non-coding sequence regions of the FOXP3 locus, thus regulating the T cells, resulting in the reduction of the inflammatory response and allergies.

Intestinal microbiota

The large intestine contains multiple types of bacteria that can break down molecules the human body cannot process alone, demonstrating a symbiotic relationship. These bacteria are responsible for gas production at host–pathogen interface, which is released as flatulence. However, the primary function of the large intestine is water absorption from digested material (regulated by the hypothalamus) and the reabsorption of sodium and nutrients.

Beneficial intestinal bacteria compete with potentially harmful bacteria for space and "food", as the intestinal tract has limited resources. A ratio of 80–85% beneficial to 15–20% potentially harmful bacteria is proposed for maintaining homeostasis. An imbalanced ratio results in dysbiosis.

Detoxification and drug metabolism

Enzymes such as CYP3A4, along with the antiporter activities, are also instrumental in the intestine's role of drug metabolism in the detoxification of antigens and xenobiotics.

Other animals

In most vertebrates, including fishes, amphibians, birds, reptiles, and egg-laying mammals, the gastrointestinal tract ends in a cloaca and not an anus. In the cloaca, the urinary system is fused with the genito-anal pore. Therians (all mammals that do not lay eggs, including humans) possess separate anal and uro-genital openings. The females of the subgroup placentalia have even separate urinary and genital openings.

During early development the asymmetric position of the bowels and inner organs is initiated (see also axial twist theory).

Ruminants show many specializations for digesting and fermenting tough plant material, consisting of additional stomach compartments.

Many birds and other animals have a specialised stomach in the digestive tract called a gizzard used for grinding up food.

Another feature found in a range of animals is the crop. In birds this is found as a pouch alongside the esophagus.

In 2020, the oldest known fossil digestive tract, of an extinct wormlike organism in the Cloudinidae was discovered; it lived during the late Ediacaran period about 550 million years ago.

A through-gut (one with both mouth and anus) is thought to have evolved within the nephrozoan clade of Bilateria, after their ancestral ventral orifice (single, as in cnidarians and acoels; re-evolved in nephrozoans like flatworms) stretched antero-posteriorly, before the middle part of the stretch would get narrower and closed fully, leaving an anterior orifice (mouth) and a posterior orifice (anus plus genital opening). A stretched gut without the middle part closed is present in another branch of bilaterians, the extinct proarticulates. This and the amphistomic development (when both mouth and anus develop from the gut stretch in the embryo) present in some nephrozoans (e.g. roundworms) are considered to support this hypothesis.

Clinical significance

This section discusses related diseases, medical associations with the gastrointestinal tract, and use in surgery.

Main articles: Gastrointestinal disease and Gastroenterology

Main article: Clinical significance

Diseases

There are many diseases and conditions that can affect the gastrointestinal system, including infections, inflammation and cancer.

Various pathogens, such as bacteria that cause foodborne illnesses, can induce gastroenteritis which results from inflammation of the stomach and small intestine. Antibiotics to treat such bacterial infections can decrease the microbiome diversity of the gastrointestinal tract, and further enable inflammatory mediators. Gastroenteritis is the most common disease of the GI tract.

• Gastrointestinal cancer may occur at any point in the gastrointestinal tract, and includes mouth cancer, tongue cancer, oesophageal cancer, stomach cancer, and colorectal cancer.
• Inflammatory conditions. Ileitis is an inflammation of the ileum, colitis is an inflammation of the large intestine.
• Appendicitis is inflammation of the appendix located at the caecum. This is a potentially fatal condition if left untreated; most cases of appendicitis require surgical intervention.

Diverticular disease is a condition that is very common in older people in industrialized countries. It usually affects the large intestine but has been known to affect the small intestine as well. Diverticulosis occurs when pouches form on the intestinal wall. Once the pouches become inflamed it is known as diverticulitis.

Inflammatory bowel disease is an inflammatory condition affecting the bowel walls, and includes the subtypes Crohn's disease and ulcerative colitis. While Crohn's can affect the entire gastrointestinal tract, ulcerative colitis is limited to the large intestine. Crohn's disease is widely regarded as an autoimmune disease. Although ulcerative colitis is often treated as though it were an autoimmune disease, there is no consensus that it actually is such.

Functional gastrointestinal disorders the most common of which is irritable bowel syndrome. Functional constipation and chronic functional abdominal pain are other functional disorders of the intestine that have physiological causes but do not have identifiable structural, chemical, or infectious pathologies.

Symptoms

Several symptoms can indicate problems with the gastrointestinal tract, including:

• Vomiting, which may include regurgitation of food or the vomiting of blood
• Diarrhea, or the passage of liquid or more frequent stools
• Constipation, which refers to the passage of fewer and hardened stools
• Blood in stool, which includes fresh red blood, maroon-coloured blood, and tarry-coloured blood

Treatment

Gastrointestinal surgery can often be performed in the outpatient setting. In the United States in 2012, operations on the digestive system accounted for 3 of the 25 most common ambulatory surgery procedures and constituted 9.1 percent of all outpatient ambulatory surgeries.

Imaging

Various methods of imaging the gastrointestinal tract include the upper and lower gastrointestinal series:

• Radioopaque dyes may be swallowed to produce a barium swallow
• Parts of the tract may be visualised by camera. This is known as endoscopy if examining the upper gastrointestinal tract and colonoscopy or sigmoidoscopy if examining the lower gastrointestinal tract. Capsule endoscopy is where a capsule containing a camera is swallowed in order to examine the tract. Biopsies may also be taken when examined.
• An abdominal x-ray may be used to examine the lower gastrointestinal tract.

Other related diseases

• Cholera
• Enteric duplication cyst
• Giardiasis
• Pancreatitis
• Peptic ulcer disease
• Yellow fever
• Helicobacter pylori is a gram-negative spiral bacterium. Over half the world's population is infected with it, mainly during childhood; it is not certain how the disease is transmitted. It colonizes the gastrointestinal system, predominantly the stomach. The bacterium has specific survival conditions that are specific to the human gastric microenvironment: it is both capnophilic and microaerophilic. Helicobacter also exhibits a tropism for gastric epithelial lining and the gastric mucosal layer about it. Gastric colonization of this bacterium triggers a robust immune response leading to moderate to severe inflammation, known as gastritis. Signs and symptoms of infection are gastritis, burning abdominal pain, weight loss, loss of appetite, bloating, burping, nausea, bloody vomit, and black tarry stools. Infection can be detected in a number of ways: GI X-rays, endoscopy, blood tests for anti-Helicobacter antibodies, a stool test, and a urease breath test (which is a by-product of the bacteria). If caught soon enough, it can be treated with three doses of different proton pump inhibitors as well as two antibiotics, taking about a week to cure. If not caught soon enough, surgery may be required.
• Intestinal pseudo-obstruction is a syndrome caused by a malformation of the digestive system, characterized by a severe impairment in the ability of the intestines to push and assimilate. Symptoms include daily abdominal and stomach pain, nausea, severe distension, vomiting, heartburn, dysphagia, diarrhea, constipation, dehydration and malnutrition. There is no cure for intestinal pseudo-obstruction. Different types of surgery and treatment managing life-threatening complications such as ileus and volvulus, intestinal stasis which lead to bacterial overgrowth, and resection of affected or dead parts of the gut may be needed. Many patients require parenteral nutrition.
• Ileus is a blockage of the intestines.
• Coeliac disease is a common form of malabsorption, affecting up to 1% of people of northern European descent. An autoimmune response is triggered in intestinal cells by digestion of gluten proteins. Ingestion of proteins found in wheat, barley and rye, causes villous atrophy in the small intestine. Lifelong dietary avoidance of these foodstuffs in a gluten-free diet is the only treatment.
• Enteroviruses are named by their transmission-route through the intestine (enteric meaning intestinal), but their symptoms are not mainly associated with the intestine.
• Endometriosis can affect the intestines, with similar symptoms to IBS.
• Bowel twist (or similarly, bowel strangulation) is a comparatively rare event (usually developing sometime after major bowel surgery). It is, however, hard to diagnose correctly, and if left uncorrected can lead to bowel infarction and death. (The singer Maurice Gibb is understood to have died from this.)
• Angiodysplasia of the colon
• Constipation
• Diarrhea
• Hirschsprung's disease (aganglionosis)
• Intussusception
• Polyp (medicine) (see also colorectal polyp)
• Pseudomembranous colitis
• Toxic megacolon usually a complication of ulcerative colitis

動物の腸の用途

人間以外の動物の腸はさまざまな方法で利用されている。 乳の供給源となる家畜の各種からは、乳を与えたcalves/jaの腸から対応するレンネットが得られる。 豚や子牛の腸は食用にされ、豚の腸はソーセージのケーシングとして使われる。子牛の腸は子牛腸アルカリホスファターゼ（CIP）を供給し、金平糖の皮を作るのに使われる。 その他の用途：

• 音楽家によるストリングスの使用はエジプト第3王朝まで遡ることができる。近年ではラムのガットで弦が作られていた。近代に入り、音楽家は絹やナイロン、スチールなどの合成素材の弦を使用する傾向にある。しかし、一部の楽器奏者は、古い音質を呼び起こすために、いまだにガット弦を使用している。このような弦は一般に「キャットガット」弦と呼ばれたが、猫がガット弦の材料として使われたことはない。
• シープガットはテニスなどのラケットに使われる天然ガットストリングの元祖である。 今日では合成弦の方がはるかに一般的だが、現在では最高のガット弦は牛ガットで作られている。
• ガット・コードはスネア・ドラムの特徴であるブンブンという音を出すスネア用の弦にも使われてきた。現代のスネアドラムはほとんどガット・コードではなく金属線を使用しているが、北アフリカのベンディルフレーム・ドラムは未だにこの目的のためにガットを使用している。
• "天然"ソーセージの外皮、またはケーシングは動物の腸、特に豚、牛、羊の腸で作られる。
• ココレッツィ、ガルドバキア、トルシーネロの包装は羊（またはヤギ）の腸でできている。
• ハギスは伝統的に羊の胃袋で茹でて食べる。
• 豚の腸をよく洗ったチリリングという食べ物がある。
• 動物の腸はロングケースクロックのコードラインやブラケットクロックのフュジームーブメントに使われていたが、金属線に取って代わられることもある。
• 西暦1640年の最古のコンドームは動物の腸から作られていた。

こちらも参照

この記事は解剖学用語を使用している。

• Gastrointestinal physiology/ja
• Gut-on-a-chip/ja
• All pages with titles beginning with Gastrointestinal/ja
• All pages with titles containing Gastrointestinal/ja

外部リンク

• The gastro intestinal tract in the Human Protein Atlas
• Your Digestive System and How It Works at National Institutes of Health