出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/04/19 10:03:05」(JST)
| Large intestine | |
|---|---|
| Front of abdomen, showing the large intestine, with the stomach and small intestine in gray outline. | |
| Front of abdomen, showing surface markings for liver (red), and the stomach and large intestine (blue) | |
| Latin | intestinum crassum |
| Gray's | subject #249 1177 |
| Lymph | inferior mesenteric lymph nodes |
| Dorlands/Elsevier | Large intestine |
The large intestine (or bowel, colon) is the last part of the digestive system 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.[1] This article is primarily about the human gut, though the information about its processes are directly applicable to most mammals.
The large intestine consists of the cecum, rectum and anal canal.[1][2][3][4] It starts in the right iliac region of the pelvis, just at or below the right waist, where it is joined to the bottom end of the small intestine. From here it continues up the abdomen, then across the width of the abdominal cavity, and then it turns down, continuing to its endpoint at the anus.
The large intestine is about 4.9 feet (1.5 m) long, which is about one-fifth of the whole length of the intestinal canal.
In Terminologia Anatomica the large intestine includes the cecum, colon, rectum, and anal canal. However, some sources exclude the anal canal.[5]
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The large intestine takes about 16 hours to finish the digestion of the food. It removes water and any remaining absorbable nutrients from the food before sending the indigestible matter to the rectum. The colon absorbs vitamins which are created by the colonic bacteria - such as vitamin K (especially important as the daily ingestion of vitamin K is not normally enough to maintain adequate blood coagulation), vitamin B12, thiamine and riboflavin. It also compacts feces, and stores fecal matter in the rectum until it can be discharged via the anus in defecation.
The large intestine differs in physical form from the small intestine in being much wider and in showing the longitudinal layer of the muscularis have been reduced to 3 strap-like structures known as the taeniae coli. The wall of the large intestine is lined with simple columnar epithelium. Instead of having the evaginations of the small intestine (villi), the large intestine has invaginations (the intestinal glands). While both the small intestine and the large intestine have goblet cells, they are abundant in the large intestine.
The appendix is attached to its inferior surface of the cecum. It contains the least of lymphoid tissue. It is a part of mucosa-associated lymphoid tissue, which gives the appendix an important role in immunity. Appendicitis is the result of a blockage that traps infectious material in the lumen. The appendix can be removed with no apparent damage or consequence to the patient. The large intestine extends from the ileocecal junction to the anus and is about 4.9 ft long. On the surface, bands of longitudinal muscle fibers called taeniae coli, each about 1/5 in wide, can be identified. There are three bands, and they start at the base of the appendix and extend from the cecum to the rectum. Along the sides of the taeniae, tags of peritoneum filled with fat, called epiploic appendages (or appendices epiploicae) are found. The sacculations, called haustra, are characteristic features of the large intestine, and distinguish it from the small intestine.
Parts of the large intestine are:
Cecum – the first part of the large intestine
Locations along the colon are:
The large intestine houses over 700 species of bacteria that perform a variety of functions.
The large intestine absorbs some of the products formed by the bacteria inhabiting this region. Undigested polysaccharides (fiber) are metabolized to short-chain fatty acids by bacteria in the large intestine and absorbed by passive diffusion. The bicarbonate that the large intestine secretes helps to neutralize the increased acidity resulting from the formation of these fatty acids.
These bacteria also produce large amounts of vitamins, especially vitamin K and biotin (a B vitamin), for absorption into the blood. Although this source of vitamins, in general, provides only a small part of the daily requirement, it makes a significant contribution when dietary vitamin intake is low. An individual who depends on absorption of vitamins formed by bacteria in the large intestine may become vitamin-deficient if treated with antibiotics that inhibit other species of bacteria as well as the disease-causing bacteria.
Other bacterial products include gas (flatus), which is a mixture of nitrogen and carbon dioxide, with small amounts of the gases hydrogen, methane, and hydrogen sulphide. Bacterial fermentation of undigested polysaccharides produces these. The normal flora is also essential in the development of certain tissues, including the cecum and lymphatics.
They are also involved in the production of cross-reactive antibodies. These are antibodies produced by the immune system against the normal flora, that are also effective against related pathogens, thereby preventing infection or invasion.
The most prevalent bacteria are the bacteroides, which have been implicated in the initiation of colitis and colon cancer. Bifidobacteria are also abundant, and are often described as 'friendly bacteria'.
A mucus layer protects the large intestine from attacks from colonic commensal bacteria.[6]
The large intestine is truly distinct only in tetrapods, in which it is almost always separated from the small intestine by an ileocaecal valve. In most vertebrates, however, it is a relatively short structure running directly to the anus, although noticeably wider than the small intestine. Although the caecum is present in most amniotes, only in mammals does the remainder of the large intestine develop into a true colon.[7]
In some small mammals, the colon is straight, as it is in other tetrapods, but, in the majority of mammalian species, it is divided into ascending and descending portions; a distinct transverse colon is typically present only in primates. However, the taeniae coli and accompanying haustra are not found in either carnivorans or ruminants. The rectum of mammals (other than monotremes) is derived from the cloaca of other vertebrates, and is, therefore, not truly homologous with the "rectum" found in these species.[7]
In fish, there is no true large intestine, but simply a short rectum connecting the end of the digestive part of the gut to the cloaca. In sharks, this includes a rectal gland that secretes salt to help the animal maintain osmotic balance with the seawater. The gland somewhat resembles a caecum in structure, but is not a homologous structure.[7]
| Look up large intestine in Wiktionary, the free dictionary. |
This article incorporates text from a public domain edition of Gray's Anatomy.
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| リンク元 | 「大腸」 |
1)膨起性往復運動 haustralshuttling movement 2)(単一)膨起性移送運動 segmentalhaustralpropulsion 3)多膨起性移送運動 multihaustralpropulsion 1) 2)により内容物のゆっくりした移動(5cm/hr) → 48hrで上行結腸よりS状結腸へ 4)総蠕動mass movement(mass peristalsis，maSS PrePulsion) 1-3回/日、強い蠕動→結腸内容物が直腸へ移動(→排便誘発) 5)収縮回数：直腸 ＞ S状結腸 のため内容物はS状結腸へ移動 (通常は、直腸に内容物(-)) 6)胃大腸反射 gastro-colonic reflex 小腸大腸反射 ileo-colonic reflex：胃、小腸に内容物－→結腸に総蠕動(+) *排便 1)解剖 ①内肛門括約筋internalanal ②外肛門括約筋externalanal sphincter－－－平滑筋 sphincter山－一横紋筋 2)排便のメカニズム i)総蠕動一糞便直腸へ ii)直腸内圧〉20Ⅷ舶g ⇒ 直腸壁伸展⇒ 仙髄排便中枢(S2-4) ⇒ ①高位中枢(便奇形成) ②排便反射defecation reflex 内肛門筋弛緩 外肛門筋収縮(一過性) 直腸蠕動運動(⇒内圧をさらに高める) iii) 内圧45－55mmHg以上 内容物200ml以上 便意による排便動作 外肛門筋弛緩 腹筋、横隔膜収縮