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Large intestine
Large intestine
Large intestine
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Large intestine
Front of abdomen, showing the large intestine, with the stomach and small intestine in gray.
Details
Part ofGastrointestinal tract
SystemDigestive system
ArterySuperior mesenteric, inferior mesenteric and iliac arteries
VeinSuperior and inferior mesenteric vein
LymphInferior mesenteric lymph nodes
Identifiers
Latincolon or intestinum crassum
MeSHD007420
TA98A05.7.01.001
TA22963
FMA7201
Anatomical terminology
Major parts of the
Gastrointestinal tract
Upper gastrointestinal tract
Lower gastrointestinal tract
See also

The large intestine, also known as the large bowel, is the last part of the gastrointestinal tract and of the digestive system in tetrapods. Water is absorbed here and the remaining waste material is stored in the rectum as feces before being removed by defecation.[1] The colon (progressing from the ascending colon to the transverse, the descending and finally the sigmoid colon) is the longest portion of the large intestine, and the terms "large intestine" and "colon" are often used interchangeably, but most sources define the large intestine as the combination of the cecum, colon, rectum, and anal canal.[1][2][3] Some other sources exclude the anal canal.[4][5][6]

In humans, the large intestine begins in the right iliac region of the pelvis, just at or below the waist, where it is joined to the end of the small intestine at the cecum, via the ileocecal valve. It then continues as the colon ascending the abdomen, across the width of the abdominal cavity as the transverse colon, and then descending to the rectum and its endpoint at the anal canal.[7] Overall, in humans, the large intestine is about 1.5 metres (5 ft) long, which is about one-fifth of the whole length of the human gastrointestinal tract.[8]

Structure

[edit]
Illustration of the large intestine.

The colon of the large intestine is the last part of the digestive system. It has a segmented appearance due to a series of saccules called haustra.[9] It extracts water and salt from solid wastes before they are eliminated from the body and is the site in which the fermentation of unabsorbed material by the gut microbiota occurs. Unlike the small intestine, the colon does not play a major role in absorption of foods and nutrients. About 1.5 litres or 45 ounces of water arrives in the colon each day.[10]

The colon is the longest part of the large intestine and its average length in the adult human is 65 inches or 166 cm (range of 80 to 313 cm) for males, and 61 inches or 155 cm (range of 80 to 214 cm) for females.[11]

Sections

[edit]
Inner diameters of colon sections

In mammals, the large intestine consists of the cecum (including the appendix), colon (the longest part), rectum, and anal canal.[1]

The four sections of the colon are: the ascending colon, transverse colon, descending colon, and sigmoid colon. These sections turn at the colic flexures.

The parts of the colon are either intraperitoneal or behind it in the retroperitoneum. Retroperitoneal organs, in general, do not have a complete covering of peritoneum, so they are fixed in location. Intraperitoneal organs are completely surrounded by peritoneum and are therefore mobile.[12] Of the colon, the ascending colon, descending colon and rectum are retroperitoneal, while the cecum, appendix, transverse colon and sigmoid colon are intraperitoneal.[13] This is important as it affects which organs can be easily accessed during surgery, such as a laparotomy.

In terms of diameter, the cecum is the widest, averaging slightly less than 9 cm in healthy individuals, and the transverse colon averages less than 6 cm in diameter.[14] The descending and sigmoid colon are slightly smaller, with the sigmoid colon averaging 4–5 cm (1.6–2.0 in) in diameter.[14][15] Diameters larger than certain thresholds for each colonic section can be diagnostic for megacolon.

3D file generated from computed tomography of large intestine

Cecum and appendix

[edit]
Main articles: Cecum and Appendix (anatomy)

The cecum is the first section of the large intestine and is involved in digestion, while the appendix which develops embryologically from it, is not involved in digestion and is considered to be part of the gut-associated lymphoid tissue. The function of the appendix is uncertain, but some sources believe that it has a role in housing a sample of the gut microbiota, and is able to help to repopulate the colon with microbiota if depleted during the course of an immune reaction. The appendix has also been shown to have a high concentration of lymphatic cells.

Ascending colon

[edit]
Main article: Ascending colon

The ascending colon is the first of four main sections of the large intestine. It is connected to the small intestine by a section of bowel called the cecum. The ascending colon runs upwards through the abdominal cavity toward the transverse colon for approximately eight inches (20 cm).

One of the main functions of the colon is to remove the water and other key nutrients from waste material and recycle it. As the waste material exits the small intestine through the ileocecal valve, it will move into the cecum and then to the ascending colon where this process of extraction starts. The waste material is pumped upwards toward the transverse colon by peristalsis. The ascending colon is sometimes attached to the appendix via Gerlach's valve. In ruminants, the ascending colon is known as the spiral colon.[16][17]

Transverse colon

[edit]
Main article: Transverse colon

The transverse colon is the part of the colon from the hepatic flexure, also known as the right colic, (the turn of the colon by the liver) to the splenic flexure also known as the left colic, (the turn of the colon by the spleen). The transverse colon hangs off the stomach, attached to it by a large fold of peritoneum called the greater omentum. On the posterior side, the transverse colon is connected to the posterior abdominal wall by a mesentery known as the transverse mesocolon.

The transverse colon is encased in peritoneum, and is therefore mobile (unlike the parts of the colon immediately before and after it).

The proximal two-thirds of the transverse colon is perfused by the middle colic artery, a branch of the superior mesenteric artery (SMA), while the latter third is supplied by branches of the inferior mesenteric artery (IMA). The "watershed" area between these two blood supplies, which represents the embryologic division between the midgut and hindgut, is an area sensitive to ischemia.

Descending colon

[edit]
Main article: Descending colon

The descending colon is the part of the colon from the splenic flexure to the beginning of the sigmoid colon. One function of the descending colon in the digestive system is to store feces that will be emptied into the rectum. It is retroperitoneal in two-thirds of humans. In the other third, it has a (usually short) mesentery.[18] The arterial supply comes via the left colic artery. The descending colon is also called the distal gut, as it is further along the gastrointestinal tract than the proximal gut. Gut flora are very dense in this region.

Sigmoid colon

[edit]
Main article: Sigmoid colon

The sigmoid colon is the part of the large intestine after the descending colon and before the rectum. The name sigmoid means S-shaped (see sigmoid; cf. sigmoid sinus). The walls of the sigmoid colon are muscular and contract to increase the pressure inside the colon, causing the stool to move into the rectum.

The sigmoid colon is supplied with blood from several branches (usually between 2 and 6) of the sigmoid arteries, a branch of the IMA. The IMA terminates as the superior rectal artery.

Sigmoidoscopy is a common diagnostic technique used to examine the sigmoid colon.

Rectum

[edit]
Main article: Rectum

The rectum is the last section of the large intestine. It holds the formed feces awaiting elimination via defecation. It is about 12 cm long.[19]

Appearance

[edit]

The cecum – the first part of the large intestine

The taenia coli run the length of the large intestine. Because the taenia coli are shorter than the large bowel itself, the colon becomes sacculated, forming the haustra of the colon which are the shelf-like intraluminal projections.[20]

Blood supply

[edit]

Arterial supply to the colon comes from branches of the superior mesenteric artery (SMA) and inferior mesenteric artery (IMA). Flow between these two systems communicates via the marginal artery of the colon that runs parallel to the colon for its entire length. Historically, a structure variously identified as the arc of Riolan or meandering mesenteric artery (of Moskowitz) was thought to connect the proximal SMA to the proximal IMA. This variably present structure would be important if either vessel were occluded. However, at least one review of the literature questions the existence of this vessel, with some experts calling for the abolition of these terms from future medical literature.[21]

Venous drainage usually mirrors colonic arterial supply, with the inferior mesenteric vein draining into the splenic vein, and the superior mesenteric vein joining the splenic vein to form the hepatic portal vein that then enters the liver. Middle rectal veins are an exception, delivering blood to inferior vena cava and bypassing the liver.[22]

Lymphatic drainage

[edit]

Lymphatic drainage from the ascending colon and proximal two-thirds of the transverse colon is to the ileocolic lymph nodes and the superior mesenteric lymph nodes, which drain into the cisterna chyli.[23] The lymph from the distal one-third of the transverse colon, the descending colon, the sigmoid colon, and the upper rectum drain into the inferior mesenteric and colic lymph nodes.[23] The lower rectum to the anal canal above the pectinate line drain to the internal ileocolic nodes.[24] The anal canal below the pectinate line drains into the superficial inguinal nodes.[24] The pectinate line only roughly marks this transition.

Nerve supply

[edit]

Sympathetic supply: superior & inferior mesenteric ganglia; parasympathetic supply: vagus & sacral plexus (S2-S4)[citation needed]

Development

[edit]
See also: Development of the digestive system

The endoderm, mesoderm and ectoderm are germ layers that develop in a process called gastrulation. Gastrulation occurs early in human development. The gastrointestinal tract is derived from these layers.[25]

Variation

[edit]

One variation on the normal anatomy of the colon occurs when extra loops form, resulting in a colon that is up to five metres longer than normal. This condition, referred to as redundant colon, typically has no direct major health consequences, though rarely volvulus occurs, resulting in obstruction and requiring immediate medical attention.[26][27] A significant indirect health consequence is that use of a standard adult colonoscope is difficult and in some cases impossible when a redundant colon is present, though specialized variants on the instrument (including the pediatric variant) are useful in overcoming this problem.[28]

Microanatomy

[edit]
Further information: Gastrointestinal wall

Colonic crypts

[edit]
Colonic crypts (intestinal glands) within four tissue sections. The cells have been stained to show a brown-orange color if the cells produce the mitochondrial protein cytochrome c oxidase subunit I (CCOI), and the nuclei of the cells (located at the outer edges of the cells lining the walls of the crypts) are stained blue-gray with haematoxylin. Panels A, B were cut across the long axes of the crypts and panels C, D were cut parallel to the long axes of the crypts. In panel A the bar shows 100 μm and allows an estimate of the frequency of crypts in the colonic epithelium. Panel B includes three crypts in cross-section, each with one segment deficient for CCOI expression and at least one crypt, on the right side, undergoing fission into two crypts. Panel C shows, on the left side, a crypt fissioning into two crypts. Panel D shows typical small clusters of two and three CCOI deficient crypts (the bar shows 50 μm). The images were made from original photomicrographs, but panels A, B and D were also included in an article[29] and illustrations were published with Creative Commons Attribution-Noncommercial License allowing re-use.

The wall of the large intestine is lined with simple columnar epithelium with invaginations. The invaginations are called the intestinal glands or colonic crypts.

  • Micrograph of normal large instestinal crypts.
    Micrograph of normal large instestinal crypts.
  • Anatomy of normal large intestinal crypts
    Anatomy of normal large intestinal crypts

The colon crypts are shaped like microscopic thick walled test tubes with a central hole down the length of the tube (the crypt lumen). Four tissue sections are shown here, two cut across the long axes of the crypts and two cut parallel to the long axes. In these images the cells have been stained by immunohistochemistry to show a brown-orange color if the cells produce a mitochondrial protein called cytochrome c oxidase subunit I (CCOI). The nuclei of the cells (located at the outer edges of the cells lining the walls of the crypts) are stained blue-gray with haematoxylin. As seen in panels C and D, crypts are about 75 to about 110 cells long. Baker et al.[30] found that the average crypt circumference is 23 cells. Thus, by the images shown here, there are an average of about 1,725 to 2,530 cells per colonic crypt. Nooteboom et al.[31] measuring the number of cells in a small number of crypts reported a range of 1,500 to 4,900 cells per colonic crypt. Cells are produced at the crypt base and migrate upward along the crypt axis before being shed into the colonic lumen days later.[30] There are 5 to 6 stem cells at the bases of the crypts.[30]

As estimated from the image in panel A, there are about 100 colonic crypts per square millimeter of the colonic epithelium.[32] Since the average length of the human colon is 160.5 cm[11] and the average inner circumference of the colon is 6.2 cm,[32] the inner surface epithelial area of the human colon has an average area of about 995 cm2, which includes 9,950,000 (close to 10 million) crypts.

In the four tissue sections shown here, many of the intestinal glands have cells with a mitochondrial DNA mutation in the CCOI gene and appear mostly white, with their main color being the blue-gray staining of the nuclei. As seen in panel B, a portion of the stem cells of three crypts appear to have a mutation in CCOI, so that 40% to 50% of the cells arising from those stem cells form a white segment in the cross cut area.

Overall, the percent of crypts deficient for CCOI is less than 1% before age 40, but then increases linearly with age.[29] Colonic crypts deficient for CCOI in women reaches, on average, 18% in women and 23% in men by 80–84 years of age.[29]

Crypts of the colon can reproduce by fission, as seen in panel C, where a crypt is fissioning to form two crypts, and in panel B where at least one crypt appears to be fissioning. Most crypts deficient in CCOI are in clusters of crypts (clones of crypts) with two or more CCOI-deficient crypts adjacent to each other (see panel D).[29]

Mucosa

[edit]

About 150 of the many thousands of protein coding genes expressed in the large intestine, some are specific to the mucous membrane in different regions and include CEACAM7.[33]

Function

[edit]
Histological section.

The large intestine absorbs water and any remaining absorbable nutrients from the food before sending the indigestible matter to the rectum.[34] The colon absorbs vitamins that are created by the colonic bacteria, such as thiamine, riboflavin, and vitamin K.[35][36] It also compacts feces, and stores fecal matter in the rectum until it can be discharged via the anus in defecation.

The large intestine also secretes potassium and chloride. Recycling of various nutrients takes place in the colon. Examples include fermentation of carbohydrates, short chain fatty acids, and urea cycling.[34]

The appendix contains a small amount of mucosa-associated lymphoid tissue which gives the appendix an undetermined role in immunity. However, the appendix is known to be important in fetal life as it contains endocrine cells that release biogenic amines and peptide hormones important for homeostasis during early growth and development.[37]

By the time chyme enters the large intestine, the small intestine has absorbed nearly all digestible nutrients and approximately 90% of the ingested water, based on volume measurements showing that only about 1–2 litres of fluid pass into the colon from a daily intestinal load of roughly 9–10 litres.[38][39] Indeed, as demonstrated by the commonality of ileostomy procedures, it is possible for many people to live without large portions of their large intestine, or even without it completely. At this point only some electrolytes like sodium, magnesium, and chloride are left as well as indigestible parts of ingested food (e.g., a large part of ingested amylose, starch which has been shielded from digestion heretofore, and dietary fiber, which is largely indigestible carbohydrate in either soluble or insoluble form). As the chyme moves through the large intestine, most of the remaining water is removed, while the chyme is mixed with mucus and bacteria (known as gut flora), and becomes feces. The ascending colon receives fecal material as a liquid. The muscles of the colon then move the watery waste material forward and slowly absorb all the excess water, causing the stools to gradually solidify as they move along into the descending colon.[34]

The bacteria break down some of the fiber for their own nourishment and create acetate, propionate, and butyrate as waste products, which in turn are used by the cell lining of the colon for nourishment.[40] No protein is made available. In humans, perhaps 10% of the undigested carbohydrate thus becomes available, though this may vary with diet;[41] in other animals, including other apes and primates, who have proportionally larger colons, more is made available, thus permitting a higher portion of plant material in the diet. The large intestine produces no digestive enzymes — chemical digestion is completed in the small intestine before the chyme reaches the large intestine. The pH in the colon varies between 5.5 and 7 (slightly acidic to neutral).[34]

Standing gradient osmosis

[edit]

Water absorption at the colon typically proceeds against a transmucosal osmotic pressure gradient. The standing gradient osmosis is the reabsorption of water against the osmotic gradient in the intestines. Cells occupying the intestinal lining pump sodium ions into the intercellular space, raising the osmolarity of the intercellular fluid. This hypertonic fluid creates an osmotic pressure that drives water into the lateral intercellular spaces by osmosis via tight junctions and adjacent cells, which then in turn moves across the basement membrane and into the capillaries, while more sodium ions are pumped again into the intercellular fluid.[42] Although water travels down an osmotic gradient in each individual step, overall, water usually travels against the osmotic gradient due to the pumping of sodium ions into the intercellular fluid. This allows the large intestine to absorb water despite the blood in capillaries being hypotonic compared to the fluid within the intestinal lumen.

Gut flora

[edit]
Main article: Gut microbiota

The large intestine houses over 700 species of bacteria that perform a variety of functions, as well as fungi, protozoa, and archaea. Species diversity varies by geography and diet.[43] The microbes in a human distal gut often number in the vicinity of 100 trillion, and can weigh around 200 grams (0.44 pounds). This mass of mostly symbiotic microbes has recently been called the latest human organ to be "discovered" or in other words, the "forgotten organ".[44]

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.[45]

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 the vitamin producing species of bacteria as well as the intended disease-causing bacteria.[46]

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 sulfide. Bacterial fermentation of undigested polysaccharides produces these. Some of the fecal odor is due to indoles, metabolized from the amino acid tryptophan. The normal flora is also essential in the development of certain tissues, including the cecum and lymphatics.[citation needed]

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 two most prevalent phyla of the colon are Bacillota and Bacteroidota. The ratio between the two seems to vary widely as reported by the Human Microbiome Project.[47] Bacteroides are implicated in the initiation of colitis and colon cancer. Bifidobacteria are also abundant, and are often described as 'friendly bacteria'.[48][49]

A mucus layer protects the large intestine from attacks from colonic commensal bacteria.[50]

Clinical significance

[edit]

Disease

[edit]
Main article: Gastrointestinal disease

Following are the most common diseases or disorders of the colon:

Colonoscopy

[edit]
Main article: Colonoscopy
Colonoscopy image, splenic flexure,
normal mucosa. The spleen can be seen through it

Colonoscopy is the endoscopic examination of the large intestine and the distal part of the small bowel with a CCD camera or a fiber optic camera on a flexible tube passed through the anus. It can provide a visual diagnosis (e.g. ulceration, polyps) and grants the opportunity for biopsy or removal of suspected colorectal cancer lesions. Colonoscopy can remove polyps as small as one millimetre or less. Once polyps are removed, they can be studied with the aid of a microscope to determine if they are precancerous or not. It takes 15 years or fewer for a polyp to turn cancerous.

Colonoscopy is similar to sigmoidoscopy—the difference being related to which parts of the colon each can examine. A colonoscopy allows an examination of the entire colon (1200–1500 mm in length). A sigmoidoscopy allows an examination of the distal portion (about 600 mm) of the colon, which may be sufficient because benefits to cancer survival of colonoscopy have been limited to the detection of lesions in the distal portion of the colon.[51][52][53]

A sigmoidoscopy is often used as a screening procedure for a full colonoscopy, often done in conjunction with a stool-based test such as a fecal occult blood test (FOBT), fecal immunochemical test (FIT), or multi-target stool DNA test (Cologuard) or blood-based test, SEPT9 DNA methylation test (Epi proColon).[54] About 5% of these screened patients are referred to colonoscopy.[55]

Virtual colonoscopy, which uses 2D and 3D imagery reconstructed from computed tomography (CT) scans or from nuclear magnetic resonance (MR) scans, is also possible, as a totally non-invasive medical test, although it is not standard and still under investigation regarding its diagnostic abilities. Furthermore, virtual colonoscopy does not allow for therapeutic maneuvers such as polyp/tumour removal or biopsy nor visualization of lesions smaller than 5 millimeters. If a growth or polyp is detected using CT colonography, a standard colonoscopy would still need to be performed. Additionally, surgeons have lately been using the term pouchoscopy to refer to a colonoscopy of the ileo-anal pouch.

Other animals

[edit]

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.[56]

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.[56]

In some 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.[56]

Additional images

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  • Intestines
    Intestines
  • Colon. Deep dissection. Anterior view.
    Colon. Deep dissection. Anterior view.

See also

[edit]
This article uses anatomical terminology.

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Large intestine

View on Grokipedia
from Grokipedia
The large intestine, also known as the colon or large bowel, is the terminal portion of the in humans and other vertebrates, measuring approximately 1.5 meters (5 feet) in length and comprising about one-fifth of the total length. It receives undigested material from the via the and primarily functions to absorb and electrolytes, produce and absorb vitamins through bacterial fermentation, and form and propel toward the for elimination. The organ is divided into key segments: the (a blind pouch at the junction with the ), the colon (subdivided into ascending, transverse, descending, and sigmoid portions), the , and the . Structurally, the large intestine features a wider lumen than the , with a characteristic layered wall consisting of mucosa (lined with goblet cells for mucus ), submucosa, a muscular layer (including an inner circular and outer longitudinal component gathered into three teniae coli bands), and serosa. Distinctive external features include haustra (pouch-like sacculations formed by the teniae coli) and omental appendices (fat-filled tags along the outer surface), which contribute to its segmented appearance and aid in the slow propulsion of contents via haustral contractions. The appendix, a narrow, worm-like extension of the measuring 6 to 10 cm, is attached near the ileocecal junction and contains lymphoid tissue, though its precise role remains under study. Embryologically, the large intestine develops from the (cecum to proximal ) and (distal to ), undergoing a 270-degree counterclockwise during fetal development by week 10. Functionally, the large intestine absorbs up to 90% of the remaining water and electrolytes after small intestinal processing, secretes mucus and bicarbonate to lubricate and neutralize contents, and hosts a diverse gut microbiota that ferments undigested carbohydrates to produce short-chain fatty acids, vitamins K and B, and gases. Its motility is slower than the small intestine, relying on segmental mixing (haustra contractions) and mass movements to compact residue into feces, which are stored in the rectum until defecation. Blood supply derives from the superior mesenteric artery for the midgut-derived proximal portions and the inferior mesenteric artery for the hindgut-derived distal segments, connected by the marginal artery of Drummond to ensure collateral circulation. Innervation involves the autonomic nervous system, with sympathetic input inhibiting motility and parasympathetic input enhancing it, alongside an intrinsic enteric nervous system for local control.

Gross Anatomy

Cecum and Appendix

The cecum is a blind-ended pouch forming the first segment of the large intestine, situated in the right at the junction with the terminal of the . It receives from the through an opening regulated by the and measures approximately 6 cm in length and 7.5 cm in width in adults. The , composed of two folds of mucosa and circular muscle, functions primarily to prevent reflux of cecal contents back into the while allowing unidirectional flow. The vermiform appendix arises as a narrow, tubular diverticulum from the posteromedial wall of the , typically 2 cm inferior to the at the confluence of the taeniae coli. Its length is highly variable, ranging from 2 to 20 cm with an average of 9 cm, and its diameter usually measures 6-8 mm. The appendix exhibits positional variability, with the retrocecal location being the most frequent, occurring in approximately 65% of individuals; other positions include pelvic, subcecal, and post-ileal. Histologically, the appendix possesses all four layers of the intestinal wall, but it is distinguished by a pronounced accumulation of lymphoid follicles in the mucosa and , forming that supports immune surveillance. The appendix's anatomical relation to the cecum holds significant surgical implications, particularly in , the most common cause of acute requiring surgical intervention. Obstruction of the appendiceal lumen—often by fecaliths, , or parasites—leads to bacterial overgrowth, , and potential ; the retrocecal position can alter clinical presentation, causing flank or rather than classic right lower quadrant tenderness due to its posterior orientation relative to the . Laparoscopic or open remains the standard treatment, with the appendiceal base serving as a reliable for identification during . The marks the origin of key structural features of the large intestine, including the taeniae coli—three longitudinal bands of thickened that converge at the appendiceal base and extend along the colon. Contraction of these taeniae coli shortens the cecal and colonic walls unevenly, producing haustra, the characteristic sacculations or pouches that begin in the and define the large intestine's segmented appearance. The connects proximally to the , contributing to the overall continuity of the large intestine, and houses a substantial portion of the essential for colonic function.

Colonic Segments

The large intestine, or colon, is divided into four main segments: the , , , and , each with distinct anatomical positions, peritoneal relationships, and mobilities that facilitate the progression of intestinal contents from the to the . These segments collectively form a frame-like structure around the , with the ascending and descending portions fixed along the flanks and the transverse and sigmoid portions more freely mobile. The extends from the in the right upward along the right side of the to the hepatic , measuring approximately 15 to 20 cm in length and positioned retroperitoneally, which anchors it firmly to the posterior . This fixation limits its mobility compared to other segments, contributing to its role in initial fecal consolidation. The , the longest segment at about 50 cm, spans horizontally across the upper from the hepatic on the right to the splenic on the left, suspended intraperitoneally by the transverse mesocolon attached to the and via the gastrocolic ligament, allowing greater mobility and potential descent below the umbilicus in some individuals. The runs downward along the left side of the from the splenic to the sigmoid junction, spanning 25 to 30 cm and also retroperitoneal, providing stability but with slightly more mobility than the due to partial peritoneal coverage in some cases. Finally, the forms an S-shaped loop in the lower left , approximately 40 cm long, entirely intraperitoneal with its own mesocolon, enabling high mobility to accommodate variable fecal volumes before entering the at the level of the third sacral . Key transitions between segments occur at the flexures: the hepatic (right colic) flexure, located inferior to the right lobe of the liver, marks the sharp bend where the turns medially into the ; and the splenic (left colic) flexure, positioned near the and anchored by the phrenicocolic to the diaphragm, represents a more acute angle where the descends into the descending segment, often the highest and least mobile point in the colon. These flexures influence the flow of contents and are sites of potential obstruction. Throughout all colonic segments, characteristic structural features include haustra—pouch-like sacculations formed by the circular muscle layer that give the colon its segmented appearance; taeniae coli—three longitudinal bands of that run the length of the colon, shorter than the outer muscularis and responsible for the haustral contractions; and epiploic appendages—small, fat-filled peritoneal pouches attached to the taeniae, varying in number and size but most prominent in the transverse and sigmoid regions. These elements are consistent across segments, though their prominence may vary with individual fat distribution. Length variations among individuals are common, influenced by factors such as age, , and body habitus, with total colonic length averaging 131 to 150 cm; for instance, the shows the greatest variability (up to 10 cm standard deviation), while mobility differs markedly—retroperitoneal ascending and descending segments are largely fixed (mobile in only 31-66% of cases, covering less than half their length), whereas the intraperitoneal transverse and sigmoid segments exhibit full mobility due to their mesocolic suspensions.

Rectum

The rectum serves as the terminal dilated chamber of the large intestine, measuring approximately 12 to 15 cm in length from the rectosigmoid junction to the dentate line in the . It receives fecal material as a distal continuation from the , expanding into a rectal ampulla that rests on the pelvic diaphragm and functions primarily as a temporary storage reservoir for before . Unlike the preceding colonic segments, the rectum lacks taeniae coli and haustra, with the three longitudinal muscle bands of the colon coalescing at the rectosigmoid junction to form a continuous outer longitudinal muscle layer encircling the rectal wall. The rectal wall exhibits three distinct lateral curvatures that conform to the pelvic anatomy, corresponding internally to the submucosal folds known as the , which project into the lumen and typically consist of two on the left side and one on the right. The upper and lower curvatures are convex to the right, while the middle curvature is convex to the left, aiding in the efficient storage and passage of contents. Peritoneally, the upper third of the rectum is covered anteriorly and laterally, rendering it intraperitoneal, whereas the middle third receives only anterior peritoneal coverage, and the lower third is entirely extraperitoneal, enveloped by the mesorectal fascia. At its inferior end, the rectum forms the anorectal at the level of the muscle, where it narrows and transitions into the , with the inner circular muscle layer thickening to become the . The puborectalis muscle, a component of the group within the , wraps as a U-shaped sling around the anorectal junction, accentuating the anorectal angle to help maintain fecal continence. The rectum's capacity reaches up to 500 mL in continent individuals, enabling its reservoir function, while continence is further supported by both the involuntary and the voluntary .

Microscopic Anatomy

Mucosal Layer

The mucosal layer of the large intestine forms the innermost lining, consisting of the , , and . This structure facilitates , absorption, and immune defense while interfacing briefly with the underlying muscular layers to maintain overall wall integrity. The is a simple columnar type, lacking villi but featuring numerous tubular glands known as colonic crypts of Lieberkühn that extend down to the . It comprises several specialized cell types: absorptive enterocytes, which bear apical microvilli forming a to enhance surface area for nutrient and water uptake; goblet cells, which secrete to lubricate the luminal surface and protect against mechanical stress; and enteroendocrine cells, which release hormones regulating gastrointestinal and . Within the colonic crypts, stem cells reside at the base, continuously regenerating the epithelial lining by producing transit-amplifying progenitors that differentiate into the various epithelial cell types. These crypt bases also contain Paneth cells, which are more common in the proximal colon, providing antimicrobial secretions and support stem cell maintenance, analogous to Paneth cells in the . The , a layer beneath the , is rich in immune components, including lymphocytes, plasma cells, and macrophages, which contribute to mucosal immunity and surveillance against pathogens. The , a thin sheet of at the base of the mucosa, enables localized contractions that aid in mixing contents and facilitating absorption. Regional variations in the mucosa include a higher of lymphoid follicles in the , part of the , which enhances immune sampling in this proximal segment.

Muscular and Serosal Layers

The wall of the large intestine consists of four primary histological layers, with the , muscularis externa, and forming the outer supportive and contractile components beyond the mucosa. The submucosa is a layer of that lies immediately beneath the mucosa, providing structural support and housing key vascular and neural elements. It contains numerous blood vessels and lymphatics that supply the overlying mucosa, as well as the submucosal (Meissner's) plexus, a network of neurons and ganglia that regulates local glandular secretion, blood flow, and mucosal motility. The muscularis externa, also known as the muscularis propria, comprises two distinct layers of : an inner circular layer and an outer longitudinal layer, which together facilitate and segmentation. In the colon, the longitudinal muscle layer is not uniformly distributed but condenses into three thickened bands called taeniae coli, which run along the antimesenteric surface and contribute to the formation of haustra (pouches) by gathering the wall into folds. Unlike the , the large intestine's muscularis externa features a thicker circular muscle layer, enhancing its role in slower, mixing-type contractions, while the myenteric (Auerbach's) plexus embedded between the muscle layers coordinates propulsion. In the , the longitudinal layer becomes more complete and uniform, forming a continuous sheath without distinct taeniae. The outermost layer of the large intestine is the serosa or , which provides peritoneal covering and protection. For intraperitoneal segments such as the transverse and , the serosa consists of a thin layer of visceral —a simple squamous supported by —that allows mobility within the . In contrast, retroperitoneal portions like the colon are covered by , a fibrous layer that lacks and blends directly with surrounding retroperitoneal structures, anchoring these segments in place. This distinction in outer coverings influences surgical approaches and the organ's intraperitoneal versus retroperitoneal positioning.

Vascular and Nervous Supply

Blood Supply

The blood supply to the large intestine is primarily derived from the superior mesenteric artery (SMA) and the inferior mesenteric artery (IMA), which provide oxygenated blood to its various segments. The SMA, arising from the abdominal aorta at the level of the L1 vertebra, supplies the midgut-derived portions, including the cecum, appendix, ascending colon, and proximal two-thirds of the transverse colon. Its key branches include the ileocolic artery, which vascularizes the cecum and appendix via the appendicular branch; the right colic artery, serving the ascending colon and hepatic flexure; and the middle colic artery, which supplies the transverse colon up to the splenic flexure. In contrast, the IMA, originating from the aorta about 3-4 cm above its bifurcation at L3, perfuses the hindgut structures: the distal transverse colon, descending colon, sigmoid colon, and upper rectum. Branches from the IMA consist of the left colic artery for the descending colon and splenic flexure; multiple sigmoid arteries for the sigmoid colon; and the superior rectal artery, which continues as the main supply to the rectum. The rectum also receives supplemental arterial input from the middle rectal arteries (from the internal iliac arteries) and inferior rectal arteries (from the internal pudendal arteries), ensuring robust perfusion in this distal region. Venous drainage of the large intestine closely parallels the arterial supply for the colon, facilitating the return of deoxygenated to the liver via the portal system, while the rectum exhibits mixed drainage. Veins from the SMA territory converge into the (SMV), which ascends to join the and form the . Similarly, veins draining the IMA territory empty into the (IMV), which typically joins the before contributing to the ; the IMV runs anterior and to the left of its corresponding artery. For the rectum, venous drainage is mixed, with the superior rectal veins draining to the IMV (portal system) and the middle and inferior rectal veins draining to the internal iliac veins (systemic circulation). This parallel arrangement supports efficient nutrient transport from the intestinal mucosa to the hepatic portal circulation. The lymphatic drainage also follows the venous pathways, aiding in immune surveillance along the same routes. Critical anastomoses between the SMA and IMA branches provide collateral circulation, mitigating risks of ischemia. The marginal artery of Drummond, a continuous arcade running parallel to the colon approximately 2-3 cm from its wall, interconnects the ileocolic, right , middle , and left arteries, extending from the to the . Additionally, the arc of Riolan serves as a deeper mesenteric between the middle (SMA) and left (IMA) arteries, present in a of individuals to enhance redundancy. Watershed areas, where arterial territories meet and collateral flow may be insufficient, are particularly vulnerable to hypoperfusion; the splenic flexure (Griffith's point) represents the primary such zone due to the often tenuous there, predisposing it to .

Lymphatic and Nerve Supply

The lymphatic drainage of the large intestine occurs through a hierarchical system of lymph nodes that parallels the arterial supply, beginning with epicolic nodes located directly on the serosal surface of the , followed by paracolic nodes along the mesenteric border of the colon. From there, flows to intermediate nodes situated along the branches of the colic arteries and then to principal (preterminal) nodes at the origin of the main (SMA) or (IMA). Ultimately, this converges into the and enters the for return to the systemic circulation. Regional variations in drainage reflect the embryologic divisions of the large intestine. The right colon (cecum, ascending colon, and proximal two-thirds of the transverse colon) drains sequentially to ileocolic, right colic, and middle colic nodes associated with the SMA. In contrast, the left colon (distal one-third of the transverse colon, descending colon, and sigmoid colon) drains to left colic and sigmoid nodes linked to the IMA. The rectum exhibits a more complex pattern, with upper rectal lymphatics following the to IMA nodes, middle rectal lymphatics draining to internal iliac nodes, and lower rectal lymphatics to sacral and superficial inguinal nodes in some cases. The large intestine receives dual autonomic innervation, with parasympathetic fibers promoting and while sympathetic fibers exert inhibitory effects. Parasympathetic supply to the proximal large intestine ( to proximal transverse colon) arises from the (cranial nerve X), traveling via the SMA plexus to stimulate enteric neurons. Distally, from the distal transverse colon to the , parasympathetic innervation comes from (S2-S4 segments), which join the inferior mesenteric and pelvic plexuses to enhance and glandular . Sympathetic innervation originates from preganglionic fibers in the thoracic (T5-T12) and (L1-L2) levels, synapsing in the celiac, superior mesenteric, and inferior mesenteric ganglia before distributing along arterial plexuses to inhibit contraction and vasoconstrict. Intrinsic control is provided by the , consisting of the myenteric (Auerbach's) located between the longitudinal and circular muscle layers of the muscularis externa, which coordinates peristaltic motility, and the submucosal (Meissner's) in the submucosa, which regulates local , absorption, and blood flow. Sensory afferents, comprising visceral mechanoreceptors and chemoreceptors, detect distension and chemical stimuli in the colon and ; these signals travel primarily via sympathetic pathways for pain referral and parasympathetic pathways for reflexive responses like . This innervation facilitates immune and coordinated function, with lymphatic pathways aligning closely to vascular structures for efficient fluid and cellular transport.

Embryonic Development

Origin and Formation

The large intestine primarily derives from the , which forms the distal third of the , , , , and superior portion of the . The proximal portions, including the , appendix, , and proximal two-thirds of the , originate from the . These endodermal contributions establish the foundational epithelial lining during early gut tube formation around week 4 of embryogenesis. Key formative events occur between weeks 5 and 10. At approximately week 6, the cecal diverticulum emerges as an outgrowth from the caudal limb of the loop, marking the initial development of the . By week 8, the appendix forms as a further outgrowth from the . Concurrently, the colon undergoes significant elongation, accompanied by a 270-degree counterclockwise : an initial 90 degrees around the axis during herniation (week 5), followed by an additional 180 degrees as the returns to the by week 10. This positions the in the right lower quadrant and influences the final vascular patterns in the adult colon. Around week 7, the cloaca—a common chamber for the hindgut and urogenital systems—is septated by the descending urorectal septum, dividing it into the anterior urogenital sinus and the posterior anorectal canal, thereby delineating the rectal portion of the large intestine. Mesodermal tissues play a crucial role in supporting these endodermal structures. The splanchnic mesoderm surrounding the gut tube differentiates into the smooth muscular layers (muscularis externa and muscularis mucosae) and the serosa, providing structural integrity and motility potential to the developing large intestine.

Congenital Variations

Congenital variations in the large intestine arise from disruptions in embryonic gut , cell migration, and fixation processes, leading to structural anomalies that can affect function or predispose to complications. represents a primary such variation, occurring when the fails to complete its normal 270-degree counterclockwise around the during weeks 5-10 of gestation, resulting in abnormal positioning of the and . This incomplete rotation often leaves the cecum in the upper or left side, with a left-sided cecum observed in approximately 12% of malrotation cases. The overall incidence of malrotation is estimated at 0.2% to 1% of live births, though many cases remain until adulthood. Hirschsprung's disease constitutes another significant congenital anomaly, defined by segmental aganglionosis in the distal large intestine due to arrested migration, proliferation, or differentiation of neural crest-derived enteric neurons. This failure typically halts at the , leaving the and without parasympathetic innervation and resulting in tonic contraction and functional obstruction. The condition affects the submucosal and myenteric plexuses and has an incidence of about 1 in 5,000 live births, with a male predominance (4:1 ratio). Short-segment disease, involving only the and sigmoid, accounts for 80% of cases, while longer segments extending into the occur less frequently. The vermiform appendix, as a cecal outgrowth, exhibits congenital positional and structural variations tied to broader gut maldevelopment. In , a mirror-image reversal of abdominal viscera places the appendix in the left , mirroring normal right-sided anatomy; this condition arises from ciliary dysfunction or genetic factors disrupting left-right asymmetry during embryogenesis and occurs in roughly 1 in 10,000 individuals. Other rare positional anomalies include intrahepatic appendix placement, where the organ herniates into the liver due to rotational errors, documented in isolated case reports as an extreme malrotation variant. Length variations also occur congenitally, ranging from absent or rudimentary forms ( in <0.1% of cases) to elongated structures over 15 cm, often linked to incomplete cecal fixation. Meckel's diverticulum, a persistent remnant of the vitelline duct from midgut development, manifests as a true diverticulum on the antimesenteric border of the distal , approximately 60-80 cm proximal to the , and indirectly relates to large intestine anomalies through potential ileocecal involvement. It contains all bowel wall layers and has a prevalence of about 2% in the general population, with higher detection in series (up to 4%). This outpouching, present in a 2:1 male-to-female ratio, stems from incomplete obliteration of the omphalomesenteric duct by week 8 of .

Physiology

Absorption Mechanisms

The large intestine plays a critical role in reabsorbing and electrolytes from the ileal , which enters at approximately 1.5–2 liters per day, reclaiming about 90% of this volume to form solid . This process primarily occurs through standing , where active transport of sodium by enterocytes in the colonic generates a hypertonic . Sodium is absorbed apically via epithelial sodium channels (ENaC) and sodium-hydrogen exchangers (NHE3), while the basolateral Na⁺/K⁺- pump extrudes sodium in exchange for , maintaining the necessary for continued uptake. This solute-driven mechanism creates a local osmotic that passively draws across the , typically against a transmucosal , ensuring efficient of luminal contents. Electrolyte handling in the large intestine involves coordinated active and exchange mechanisms to support the osmotic flow. The Na⁺/K⁺-ATPase remains central, powering secondary of via apical Cl⁻/HCO₃⁻ exchangers (such as SLC26A3, also known as DRA), which facilitate electroneutral NaCl absorption by coupling with Na⁺/H⁺ exchange. Short-chain fatty acids (SCFAs), produced by bacterial of undigested carbohydrates, are absorbed primarily through nonionic diffusion of their protonated forms or via monocarboxylate transporters (MCT1) and sodium-coupled monocarboxylate transporters (SMCT1), contributing to additional sodium and uptake while providing to colonocytes. Additionally, colonic synthesize (as menaquinones) and certain B vitamins (such as and ), which are absorbed through passive diffusion across the , supplementing host nutrition, though B12 absorption is limited (approximately 7% ). Motility patterns in the large intestine enhance these absorption processes by optimizing contact time between luminal contents and the mucosa. Haustral contractions, occurring every 15–30 minutes, involve segmental mixing and slow propulsion within the haustra, promoting thorough exposure of to absorptive surfaces. Complementing this, mass movements—powerful peristaltic waves—occur 3–4 times daily, typically after meals, to consolidate and advance residue toward the while allowing sufficient residence time for upstream. These coordinated motions ensure maximal efficiency without rapid transit that could impair recovery.

Microbiota Interactions

The large intestine harbors a diverse microbial community, collectively known as the , estimated to comprise approximately 10^{14} bacterial cells, predominantly from the phyla Firmicutes and Bacteroidetes. This microbial ecosystem is densest in the proximal regions, such as the , where bacterial concentrations reach 10^{11} to 10^{12} colony-forming units per gram of content, forming structured biofilms that adhere to the mucosal surface without invading the under healthy conditions. These biofilms contribute to the stability of the microbial community, facilitating symbiotic interactions that support host . A primary metabolic role of the large intestinal microbiota involves the fermentation of undigested carbohydrates and dietary fibers that escape small intestinal digestion, producing short-chain fatty acids (SCFAs) such as acetate, propionate, and butyrate. These SCFAs serve as an energy source for the host, accounting for approximately 10% of daily caloric requirements through absorption by colonocytes and subsequent utilization in hepatic metabolism. Butyrate, in particular, fuels colonocyte proliferation and barrier integrity, while acetate and propionate influence systemic metabolism, underscoring the microbiota's contribution to host energy homeostasis. The microbiota also modulates mucosal immunity, promoting tolerance to commensal bacteria through mechanisms including secretory (IgA) production and expansion of regulatory T (T-reg) cells. IgA coats luminal bacteria to prevent epithelial , while T-reg cells suppress excessive , maintaining immune homeostasis. Disruptions in this balance, such as overgrowth of pathogens like Clostridium difficile, can arise from reduced microbial diversity, highlighting the microbiota's role in pathogen resistance. Dysbiosis, characterized by diminished microbial diversity and altered composition, is linked to various health impairments and can be induced by antibiotics, which deplete beneficial taxa and promote dominance for weeks to months. Such perturbations compromise SCFA production and immune regulation, emphasizing the need for resilience to sustain symbiotic benefits.

Clinical Aspects

Major Diseases

The large intestine is susceptible to several major pathological conditions, including inflammatory, neoplastic, and functional disorders, which can significantly impact and require long-term management. These diseases often arise from a combination of genetic, environmental, and factors, with disruptions in the contributing to in some cases, such as observed in inflammatory conditions. Inflammatory bowel disease (IBD) encompasses two primary idiopathic conditions: (UC) and (CD), both characterized by chronic inflammation but differing in extent and depth. UC involves superficial mucosal inflammation that is continuous and typically starts in the , extending proximally to varying degrees of the colon, leading to symptoms like bloody , , and urgency. In contrast, CD features transmural inflammation with discontinuous skip lesions that can affect any segment of the , including the , often resulting in complications such as fistulas, strictures, and abscesses. The prevalence of UC is approximately 1 in 198 persons in high-incidence regions like , while CD affects about 1 in 310, with both showing rising global rates. Colorectal cancer, the third most common cancer worldwide with around 1.93 million new cases in 2022, predominantly arises from the adenoma-carcinoma sequence, where over 90% of sporadic cases progress from benign adenomatous polyps through sequential genetic alterations. Incidence rates are increasing among adults under 50 years old, accounting for approximately 10-20% of cases as of 2025. Key risk factors include advancing age (most cases occur after 50 years), diets high in red and processed meats, and genetic predispositions such as mutations in the gene, which underlie and initiate polyp formation. Staging relies on systems like the older Dukes classification, which assesses tumor depth, nodal involvement, and (A: mucosa/; B: muscularis; C: nodes; D: distant), or the current AJCC TNM system, categorizing from stage 0 () to IV (metastatic). Diverticular disease involves the formation of pouch-like diverticula, most commonly in the due to high intraluminal pressure, affecting up to 50% of individuals over 60 years old in Western populations. While often asymptomatic (), complications arise in 10-25% of cases as acute , characterized by and microperforation, potentially leading to formation, , or . Risk factors include low-fiber diets and , with abscesses occurring in about 17% of hospitalized diverticulitis patients. Irritable bowel syndrome (IBS) is a common affecting 10-15% of adults, defined by recurrent associated with altered bowel habits in the absence of structural or biochemical abnormalities. It stems from disordered gut and brain-gut axis dysfunction, involving heightened visceral sensitivity and altered serotonin signaling, without evidence of or organic changes on or . Subtypes include constipation-predominant IBS (IBS-C), marked by infrequent, hard stools; diarrhea-predominant IBS (IBS-D), with loose, frequent stools; and mixed IBS (IBS-M), alternating between the two.

Diagnostic and Therapeutic Procedures

Diagnostic procedures for disorders of the large intestine primarily involve endoscopic and techniques to visualize the mucosal surface, detect abnormalities such as polyps or strictures, and facilitate or intervention. is the gold standard for direct examination, allowing full visualization of the large intestine from the to the using a flexible equipped with a camera, , and channels for instruments. This procedure enables the identification of inflammatory conditions, polyps, and neoplasms, with capabilities for real-time collection and polyp removal via polypectomy, which can be therapeutic during screening. Routine screening is recommended starting at age 45 for average-risk individuals, as it has been associated with a reduction in mortality by approximately 60%. Imaging modalities complement endoscopy when full visualization is not feasible due to patient factors or anatomical limitations. Computed tomography (CT) colonography, also known as , provides noninvasive, three-dimensional views of the colon after bowel preparation and insufflation with air or , offering high sensitivity (around 90%) for detecting polyps larger than 10 mm. (MRI) colonography similarly generates virtual images without , though it is less commonly used due to longer scan times and higher costs, but it is valuable for patients requiring repeated imaging. Barium , a traditional radiographic technique involving contrast instillation into the , is particularly useful for evaluating colonic strictures by outlining luminal narrowing and assessing patency, especially in cases where cannot pass obstructions. Therapeutic interventions for large intestine disorders range from pharmacological to surgical resection, tailored to the underlying . Surgical options include , the removal of part or all of the colon, which can be segmental (limited to affected areas) or total (entire colon), often performed laparoscopically or via open to address conditions like cancer or severe . Hemicolectomy, a subtype involving removal of the right or left half of the colon along with associated nodes, is a standard procedure for localized to achieve curative intent. In cases requiring fecal diversion, creates an opening in the to which a segment of the colon is brought out, allowing waste elimination into an external pouch when primary is not possible. Pharmacotherapy targets symptom relief and disease modification in inflammatory and functional disorders. For (IBD) affecting the large intestine, such as , 5-aminosalicylic acid (5-ASA) compounds like mesalamine serve as first-line therapy for mild to moderate cases, reducing inflammation through topical and systemic effects on the colonic mucosa. Biologic agents, particularly anti-tumor necrosis factor (anti-TNF) therapies such as and , are employed for moderate to severe IBD refractory to conventional treatments, inhibiting inflammatory cytokines to induce and maintain remission. In (IBS), laxatives like address constipation-predominant symptoms by softening stool and promoting bowel movements, while antispasmodics such as dicyclomine alleviate and cramping by relaxing intestinal smooth muscle.

Comparative Anatomy

In Non-Human Mammals

In non-human mammals, the large intestine exhibits significant variations in and function, primarily driven by dietary adaptations that influence microbial , water absorption, and transit time. Herbivores, which rely on high-fiber plant material, typically feature an elongated and colon to facilitate , while carnivores possess a short, simple structure suited for rapid processing of protein-rich diets. Omnivores display intermediate forms that balance and quick transit, and specialized cases further highlight evolutionary tweaks for unique ecological niches. Among herbivorous mammals, fermenters like horses have a prominently enlarged large intestine adapted for microbial breakdown of fibrous . The horse's measures approximately 1 meter in length with a capacity of 30-34 liters, comprising approximately 60% of the total gastrointestinal volume in a 500 kg adult, alongside the large colon (3-3.7 meters, 50-60 liters) and small colon (3 meters, 18-19 liters). This structure hosts symbiotic microbes that non-starch into volatile fatty acids, providing up to 70% of the horse's energy needs from . In contrast, foregut-fermenting ruminants such as cows exhibit a reduced large intestine due to the rumen's dominant role in initial ; their is about 3 feet long with a 2-gallon (roughly 7.6-liter) capacity, serving mainly for secondary absorption rather than primary microbial action. Carnivorous mammals, exemplified by dogs, possess a short and relatively simple colon optimized for swift passage of easily digestible . The dog's ascending spans about 5 cm from the ileocolic , with the descending colon as the longest segment but overall comprising a brief portion of the gut; it features haustral sacculations for water and reabsorption while minimizing retention time to prevent of protein residues. This design supports , typically completing in 12-30 hours, aligning with a diet low in fiber and high in nutrients absorbed earlier in the . Omnivorous species like pigs demonstrate an intermediate large intestine with adaptations for both fermentation and efficient mixing. The pig's colon forms a distinctive spiral coil, commencing from the and terminating in the , which enhances surface area for water absorption and microbial processing of mixed plant-animal diets, including breakdown into usable energy sources. This coiled structure promotes thorough mixing of digesta, allowing for omnivorous flexibility without the extremes of elongation or brevity. Specialized adaptations appear in mammals with niche diets, such as the , a eucalyptus , whose is greatly enlarged—boasting the highest cecum-to-body-size ratio among mammals—to house microbes that detoxify and ferment the leaves' and lignocellulose over an extended retention time of about 8 days. Similarly, cetaceans like the have a shortened, undifferentiated large intestine lacking a distinct , forming a uniform tube that minimizes gas-producing ; this fused, compact structure (with an elongated compensating for whole-prey digestion) suits their aquatic, high-protein and diet by reducing buoyancy risks from undigested residue.

Evolutionary Adaptations

The large intestine, or hindgut, in early vertebrates such as fish and amphibians, consists of a simple tubular structure serving primarily for water reabsorption and waste concentration, with minimal compartmentalization. Fossil evidence from Triassic actinopterygian fishes like Saurichthys reveals a gastrointestinal tract with a spiral-shaped intestine and a short posterior intestine, underscoring the primitive, undifferentiated nature of this region before the diversification of higher vertebrates. In reptiles and birds, the hindgut evolves greater complexity, incorporating distinct compartments such as the cecum for limited fermentation, often integrated with the cloaca as a multifunctional chamber for digestive, urinary, and reproductive outputs. This compartmentalization supports microbial activity in species with vegetarian diets, marking an adaptive shift toward enhanced nutrient extraction in terrestrial environments. Following the Cretaceous-Paleogene extinction event, mammalian radiation saw significant diversification tied to dietary shifts. Herbivorous mammals developed enlarged , including expanded ceca and colons, to facilitate microbial breakdown of fibrous material, a response to the post-Cretaceous proliferation of angiosperms that provided more abundant, digestible foliage. In contrast, carnivorous mammals exhibit reduced hindgut sizes for rapid transit and efficient processing of protein-rich diets, minimizing needs. These adaptations reflect phylogenetic and ecological pressures, with hindgut morphology correlating to trophic levels across mammalian orders. In humans, the large intestine is notably shortened compared to that of great apes, comprising only about 20% of the total gastrointestinal volume versus 50% in apes, linked to the evolutionary adoption of cooked foods around 2 million years ago. This reduction, part of a broader 60% decrease in gut size relative to body mass, freed metabolic resources for encephalization, as cooking enhanced caloric yield and reduced the demands of . Genetic evidence indicates positive selection in genes related to immune and digestive adaptation to processed diets, predating modern Homo sapiens. Key evolutionary adaptations in the mammalian large intestine include haustra, the sacculations formed by the taeniae coli that segment the colon to slow digesta transit and optimize and absorption. The vermiform appendix, a vestigial , has convergently evolved multiple times and functions as a lymphoid reservoir, harboring beneficial and supporting immune maturation, particularly in early life. These features underscore the hindgut's role in balancing hydration, , and immunity amid dietary and environmental changes.

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