what structure controls the release of material from the stomach to the small intestine

Learning Objectives

By the cease of this section, yous will be able to:

  • Compare and contrast the location and gross anatomy of the small and big intestines
  • Place three primary adaptations of the minor intestine wall that increase its absorptive capacity
  • Describe the mechanical and chemic digestion of chyme upon its release into the small intestine
  • List three features unique to the wall of the large intestine and identify their contributions to its function
  • Place the beneficial roles of the bacterial flora in digestive system functioning
  • Trace the pathway of nutrient waste from its signal of entry into the large intestine through its exit from the body as carrion

The word intestine is derived from a Latin root significant "internal," and indeed, the two organs together most fill the interior of the abdominal cavity. In addition, called the small and large bowel, or colloquially the "guts," they constitute the greatest mass and length of the comestible canal and, with the exception of ingestion, perform all digestive arrangement functions.

The Modest Intestine

Chyme released from the tummy enters the pocket-sized intestine, which is the principal digestive organ in the body. Not simply is this where virtually digestion occurs, it is also where practically all absorption occurs. The longest part of the alimentary canal, the modest intestine is about 3.05 meters (x feet) long in a living person (merely about twice as long in a cadaver due to the loss of musculus tone). Since this makes it near five times longer than the large intestine, yous might wonder why it is called "pocket-size." In fact, its name derives from its relatively smaller bore of only nigh 2.54 cm (1 in), compared with vii.62 cm (3 in) for the big intestine. Every bit nosotros'll see shortly, in addition to its length, the folds and projections of the lining of the small intestine work to give it an enormous surface area, which is approximately 200 m2, more than 100 times the surface area of your skin. This large surface area is necessary for complex processes of digestion and absorption that occur within it.

Structure

The coiled tube of the pocket-size intestine is subdivided into three regions. From proximal (at the tum) to distal, these are the duodenum, jejunum, and ileum.

The shortest region is the 25.4-cm (ten-in) duodenum, which begins at the pyloric sphincter. Just by the pyloric sphincter, it bends posteriorly backside the peritoneum, becoming retroperitoneal, and then makes a C-shaped curve around the head of the pancreas earlier ascending anteriorly again to return to the peritoneal crenel and join the jejunum. The duodenum can therefore be subdivided into four segments: the superior, descending, horizontal, and ascending duodenum.

Of item interest is the hepatopancreatic ampulla (ampulla of Vater). Located in the duodenal wall, the ampulla marks the transition from the inductive portion of the alimentary canal to the mid-region, and is where the bile duct (through which bile passes from the liver) and the chief pancreatic duct (through which pancreatic juice passes from the pancreas) bring together. This ampulla opens into the duodenum at a tiny volcano-shaped construction called the major duodenal papilla. The hepatopancreatic sphincter (sphincter of Oddi) regulates the flow of both bile and pancreatic juice from the ampulla into the duodenum.

This diagram shows the small intestine. The different parts of the small intestine are labeled.

Effigy 1. The iii regions of the minor intestine are the duodenum, jejunum, and ileum.

The jejunum is most 0.9 meters (iii feet) long (in life) and runs from the duodenum to the ileum. Jejunum means "empty" in Latin and supposedly was so named by the ancient Greeks who noticed it was e'er empty at death. No clear demarcation exists between the jejunum and the concluding segment of the small intestine, the ileum.

The ileum is the longest part of the small intestine, measuring most 1.viii meters (half-dozen feet) in length. It is thicker, more vascular, and has more developed mucosal folds than the jejunum. The ileum joins the cecum, the beginning portion of the large intestine, at the ileocecal sphincter (or valve). The jejunum and ileum are tethered to the posterior intestinal wall by the mesentery. The large intestine frames these three parts of the minor intestine.

Parasympathetic nerve fibers from the vagus nerve and sympathetic nerve fibers from the thoracic splanchnic nerve provide extrinsic innervation to the small intestine. The superior mesenteric artery is its principal arterial supply. Veins run parallel to the arteries and bleed into the superior mesenteric vein. Nutrient-rich blood from the small-scale intestine is then carried to the liver via the hepatic portal vein.

Histology

The wall of the small intestine is composed of the same iv layers typically present in the alimentary arrangement. However, three features of the mucosa and submucosa are unique. These features, which increase the absorptive surface area of the small intestine more than 600-fold, include circular folds, villi, and microvilli. These adaptations are most arable in the proximal two-thirds of the small intestine, where the majority of absorption occurs.

Illustration (a) shows the histological cross-section of the small intestine. The left panel shows a small region of the small intestine, along with the blood vessels and the muscle layers. The middle panel shows a magnified view of a small region of the small intestine, highlighting the absorptive cells, the lacteal and the goblet cells. The right panel shows a further magnified view of the epithelial cells including the microvilli. Illustrations (b) shows a micrograph of the circular folds, and illustration (c) shows a micrograph of the villi. Illustration (d) shows an electron micrograph of the microvilli.

Figure ii. (a) The absorptive surface of the small intestine is vastly enlarged past the presence of round folds, villi, and microvilli. (b) Micrograph of the circular folds. (c) Micrograph of the villi. (d) Electron micrograph of the microvilli. From left to right, LM x 56, LM x 508, EM ten 196,000. (credit b-d: Micrograph provided past the Regents of University of Michigan Medical School © 2012)

Circular folds

Also called a plica circulare, a circular fold is a deep ridge in the mucosa and submucosa. Commencement near the proximal role of the duodenum and ending near the heart of the ileum, these folds facilitate assimilation. Their shape causes the chyme to spiral, rather than motion in a straight line, through the small-scale intestine. Spiraling slows the motion of chyme and provides the time needed for nutrients to be fully absorbed.

Villi

Within the circular folds are small (0.5–one mm long) hairlike vascularized projections chosen villi (singular = villus) that give the mucosa a furry texture. In that location are most 20 to twoscore villi per square millimeter, increasing the surface surface area of the epithelium tremendously. The mucosal epithelium, primarily composed of absorbent cells, covers the villi. In addition to muscle and connective tissue to support its structure, each villus contains a capillary bed composed of ane arteriole and one venule, equally well as a lymphatic capillary called a lacteal. The breakdown products of carbohydrates and proteins (sugars and amino acids) can enter the bloodstream straight, just lipid breakdown products are absorbed by the lacteals and transported to the bloodstream via the lymphatic system.

Microvilli

As their name suggests, microvilli (atypical = microvillus) are much smaller (i µyard) than villi. They are cylindrical upmost surface extensions of the plasma membrane of the mucosa's epithelial cells, and are supported by microfilaments inside those cells. Although their pocket-sized size makes it hard to see each microvillus, their combined microscopic appearance suggests a mass of bristles, which is termed the brush border. Fixed to the surface of the microvilli membranes are enzymes that finish digesting carbohydrates and proteins. There are an estimated 200 one thousand thousand microvilli per square millimeter of small intestine, greatly expanding the surface area of the plasma membrane and thus greatly enhancing assimilation.

Abdominal Glands

In addition to the 3 specialized absorptive features merely discussed, the mucosa between the villi is dotted with deep crevices that each lead into a tubular intestinal gland (crypt of Lieberkühn), which is formed by cells that line the crevices. These produce intestinal juice, a slightly alkali metal (pH 7.4 to 7.eight) mixture of water and mucus. Each 24-hour interval, most 0.95 to 1.ix liters (i to 2 quarts) are secreted in response to the distention of the small intestine or the irritating effects of chyme on the intestinal mucosa.

The submucosa of the duodenum is the merely site of the complex mucus-secreting duodenal glands (Brunner'due south glands), which produce a bicarbonate-rich alkaline fungus that buffers the acidic chyme as it enters from the tum.

The roles of the cells in the small intestinal mucosa are detailed in Table ane.

Table 1. Cells of the Small Abdominal Mucosa
Jail cell blazon Location in the mucosa Function
Absorptive Epithelium/intestinal glands Digestion and absorption of nutrients in chyme
Goblet Epithelium/intestinal glands Secretion of mucus
Paneth Intestinal glands Secretion of the bactericidal enzyme lysozyme; phagocytosis
Chiliad cells Intestinal glands of duodenum Secretion of the hormone intestinal gastrin
I cells Intestinal glands of duodenum Secretion of the hormone cholecystokinin, which stimulates release of pancreatic juices and bile
Yard cells Intestinal glands Secretion of the hormone glucose-dependent insulinotropic peptide, which stimulates the release of insulin
K cells Intestinal glands of duodenum and jejunum Secretion of the hormone motilin, which accelerates gastric emptying, stimulates abdominal peristalsis, and stimulates the product of pepsin
Due south cells Intestinal glands Secretion of the hormone secretin

Intestinal MALT

The lamina propria of the small intestine mucosa is studded with quite a bit of MALT. In improver to solitary lymphatic nodules, aggregations of intestinal MALT, which are typically referred to equally Peyer's patches, are full-bodied in the distal ileum, and serve to proceed bacteria from entering the bloodstream. Peyer's patches are nearly prominent in young people and go less distinct as you age, which coincides with the general action of our allowed organization.

Watch this animation that depicts the construction of the small intestine, and, in particular, the villi. Epithelial cells continue the digestion and absorption of nutrients and ship these nutrients to the lymphatic and circulatory systems. In the small intestine, the products of food digestion are absorbed by different structures in the villi. Which structure absorbs and transports fats?

Mechanical Digestion in the Small Intestine

The motility of abdominal smooth muscles includes both sectionalization and a form of peristalsis called migrating move complexes. The kind of peristaltic mixing waves seen in the tum are non observed hither.

This diagram shows the process of segmentation in the intestines. The left panel shows the separation of chime, the middle panel shows the remixing of the chime by pushing it back together and the right panel indicates that the chime is being digested and absorbed.

Figure iii. Segmentation separates chyme and then pushes it back together, mixing information technology and providing fourth dimension for digestion and assimilation.

If yous could see into the small intestine when information technology was going through partition, it would look every bit if the contents were being shoved incrementally back and forth, as the rings of shine musculus repeatedly contract and then relax. Segmentation in the minor intestine does not force chyme through the tract. Instead, it combines the chyme with digestive juices and pushes nutrient particles confronting the mucosa to be absorbed. The duodenum is where the well-nigh rapid segmentation occurs, at a rate of most 12 times per minute. In the ileum, segmentations are only almost eight times per minute.

When most of the chyme has been absorbed, the pocket-sized abdominal wall becomes less distended. At this point, the localized sectionalisation process is replaced by transport movements. The duodenal mucosa secretes the hormone motilin, which initiates peristalsis in the form of a migrating movement complex. These complexes, which begin in the duodenum, forcefulness chyme through a curt department of the small intestine and then end. The side by side contraction begins a trivial chip farther downwardly than the showtime, forces chyme a bit further through the pocket-size intestine, then stops. These complexes move slowly down the modest intestine, forcing chyme on the style, taking effectually 90 to 120 minutes to finally reach the end of the ileum. At this bespeak, the process is repeated, starting in the duodenum.

The ileocecal valve, a sphincter, is usually in a constricted state, simply when motility in the ileum increases, this sphincter relaxes, allowing food residual to enter the first portion of the big intestine, the cecum. Relaxation of the ileocecal sphincter is controlled past both nerves and hormones. Offset, digestive activity in the stomach provokes the gastroileal reflex, which increases the strength of ileal division. 2nd, the stomach releases the hormone gastrin, which enhances ileal move, thus relaxing the ileocecal sphincter. Later on chyme passes through, backward pressure helps close the sphincter, preventing backflow into the ileum. Because of this reflex, your dejeuner is completely emptied from your stomach and minor intestine by the fourth dimension you eat your dinner. It takes about 3 to 5 hours for all chyme to leave the small intestine.

Chemical Digestion in the Small Intestine

The digestion of proteins and carbohydrates, which partially occurs in the tummy, is completed in the small intestine with the aid of intestinal and pancreatic juices. Lipids arrive in the intestine largely undigested, so much of the focus here is on lipid digestion, which is facilitated by bile and the enzyme pancreatic lipase.

Moreover, abdominal juice combines with pancreatic juice to provide a liquid medium that facilitates absorption. The intestine is besides where most water is absorbed, via osmosis. The small intestine's absorptive cells too synthesize digestive enzymes so place them in the plasma membranes of the microvilli. This distinguishes the modest intestine from the stomach; that is, enzymatic digestion occurs not only in the lumen, simply also on the luminal surfaces of the mucosal cells.

For optimal chemical digestion, chyme must be delivered from the breadbasket slowly and in small amounts. This is considering chyme from the stomach is typically hypertonic, and if large quantities were forced all at in one case into the pocket-size intestine, the resulting osmotic h2o loss from the blood into the intestinal lumen would upshot in potentially life-threatening low blood volume. In improver, connected digestion requires an upward adjustment of the depression pH of breadbasket chyme, along with rigorous mixing of the chyme with bile and pancreatic juices. Both processes take time, so the pumping action of the pylorus must be carefully controlled to prevent the duodenum from being overwhelmed with chyme.

Disorders of the Small Intestine: Lactose Intolerance

Lactose intolerance is a condition characterized by indigestion acquired by dairy products. It occurs when the absorptive cells of the pocket-size intestine do non produce plenty lactase, the enzyme that digests the milk sugar lactose. In most mammals, lactose intolerance increases with historic period. In contrast, some human populations, most notably Caucasians, are able to maintain the ability to produce lactase equally adults.

In people with lactose intolerance, the lactose in chyme is not digested. Leaner in the large intestine ferment the undigested lactose, a process that produces gas. In addition to gas, symptoms include abdominal cramps, bloating, and diarrhea. Symptom severity ranges from mild discomfort to severe pain; however, symptoms resolve once the lactose is eliminated in feces.

The hydrogen breath test is used to aid diagnose lactose intolerance. Lactose-tolerant people have very little hydrogen in their jiff. Those with lactose intolerance exhale hydrogen, which is one of the gases produced by the bacterial fermentation of lactose in the colon. After the hydrogen is absorbed from the intestine, it is transported through blood vessels into the lungs. There are a number of lactose-free dairy products available in grocery stores. In addition, dietary supplements are available. Taken with food, they provide lactase to assist assimilate lactose.

The Big Intestine

The big intestine is the last part of the alimentary canal. The primary function of this organ is to finish assimilation of nutrients and water, synthesize sure vitamins, form feces, and eliminate carrion from the body.

Structure

The large intestine runs from the appendix to the anus. It frames the small intestine on 3 sides. Despite its being nearly half as long as the pocket-size intestine, it is called large considering it is more twice the bore of the small intestine, most 3 inches.

Subdivisions

The large intestine is subdivided into four main regions: the cecum, the colon, the rectum, and the anus. The ileocecal valve, located at the opening between the ileum and the big intestine, controls the flow of chyme from the small intestine to the big intestine.

Cecum

The first function of the large intestine is the cecum, a sac-like structure that is suspended inferior to the ileocecal valve. It is nearly 6 cm (2.4 in) long, receives the contents of the ileum, and continues the absorption of water and salts. The appendix (or vermiform appendix) is a winding tube that attaches to the cecum. Although the 7.6-cm (3-in) long appendix contains lymphoid tissue, suggesting an immunologic function, this organ is generally considered vestigial. However, at to the lowest degree i recent study postulates a survival advantage conferred by the appendix: In diarrheal illness, the appendix may serve as a bacterial reservoir to repopulate the enteric leaner for those surviving the initial phases of the disease. Moreover, its twisted anatomy provides a oasis for the aggregating and multiplication of enteric bacteria. The mesoappendix, the mesentery of the appendix, tethers it to the mesentery of the ileum.

Colon

This image shows the large intestine; the major parts of the large intestine are labeled.

Figure 4. The big intestine includes the cecum, colon, and rectum.

The cecum blends seamlessly with the colon. Upon entering the colon, the food residue start travels upwards the ascending colon on the correct side of the belly. At the inferior surface of the liver, the colon bends to form the right colic flexure (hepatic flexure) and becomes the transverse colon. The region defined as hindgut begins with the final 3rd of the transverse colon and continues on. Food residuum passing through the transverse colon travels across to the left side of the abdomen, where the colon angles sharply immediately inferior to the spleen, at the left colic flexure (splenic flexure). From there, food residue passes through the descending colon, which runs down the left side of the posterior abdominal wall. Later on entering the pelvis inferiorly, it becomes the south-shaped sigmoid colon, which extends medially to the midline (Figure 4). The ascending and descending colon, and the rectum (discussed next) are located in the retroperitoneum. The transverse and sigmoid colon are tethered to the posterior abdominal wall past the mesocolon.

Homeostatic Imbalances: Colorectal Cancer

Each twelvemonth, approximately 140,000 Americans are diagnosed with colorectal cancer, and another 49,000 die from it, making it one of the almost deadly malignancies. People with a family unit history of colorectal cancer are at increased run a risk. Smoking, excessive alcohol consumption, and a diet loftier in animal fat and poly peptide besides increase the risk. Despite popular opinion to the contrary, studies back up the conclusion that dietary fiber and calcium do not reduce the chance of colorectal cancer.

Colorectal cancer may be signaled by constipation or diarrhea, cramping, abdominal pain, and rectal bleeding. Haemorrhage from the rectum may be either obvious or occult (hidden in feces). Since most colon cancers ascend from benign mucosal growths called polyps, cancer prevention is focused on identifying these polyps. The colonoscopy is both diagnostic and therapeutic. Colonoscopy non only allows identification of precancerous polyps, the process as well enables them to be removed before they go malignant. Screening for fecal occult blood tests and colonoscopy is recommended for those over fifty years of age.

Rectum

Food residue leaving the sigmoid colon enters the rectum in the pelvis, almost the tertiary sacral vertebra. The final 20.three cm (8 in) of the alimentary canal, the rectum extends inductive to the sacrum and coccyx. Even though rectum is Latin for "straight," this structure follows the curved contour of the sacrum and has 3 lateral bends that create a trio of internal transverse folds called the rectal valves. These valves aid carve up the feces from gas to prevent the simultaneous passage of carrion and gas.

Anal Culvert

Finally, food residue reaches the concluding part of the large intestine, the anal canal, which is located in the perineum, completely outside of the abdominopelvic cavity. This 3.viii–5 cm (1.v–2 in) long structure opens to the outside of the trunk at the anus. The anal canal includes ii sphincters. The internal anal sphincter is fabricated of smooth muscle, and its contractions are involuntary. The external anal sphincter is made of skeletal muscle, which is under voluntary control. Except when defecating, both commonly remain closed.

Histology

There are several notable differences between the walls of the large and pocket-sized intestines. For case, few enzyme-secreting cells are establish in the wall of the large intestine, and at that place are no circular folds or villi. Other than in the anal canal, the mucosa of the colon is unproblematic columnar epithelium made mostly of enterocytes (absorptive cells) and goblet cells. In addition, the wall of the large intestine has far more than intestinal glands, which contain a vast population of enterocytes and goblet cells. These goblet cells secrete mucus that eases the motion of carrion and protects the intestine from the effects of the acids and gases produced by enteric bacteria. The enterocytes absorb water and salts likewise as vitamins produced past your intestinal bacteria.

This image shows the histological cross section of the large intestine. The left panel shows a small region of the large intestine. The center panel shows a magnified view of this region, highlighting the openings of the intestinal glands. The right panel shows a further magnified view, with the microvilli and goblet cells.

Effigy 5. (a) The histologies of the large intestine and small intestine (not shown) are adapted for the digestive functions of each organ. (b) This micrograph shows the colon'due south unproblematic columnar epithelium and goblet cells. LM 10 464. (credit b: Micrograph provided by the Regents of University of Michigan Medical Schoolhouse © 2012)

Anatomy

This image shows the Taenia Coli, haustra and epiploic appendages, which are parts of the large intestine.

Effigy 6. Teniae Coli, Haustra, and Epiploic Appendages

Three features are unique to the large intestine: teniae coli, haustra, and epiploic appendages (Effigy 6). The teniae coli are three bands of shine muscle that brand up the longitudinal muscle layer of the muscularis of the large intestine, except at its terminal end. Tonic contractions of the teniae coli bunch up the colon into a succession of pouches called haustra (singular = hostrum), which are responsible for the wrinkled advent of the colon. Fastened to the teniae coli are small, fat-filled sacs of visceral peritoneum called epiploic appendages. The purpose of these is unknown. Although the rectum and anal canal accept neither teniae coli nor haustra, they practise accept well-developed layers of muscularis that create the strong contractions needed for defecation.

The stratified squamous epithelial mucosa of the anal culvert connects to the skin on the outside of the anus. This mucosa varies considerably from that of the residual of the colon to accommodate the high level of abrasion every bit feces pass through. The anal culvert's mucous membrane is organized into longitudinal folds, each called an anal column, which house a filigree of arteries and veins. Two superficial venous plexuses are found in the anal canal: one within the anal columns and 1 at the anus.

Depressions between the anal columns, each called an anal sinus, secrete mucus that facilitates defecation. The pectinate line (or dentate line) is a horizontal, jagged band that runs circumferentially just below the level of the anal sinuses, and represents the junction between the hindgut and external skin. The mucosa above this line is fairly insensitive, whereas the area beneath is very sensitive. The resulting difference in pain threshold is due to the fact that the upper region is innervated by visceral sensory fibers, and the lower region is innervated past somatic sensory fibers.

Bacterial Flora

Nigh bacteria that enter the alimentary canal are killed by lysozyme, defensins, HCl, or protein-digesting enzymes. However, trillions of bacteria live within the large intestine and are referred to equally the bacterial flora. Almost of the more than than 700 species of these bacteria are nonpathogenic commensal organisms that cause no harm as long as they stay in the gut lumen. In fact, many facilitate chemical digestion and absorption, and some synthesize certain vitamins, mainly biotin, pantothenic acid, and vitamin K. Some are linked to increased immune response. A refined system prevents these bacteria from crossing the mucosal bulwark. First, peptidoglycan, a component of bacterial jail cell walls, activates the release of chemicals by the mucosa's epithelial cells, which draft immune cells, peculiarly dendritic cells, into the mucosa. Dendritic cells open up the tight junctions between epithelial cells and extend probes into the lumen to evaluate the microbial antigens. The dendritic cells with antigens then travel to neighboring lymphoid follicles in the mucosa where T cells audit for antigens. This process triggers an IgA-mediated response, if warranted, in the lumen that blocks the commensal organisms from infiltrating the mucosa and setting off a far greater, widespread systematic reaction.

Digestive Functions of the Big Intestine

The residue of chyme that enters the large intestine contains few nutrients except h2o, which is reabsorbed as the balance lingers in the large intestine, typically for 12 to 24 hours. Thus, it may non surprise yous that the large intestine tin can be completely removed without significantly affecting digestive operation. For example, in severe cases of inflammatory bowel disease, the large intestine can be removed past a procedure known every bit a colectomy. Often, a new fecal pouch can be crafted from the small intestine and sutured to the anus, but if not, an ileostomy tin be created by bringing the distal ileum through the abdominal wall, allowing the watery chyme to be collected in a bag-like agglutinative appliance.

Mechanical Digestion

In the big intestine, mechanical digestion begins when chyme moves from the ileum into the cecum, an activeness regulated by the ileocecal sphincter. Correct after you eat, peristalsis in the ileum forces chyme into the cecum. When the cecum is distended with chyme, contractions of the ileocecal sphincter strengthen. Once chyme enters the cecum, colon movements brainstorm.

Mechanical digestion in the big intestine includes a combination of three types of movements. The presence of food residues in the colon stimulates a ho-hum-moving haustral contraction. This type of movement involves sluggish sectionalization, primarily in the transverse and descending colons. When a haustrum is distended with chyme, its muscle contracts, pushing the remainder into the side by side haustrum. These contractions occur about every 30 minutes, and each terminal about i minute. These movements too mix the food residue, which helps the large intestine absorb water. The 2nd type of movement is peristalsis, which, in the large intestine, is slower than in the more proximal portions of the alimentary canal. The tertiary blazon is a mass movement. These strong waves beginning midway through the transverse colon and chop-chop force the contents toward the rectum. Mass movements commonly occur three or four times per day, either while you eat or immediately later on. Distension in the tummy and the breakup products of digestion in the small-scale intestine provoke the gastrocolic reflex, which increases movement, including mass movements, in the colon. Fiber in the nutrition both softens the stool and increases the power of colonic contractions, optimizing the activities of the colon.

Chemical Digestion

Although the glands of the large intestine secrete mucus, they do not secrete digestive enzymes. Therefore, chemical digestion in the large intestine occurs exclusively because of bacteria in the lumen of the colon. Through the process of saccharolytic fermentation, leaner suspension downwards some of the remaining carbohydrates. This results in the discharge of hydrogen, carbon dioxide, and methane gases that create flatus (gas) in the colon; flatulence is excessive flatus. Each day, upwards to 1500 mL of flatus is produced in the colon. More is produced when you consume foods such as beans, which are rich in otherwise indigestible sugars and circuitous carbohydrates like soluble dietary fiber.

Absorption, Feces Germination, and Defecation

The small intestine absorbs almost 90 percentage of the water you lot ingest (either equally liquid or within solid food). The large intestine absorbs near of the remaining water, a process that converts the liquid chyme residue into semisolid feces ("stool"). Carrion is composed of undigested food residues, unabsorbed digested substances, millions of bacteria, old epithelial cells from the GI mucosa, inorganic salts, and enough water to permit information technology pass smoothly out of the body. Of every 500 mL (17 ounces) of food rest that enters the cecum each day, about 150 mL (v ounces) become feces.

Feces are eliminated through contractions of the rectal muscles. Y'all assistance this process past a voluntary procedure chosen Valsalva's maneuver, in which you increase intra-abdominal pressure by contracting your diaphragm and abdominal wall muscles, and closing your glottis.

The procedure of defecation begins when mass movements force carrion from the colon into the rectum, stretching the rectal wall and provoking the defecation reflex, which eliminates feces from the rectum. This parasympathetic reflex is mediated past the spinal cord. Information technology contracts the sigmoid colon and rectum, relaxes the internal anal sphincter, and initially contracts the external anal sphincter. The presence of feces in the anal canal sends a signal to the encephalon, which gives you the choice of voluntarily opening the external anal sphincter (defecating) or keeping it temporarily airtight. If you decide to delay defecation, it takes a few seconds for the reflex contractions to finish and the rectal walls to relax. The next mass movement will trigger additional defecation reflexes until you defecate.

If defecation is delayed for an extended fourth dimension, boosted h2o is absorbed, making the feces firmer and potentially leading to constipation. On the other hand, if the waste affair moves also quickly through the intestines, not enough water is absorbed, and diarrhea can result. This can exist caused past the ingestion of foodborne pathogens. In full general, diet, health, and stress determine the frequency of bowel movements. The number of bowel movements varies greatly between individuals, ranging from two or three per day to iii or four per week.

By watching this animation you volition see that for the various food groups—proteins, fats, and carbohydrates—digestion begins in unlike parts of the digestion organisation, though all end in the same place. Of the three major food classes (carbohydrates, fats, and proteins), which is digested in the oral cavity, the stomach, and the small intestine?

Affiliate Review

The three main regions of the small intestine are the duodenum, the jejunum, and the ileum. The small intestine is where digestion is completed and virtually all absorption occurs. These two activities are facilitated by structural adaptations that increase the mucosal surface area by 600-fold, including round folds, villi, and microvilli. There are effectually 200 1000000 microvilli per square millimeter of small intestine, which contain brush border enzymes that consummate the digestion of carbohydrates and proteins. Combined with pancreatic juice, intestinal juice provides the liquid medium needed to further digest and absorb substances from chyme. The pocket-size intestine is also the site of unique mechanical digestive movements. Segmentation moves the chyme dorsum and forth, increasing mixing and opportunities for absorption. Migrating motion complexes propel the residual chyme toward the large intestine.

The primary regions of the big intestine are the cecum, the colon, and the rectum. The big intestine absorbs water and forms feces, and is responsible for defecation. Bacterial flora break downward additional carbohydrate residue, and synthesize certain vitamins. The mucosa of the large intestinal wall is generously endowed with goblet cells, which secrete mucus that eases the passage of feces. The entry of feces into the rectum activates the defecation reflex.

Cocky Check

Answer the question(due south) below to encounter how well y'all understand the topics covered in the previous section.

Critical Thinking Questions

  1. Explain how nutrients absorbed in the small intestine laissez passer into the full general apportionment.
  2. Why is it important that chyme from the stomach is delivered to the small intestine slowly and in small amounts?
  3. Describe three of the differences between the walls of the large and modest intestines.

Glossary

anal culvert: last segment of the big intestine

anal column: long fold of mucosa in the anal canal

anal sinus betwixt anal columns

appendix: (vermiform appendix) coiled tube attached to the cecum

ascending colon: first region of the colon

bacterial flora: bacteria in the large intestine

brush border: fuzzy appearance of the small intestinal mucosa created by microvilli

cecum: pouch forming the beginning of the large intestine

circular fold: (also, plica circulare) deep fold in the mucosa and submucosa of the small-scale intestine

colon: part of the big intestine between the cecum and the rectum

descending colon: part of the colon betwixt the transverse colon and the sigmoid colon

duodenal gland: (also, Brunner's gland) mucous-secreting gland in the duodenal submucosa

duodenum: first part of the minor intestine, which starts at the pyloric sphincter and ends at the jejunum

epiploic appendage: small sac of fat-filled visceral peritoneum fastened to teniae coli

external anal sphincter: voluntary skeletal muscle sphincter in the anal canal

feces: semisolid waste product of digestion

flatus: gas in the intestine

gastrocolic reflex: propulsive movement in the colon activated past the presence of nutrient in the tummy

gastroileal reflex: long reflex that increases the strength of division in the ileum

haustrum: small pouch in the colon created by tonic contractions of teniae coli

haustral wrinkle: tedious segmentation in the large intestine

hepatopancreatic ampulla: (also, ampulla of Vater) bulb-like point in the wall of the duodenum where the bile duct and master pancreatic duct unite

hepatopancreatic sphincter: (also, sphincter of Oddi) sphincter regulating the menstruation of bile and pancreatic juice into the duodenum

ileocecal sphincter: sphincter located where the small intestine joins with the large intestine

ileum: end of the pocket-sized intestine between the jejunum and the large intestine

internal anal sphincter: involuntary smoothen muscle sphincter in the anal canal

intestinal gland: (also, crypt of Lieberkühn) gland in the small abdominal mucosa that secretes intestinal juice

intestinal juice: mixture of water and mucus that helps absorb nutrients from chyme

jejunum: middle office of the pocket-size intestine between the duodenum and the ileum

lacteal: lymphatic capillary in the villi

large intestine: last portion of the alimentary culvert

left colic flexure: (likewise, splenic flexure) point where the transverse colon curves below the inferior end of the spleen

principal pancreatic duct: (also, duct of Wirsung) duct through which pancreatic juice drains from the pancreas

major duodenal papilla: point at which the hepatopancreatic ampulla opens into the duodenum

mass movement: long, slow, peristaltic moving ridge in the large intestine

mesoappendix: mesentery of the appendix

microvillus: minor projection of the plasma membrane of the absorbent cells of the modest abdominal mucosa

migrating motility circuitous: form of peristalsis in the small intestine

motilin: hormone that initiates migrating motility complexes

pectinate line: horizontal line that runs like a ring, perpendicular to the inferior margins of the anal sinuses

rectal valve: i of three transverse folds in the rectum where feces is separated from flatus

rectum: role of the big intestine betwixt the sigmoid colon and anal canal

right colic flexure: (also, hepatic flexure) point, at the inferior surface of the liver, where the ascending colon turns abruptly to the left

saccharolytic fermentation: anaerobic decomposition of carbohydrates

sigmoid colon: end portion of the colon, which terminates at the rectum

pocket-size intestine: section of the alimentary canal where most digestion and absorption occurs

tenia coli: ane of iii polish musculus bands that brand upward the longitudinal musculus layer of the muscularis in all of the big intestine except the last cease

transverse colon: part of the colon between the ascending colon and the descending colon

Valsalva's maneuver: voluntary contraction of the diaphragm and abdominal wall muscles and closing of the glottis, which increases intra-intestinal pressure and facilitates defecation

villus: projection of the mucosa of the small intestine

References

American Cancer Society (US). Cancer facts and figures: colorectal cancer: 2011–2013 [Net]. c2013 [cited 2013 Apr 3]. Available from: http://www.cancer.org/Inquiry/CancerFactsFigures/ColorectalCancerFactsFigures/colorectal-cancer-facts-figures-2011-2013-page.

The Diet Source. Fiber and colon cancer: following the scientific trail [Net]. Boston (MA): Harvard School of Public Health; c2012 [cited 2013 Apr 3]. Available from: http://www.hsph.harvard.edu/nutritionsource/nutrition-news/fiber-and-colon-cancer/index.html.

Centers for Disease Command and Prevention (Usa). Morbidity and mortality weekly report: notifiable diseases and bloodshed tables [Internet]. Atlanta (GA); [cited 2013 Apr iii]. Available from: http://world wide web.cdc.gov/mmwr/preview/mmwrhtml/mm6101md.htm?s_cid=mm6101md_w.

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Source: https://courses.lumenlearning.com/ap2/chapter/the-small-and-large-intestines/

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