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Introduction

The organs within the GI tract can be divided into the alimentary (nourishing) canal and the accessory organs . The alimentary canal is the tube extending from mouth to anus and includes the mouth, esophagus, stomach, small intestine, colon, rectum, anal canal, and anus.

Accessory organs are those organs located outside the alimentary canal that contribute to the ingestion and digestion of nutrients; the liver, gallbladder, and pancreas are important accessory organs.

ALIMENTARY CANAL
General Structure

The tissue structure characteristics of the alimentary canal are essentially the same throughout its length, with minor variations, and comprise four tissue layers : the mucosa, the submucosa, the muscularis, and the serosa, or adventitia

Mucosa.

The innermost layer of the gut wall is the mucosal layer. The mucosal layer has multiple mucus-secreting glands; thus it is always moist.

Submucosa .

The second layer of the gut wall is the submucosal layer ; key structures in this layer include connective tissue, blood and lymph vessels, nerve fibers, and a number of cells.

Muscularis .

The third layer of the gut wall is the muscularis ; this layer consists of two layers of smooth muscle, an inner layer that is a circular muscle and an outer layer that is a longitudinal muscle.

Serosa .

The outermost layer of the alimentary canal is known either as the serosa or as the adventitia.
Structures located within the abdominal cavity are covered by a connective tissue layer that in turn is covered by other tissue. Structures located outside the abdominal cavity, such as the esophagus, are covered with a connective tissue layer known as the adventitia.

Mouth

The mouth, also referred to as the oral cavity, represents the nearest end of the alimentary canal. It is bordered by the cheeks, the lips, the throat, the palate, and by a muscular floor.

Structure

. Key structures within the mouth include the teeth, the tongue, the hard and soft palates, and the salivary glands . The adult normally has 32 teeth, which play an important role in chewing and in speech. The tongue is a muscular organ that is covered by moist issue; the anterior surface is covered by elevations, where the taste buds are located. The tongue is supplied with both intrinsic and extrinsic muscles, and jointly these muscles control the tongue’s shape, position, and movements.The palates form the roof of the mouth, with the hard, or bony, palate and the soft, or muscular, palate. A large number of glands that contribute to production of saliva are present in the mouth. Saliva actually represents a combination of mucus and thin and watery secretions; saliva also contains amylase, which is an enzyme capable of beginning carbohydrate digestion. Usual volume of saliva production is about 1 to 1.5 L/day.

Function.

Major functions carried out by structures within the oral cavity include speech, nutrient ingestion, initiation of digestion, and swallowing.

Speech . Both the tongue and the teeth play an important role in the precise formation of words required for clear speech.

Ingestion of nutrients.

Normally nutrients are taken into the body through the mouth; disease or injury resulting in inability to eat and drink necessitates alternate routes for nutrient ingestion. The tongue provides for the sensation of taste , which is mediated by means of the taste buds. Because taste is an important stimulus to nutrient ingestion, this is an important function.

Digestion .

Mechanical digestion of nutrients begins in the mouth with the process of chewing, or mastication, which breaks large food particles into smaller particles and thereby increases the surface available for enzymatic action. Thorough chewing of food can contribute significantly to the efficiency of the digestive process. Structures that contribute to the ability to chew are the tongue, the teeth, and the muscles of mastication.
Enzymatic digestion of carbohydrates also is initiated in the mouth by the action of salivary amylase, which can reduce polysaccharides into maltose and isomaltose. Only a small percentage of carbohydrates are digested in the mouth because most starches are covered with cellulose and therefore are protected from enzymatic action.

Swallowing .

An important function of the mouth and related structures is swallowing, which begins transport of the nutrients along the digestive pathway. Any condition that interferes with the ability to swallow places the individual at risk for nutritional compromise as a result of inadequate intake. Swallowing is a complex act that involves the tongue, the soft palate, the muscles of the middle part of the pharynx, the upper esophageal sphincter , the epiglottis, the muscles of the esophagus, and gravity. Movement of food through the esophagus is accomplished by peristaltic waves; that is, the muscles away from the food bolus relax while the muscles near the food bolus contract and push the food along. Swallowing is initiated voluntarily but is completed as a reflex action. Swallowing is facilitated by the presence of saliva, which acts as a lubricant for the food particles. Saliva also helps prevent infections of the oral cavity, because it constantly bathes the mouth and because it has some degree of antibacterial action.

Esophagus

Structure.

The esophagus, is approximately 25 cm in length, is the muscular tube that connects the middle part of the pharynx to the stomach. . The esophagus generally is divided into three sections: the upper esophagus, mid and lower esophagus. The esophagus is bounded both upper and lower by sphincter mechanisms, the upper esophageal sphincter (pharyngoesophageal), and the lower esophageal sphincter(esophagogastric); these sphincters prevent reflux from the esophagus into the middle part of the pharynx and from the stomach into the esophagus.

Functions .

The major functions of the esophagus include transport of food from the mouth and middle part of the pharynx into the stomach and the production of mucus. Transport of food occurs via the aforementioned swallowing mechanism. Food in the middle part of the pharynx causes reduced pressure within the upper esophageal sphincter; the sphincter opens and allows the food or fluid to enter the esophagus. The food is moved toward the stomach by peristalsis; as the food approaches the lower esophageal sphincter, the sphincter relaxes to allow the food or fluid to enter the stomach. The lower esophageal sphincter relaxes only with the approach of a food ; at all other times it is contracted to prevent reflux of gastric contents into the esophagus. Penstaltic waves within the esophagus are stimulated by the presence of food or fluid within the esophagus, which activates the intramural plexus; the presence of food or fluid also causes nerve stimulation, which stimulates contraction of the skeletal and smooth muscles within the esophagus.

Blood Supply and Innervation . Vascular supply to the esophagus derives from the esophageal branch of the thoracic aorta and from the left gastric artery.

Abdominal Cavity and Peritoneum

Most of the organs in the gastrointestinal tract are contained in the abdominal cavity. The abdominal cavity is separated from the chest cavity by the diaphragm and actually is continuous with the pelvic cavity; the stomach, intestines, liver, spleen, pancreas, and kidneys are located within the abdominal cavity, and the pelvic cavity contains the urinary bladder, portions of the colon, and the internal reproductive organs.

The greater omentum is also known as the fatty apron because it hangs down in front of the stomach and large amounts of fat tend to accumulate in and between its double folds.

Stomach

Structure. The stomach is a expandible organ located in the left upper quadrant of the abdomen. Its size depends on its state of fullness; at capacity (about 1 L) the stomach is approximately 25 to 27.5 cm long and about 10.25 cm wide.

Regions. The stomach can be divided into several anatomic regions.

• The cardiac region is the area of the stomach that surrounds the opening from the esophagus and is so named because of its proximity to the heart.
• The fundus is the portion of the stomach that is to the left of the cardiac region.
• The body is the largest portion of the stomach; it curves toward the right, and this curve creates the lesser and greater curvatures of the stomach.
• The most distant area of the stomach is the pyloric region, it lies just above the pyloric sphincter, which controls the flow of gastric contents into the duodenum.

1. The surface mucous cells cover the gastric surface and line the gastric pits.
   The remaining four types of cells are found within the gastric glands: mucous neck cells, parietal cells, chief cells, and endocrine cells.
2. The surface mucous cells and the mucous neck cells are responsible for secreting a viscous and alkaline mucus that coats the superficial cellular tissue, cells of the stomach. This mucous blanket helps create a barrier against the proteolytic and acid gastric secretions. An additional barrier to gastric contents is provided by the surface membranes of the mucosal cells and by the tight junctions found between the mucosal cells. Ulcers may result when the barrier is damaged; substances known to be barrier breakers include aspirin, alcohol, and bile salts, which contact the gastric mucosa when duodenal contents reflux into the stomach.
   
3. The parietal cells produce hydrochloric acid (HC1) and intrinsic factor. The low pH created by the HC1 inactivates the salivary amylase and inhibits carbohydrate digestion, but it also serves to activate pepsin, which initiates protein digestion. Intrinsic factor bonds with vitamin B12 and facilitates its absorption in the ileum.
   
4. The chief cells secrete pepsinogen, which is the precursor of pepsin.
   
5. The endocrine cells secrete regulatory hormones that affect gastric secretion; for example, gastrin is produced is a powerful stimulant to the secretion of hydrochloric acid.

Gastric Secretion.

The total volume of gastric secretion normally is about 2 to 3 L/day.
Gastric secretion is known to occur in three phases:

1. the cephalic phase, in which the stimulus is the thought, smell, and taste of food;
   
2. gastric phase, in which the stimulus is food in the stomach;
   
3. intestinal phase, in which the stimulus is chyme in the intestine.
   

Gastric secretion is regulated by neural phenomena and by hormonal secretion. Neural regulation involves the autonomic and central nervous systems, in addition to local reflexes. Gastric secretion also is affected by the hormones produced by secretory cells in the stomach and the duodenum; gastrin is a powerful stimulant for gastric secretion, whereas secretin, gastric inhibitory polypeptide , and cholecystokinin have an inhibitory effect.

Factors known to increase gastric secretion include pleasant thoughts, the taste and smell of food, tactile stimulation resulting from chewing and swallowing, gastric distention and partially digested proteins in the stomach, and moderate amounts of alcohol and caffeine, which explains the positive effect of caffeinated and alcoholic beverages on appetite.

Factors known to inhibit gastric secretion include distention or irritation of the duodenum, highly acidic duodenal contents, and hypertonic or hypotonic duodenal contents. Emotions also are thought to affect gastric secretions; anger and hostility have been noted to increase gastric secretion, whereas fear and depression are considered inhibiting factors.

Functions . The stomach has several important functions:

1. It serves as a reservoir for ingested nutrients and provides controlled emptying into the duodenum.
2. It continues the digestive process begun in the mouth.
3. It contains the secretory cells for intrinsic factor.
4. It provides for limited absorption.
5. The acidity of the gastric secretions inhibits bacterial proliferation.

Nutrient reservoir.
The stomach serves as a reservoir for ingested nutrients and provides gradual and controlled emptying into the duodenum; this function of the stomach permits the healthy person to eat at intervals.
A number of factors affect gastric motility and the rate of gastric emptying ; these include the consistency of gastric contents, the acidity and fat content of the gastric contents, vagus nerve stimuli , and emotional states.

Ingested foods and fluids are retained in the stomach until they have been thoroughly mixed with the gastric secretions and converted to a semifluid material known as chyme . This is accomplished by means of a combination of mixing waves and penstaltic waves . Both mixing waves and peristaltic waves proceed from the entering end of the stomach to the exiting end; the difference is that the mixing waves are weaker and primarily act to mix the ingested contents with the gastric secretions, whereas the peristaltic waves are stronger and actively sweep the liquid chyme toward the opening, from the stomach into the duodenum . The more solid material, which is toward the center of the stomach, is pushed back toward the entering end of the stomach so that it is exposed to further mixing before it reaches the exit. This mixing and moving process continues until the gastric contents have been emptied into the duodenum.

The rate of gastric emptying is primarily determined by the pH, fat content, consistency, and temperature of the chyme. Highly acidic chyme is delivered to the duodenum at a slower rate , which allows time for the chyme to be neutralized by pancreatic secretions and mucus production ; meals with significant protein content usually result in more acidic chyme because of the increased production of HCL and pepsin.
Meals with high fat content also retard gastric emptying to allow for bile secretion and fat emulsification. Solids empty more slowly than liquids. Temperature extremes can also slow gastric emptying.
Other factors that affect gastric motility include vagus nerve stimuli, medications, pathologic states, and emotions. Vagus nerve stimulation increases gastric motility Narcotics commonly slow gastric motility, as do a number of pathologic conditions ,pain, anxiety, and depression.

Digestion.
In addition to serving as a reservoir for ingested nutrients and providing controlled emptying into the duodenum, the stomach contributes to the digestive process in the following ways.

• Mechanical digestion is continued as ingested nutrients are mixed with gastric secretions to form a semifluid chyme.
• The breakdown of starches is interrupted because the acidic pH inactivates the salivary enzymes that helps change starch into sugar:; however, the acidic pH provides the needed environment for initiation of protein digestion.
  
• The HCL produced by the parietal cells converts pepsinogen to pepsin, which is a water-soluble enzyme that breaks intact protein down into peptide chains.

Absorption.
A less important function of the stomach is its capacity for limited absorption ; substances that can be partially absorbed in the stomach include carbohydrates that have undergone chemical digestion in the mouth, as well as alcohol and some medications (e.g., aspirin).The stomach’s role in nutrient absorption is minimal.

A final function of the stomach is the antibacterial effect provided by the low gastric pH; the hydrochloric acid eliminates most of the ingested bacteria.

Small Intestine

The small intestine in the adult is approximately 22 feet in length ; in the newborn infant the small intestine is only 25% of its adult length and 13% of its adult diameter. The tremendous growth of the small bowel throughout the life cycle is illustrated by the fact that the infant has about 950 cm of absorptive surface, whereas the average adult has about 7600 cm The small intestine is the major organ for digestion and absorption of nutrients and as such is critical to life and health .
The small intestine is divided into three anatomic and functional sections:

1. the duodenum,
   
2. the jejunum,
   
3. the ileum.

Histology. The wall of the small intestine contains the four layers common to the GI tract.
Several characteristics, however, are unique to the small intestine. The submucosal and mucosal layers are arranged in folds which, together with the villi (hairlike or fingerlike vascular processes of the small intestine) serve to significantly increase the absorptive surface.

The villi are significant structures of the mucosal layer . They are fingerlike projections, 0.5 to 1 mm in length, that cover the mucosal surface and increase its absorptive area. Each villus contains a capillary network, a lymphatic vessel, and smooth muscle fibers.
The villi are covered with absorptive cells that have extensions known as microvilli; the microvilli serve to further increase the absorptive surface. The total increase in absorptive surface provided by the the villi, and the microvilli is about 600-fold. The microvilla form what is known as the brush border ; the cells that make up the brush border contain many enzymes and carrier substances that facilitate the digestion and absorption of nutrients. Enzymes present in the brush border include peptidases, disaccharidases, and nucleases.

The villi actually have the ability to elongate, or increase in size , which partially explains the phenomenon of bowel adaptation after partial bowel resection. The converse is also true; patients maintained on a regimen of nothing by mouth for more than a few days may have temporary wasting away of the villi, with resultant loss of absorptive capacity.

Function.
The small intestine is the primary organ responsible for the digestion and absorption of nutrients, vitamins, minerals, fluids and electrolytes, and miscellaneous substances such as drugs . Specific functions that contribute to the digestive process include intestinal motility, intestinal secretion, and absorption.

Motility . (Spontaneous motion) Two types of contractions occur regularly in the small bowel:

segmental contractions and peristaltic waves.

• Segmental contractions are ringlike contractions that produce a back-and-forth motion that acts to mix and churn the intestinal contents and to increase exposure of the chyme to the absorptive mucosal surface.
  
• Peristaltic waves act to propel the chyme forward. When the intestine is contracted, it stimulates a strong contraction that begins just behind the point of stimulus and sweeps along the intestine toward the ileocecal valve. Normally peristaltic waves occur almost continuously, propelling the chyme forward at a rate of 2 to 25 cm/min.

Small-bowel motility is affected by both external and internal nerves, but the internal system is far more important. Motility is most affected by local factors, such as contraction of the bowel wall, bowel contents, highly acidic chyme, and some products of digestion. Transit time from the mouth to the colon averages 4 to 9 hours.

Secretion.

The small intestine contains many intestinal glands known as Lieberkuhn’s crypts. The secretory cells of these glands secrete as much as 3 L of extracellular fluid a day This fluid contains no enzymes; the enzymes needed to complete the digestive process are found in the cells of the brush border. The watery intestinal fluid functions primarily to promote the absorptive process. In addition to the fluid secreted by the intestinal glands, large amounts of mucus are secreted by cells located throughout the mucosal layer of the small intestine.

Absorption

Nutrients undergo mechanical and chemical digestion in the mouth, stomach, and small bowel; the digestive process breaks down the complex molecules into substances that can be absorbed into the bloodstream. Absorption is then supported by the the small intestine and by the numerous carrier systems located in the brush border of the villi. Most nutrient absorption occurs in the beginning of small bowel, reflecting the tremendous absorptive capacity of this organ; however, the ileum can take over much of the absorptive function in the event of disease or surgical resection involving the small bowel near the ileum.

The intact bowel provides significant reserve capacity for nutrient absorption; thus segmental bowel resection usually is well tolerated. The effects of any particular resection depend on the length and function of bowel removed and the length and function of the remaining bowel. For example, resection involving the ileocecal valve is more likely to result in compromised absorption than is resection of an equal length of bowel that does not involve the ileocecal valve. Critical bowel length, that is, the length of absorptive small bowel that is essential for absorption of adequate nutrients is difficult to determine because of differences in measurement techniques. It has been suggested that the adult needs at least 100 to 200 cm and the infant needs at least 60 cm of absorptive small bowel; however, many factors affect an individuals response to massive small-bowel resection.

The small intestine also plays a vital role in maintenance of fluid and electrolyte balance through its reabsorption of fluids. The volume of fluid secreted into the small intestine may be as much as 7 to 9 L/day ; with the average oral intake of 2 L/day, the volume of secreted and ingested fluids may reach 11 L/day. The vast majority of this fluid is reabsorbed into the bloodstream, with less than 2 L passing through the ileocecal valve daily. Any abnormal losses, for example, losses resulting from vomiting, diarrhea, or fistula drainage, may result in fluid and electrolyte imbalance.

Blood Supply and Innervation

. The superior mesenteric artery supplies blood flow for most of the small intestine, that is, the jejunum, the ileum, and the exiting end of the duodenum. The entry end of the duodenum (the portion above Vater’s ampulla) derives its blood supply from the celiac vessels .Blood drainage for the small bowel is provided by the superior mesenteric vein; the superior mesenteric vein then empties into the portal vein, which drains into the liver. This vascular arrangement provides the necessary detoxification” of the blood that passes through the intestinal tract before it reenters the systemic circulation. This vascular arrangement also explains the frequency of metastatic disease involving the liver in a person with a malignancy of the intestinal tract.

Nerve system control of the small bowel involves both an intrinsic and an extrinsic system. Intrinsic control is provided by the plexus located within the bowel wall, Auerbach’s plexus and Meissner’s plexus; intrinsic control is the primary stimulus for intestinal secretion and motility .

Bacterial Counts .

Bacterial counts in the small intestine are relatively low , probably due to the bactericidal effect of the extremely low gastric pH and the usually rapid transit of small bowel contents, which limits the potential for bacterial proliferation. This explains why patients with large-bowel stomas usually have more flatus than do patients with small-bowel stomas.

Duodenum

Structure . The duodenum is an immobile C-shaped segment of small bowel that lies just downstream to the pylorus ( the opening, surrounded by muscular tissue, from the stomach into the duodenum ); it is secured to the region of the stomach by Treitz’s ligament, which is the dividing point between the duodenum and the jejunum. The duodenum lies in close proximity to the stomach , pancreas, liver, gallbladder, and transverse colon. The cystic duct from the gallbladder and the hepatic duct from the liver merge to form the common bile duct; the common bile duct and the pancreatic duct both empty into the duodenum. The sphincter of Oddi controls the flow of secretions into the duodenum.

Function

Chyme neutralization .

The duodenum’s major function is to neutralize the highly acidic gastric contents as they enter the duodenum. This process is accomplished partly by secretion of alkaline mucus by the duodenal . In addition, the presence of acid chyme within the duodenum causes the release of secretin , which stimulates the pancreas to secrete f luid with a high concentration of bicarbonate ions . This highly alkaline fluid drains through the pancreatic duct into the duodenum and plays a major role in neutralizing the acidic chyme. When the presence of fats in the duodenum stimulates delivery of bile, the alkaline bile also helps neutralize the acidic gastric contents.

Digestion continuation .

A second function of the duodenum is to continue the digestive process begun in the alimentary canal. Fatty acids and amino acids in the duodenum stimulate the release of cholecystokinin , which causes contraction of the gallbladder and secretion of enzymatic juices by the pancreas. Bile delivered to the duodenum acts to emulsify the fats, rendering them more susceptible to enzymatic breakdown. The pancreatic juice contains a number of digestive enzymes: amylase, which continues carbohydrate digestion; lipases, which continue the digestion of lipids; and the proteolytic enzymes trypsin, chymotrypsin, and carboxypeptidase. The higher pH in the duodenum inactivates pepsin but provides an optimal environment for pancreatic enzyme activity.

Absorption. The duodenum also plays a role in absorption; substances that are absorbed in the duodenum include carbohydrates and minerals such as iron, calcium, and magnesium.

Jejunum

The jejunum is the midportion of the small intestine ; it measures about 9 feet in length and about 1 to 1 1/2 inches in width. The jejunum is the major organ for nutrient absorption; most of the fats, proteins, and vitamins are absorbed in this area, as well as carbohydrates not absorbed in the stomach and duodenum. The enzymes in the brush border finalize nutrient digestion, and the carrier systems facilitate absorption. The large volume of intestinal secretions also promotes nutrient absorption . The jejunum has prominent villi, consistent with its role in nutrient absorption”.

Ileum

The third segment of the small intestine is the ileum, which is about 12 feet long and about 1 inch wide. Although no clear demarcation exists between the jejunum and the ileum, the ileum is narrower than the jejunum and the villi are less prominent.The ileum provides for absorption of any nutrients not absorbed by the duodenum and jejunum . The ileum also contains the only receptor sites for absorption of the intrinsic factor vitamin B12 complex and for bile salts; these sites are located in the terminal ileum. Patients who have had significant lengths of the terminal ileum resected may require lifelong replacement of vitamin B12 to prevent pernicious anemia ; fat intolerance and weight loss also may occur in these patients.
Fat intolerance results from failure to reabsorb bile salts in the terminal ileum, which retards production of bile in the liver. Normally the bile produced by the liver is concentrated in the gallbladder until gallbladder contraction is stimulated by cholecystokinin. The bile is then delivered to the duodenum, where it emulsifies the fats; the residual bile salts continue to pass through the bowel until they reach the terminal ileum. At this point the bile salts are reabsorbed into the bloodstream and delivered back to the liver, where they are again used to produce bile. This recycling of bile salts is known as the enterohepatic circulation; it promotes bile production and therefore fat absorption.

Ileocecal Valve

The ileocecal valve is a one-way valve located at the junction between the ileum and the large intestine. The ileocecal valve works in conjunction with the ileocecal sphincter , a ring of smooth muscle, to regulate emptying into the large intestine and to prevent reflux of contents back into the small intestine . Normally the sphincter is partially contracted; peristaltic waves in the ileum cause the sphincter to relax, permitting passage of chyme into the colon. In contrast, expansion of the cecum causes increased contraction of the sphincter, which protects the small bowel from reflux.

The ileocecal valve and sphincter provide some delay in the passage of chyme from the small bowel into the colon; this delay factor may be critical in the person with a short-bowel syndrome or compromised absorptive capacity because it increases the exposure of nutrients to the absorptive surface of the small bowel.

Colon

The colon is a long, stretchable tube that extends from the ileocecal valve to the anus and brackets the small intestine. In the adult the colon is 5 to 6 feet in length and has a variable width of 1 to 2 1/2 inches. The colon is subdivided into the following segments: cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal .

Secretions

Colonic secretions consist of water, mucus, potassium, and bicarbonate. Mucus, serves several functions: lubrication of the fecal bolus to aid transport of the material; protection of the mucosa from injury; and binding of the fecal material together. The secretion of bicarbonate creates an alkaline fecal matter.

Composition of Fecal Material .

Fecal material consists primarily of bile pigments, mucus, unabsorbed minerals, undigested fats, cellulose, meat protein toxins, peeled off cells, potassium, chloride, sodium, bicarbonate, and water . Fecal composition is approximately three-fourths water (50 to 150 ml) and one-fourth solid matter.
Intestinal bacteria are vital to many colonic functions. Anaerobic bacteria present in the colon serve to putrefy remaining proteins and indigestible residue; to synthesize folic acid, vitamin K, nicotinic acid, riboflavin, and some B vitamins; and to convert urea salts to ammonium salts and ammonia for absorption into the portal circulation. The concentration of bacteria in the colon is higher in the more distant portions of the colon. These bacteria are partially responsible for the odor associated with feces, which explains why an output from a sigmoid colostomy has more odor than does output from an ileostomy . Bacterial action also creates intestinal gas . In addition, swallowed air and diffusion from the blood contribute to intestinal gas production. Intestinal gas is composed of oxygen, nitrogen, carbon dioxide, methane, hydrogen, and trace gases. Such gas production also creates fecal odor.

Structure.

The cecum is the first section of colon and also is the widest segment. Measuring 2 1/2 to 3 inches in length, the cecum contains the ileocecal valve . Distention of the cecum prevents reflux of cecal contents into the ileum. This antireflux mechanism, however, is somewhat incompetent. The appendix arises from the cecum.

The ascending colon extends approximately 15 cm from the cecum to the right and is slightly narrower than the cecum. After a sharp 90-degree left turn, the colon continues as the transverse colon. Approximately 45 to 50 cm in length, the transverse colon is quite mobile because it is fixed only at its two end points, the hepatic flexure and the splenic flexure. The greater omentum lies in front of the transverse colon and must be elevated to expose the colon.

At the splenic flexure the transverse colon makes an acute, almost 180-degree turn downward and backward to continue as the descending colon.

The descending colon is 25 cm long, longer than the ascending colon.

The sigmoid colon continues in an S-shaped curve to the upper end of the rectum. Although the length of the sigmoid varies widely, the average is 40 cm.

The rectum is a hollow, angulated structure that begins at the termination of the sigmoid colon.; The rectum measures 12 to 15 cm in length and follows the curve of the sacrum and coccyx before angulating sharply downward and backward.

The rectum becomes the anal canal .

The diameter of the upper portion of the rectum is the same as that of the sigmoid colon; however, the lower part of the rectum, is dilated to facilitate storage of fecal material . The rectum, which normally is in a collapsed state, is surrounded by a continuous, strong muscular coat of longitudinal fibers.

The rectal mucosa forms three folds in the rectum known as the valves of Houston; two valves are located on the left, and one is on the right .

The anal canal , which is 2.5 to 3 cm in length, is the terminal portion of the colon . The muscles surrounding the anal canal are contracted, thus completely collapsing the anal canal. Midpoint in the anal canal is the dentate line. This is an important reference point because of the differences that exist in the tissues above and below this line. Near to this line, the mucosa assumes a pleated appearance. These longitudinal folds are created by the narrowing of the rectum into the anal canal.

The colon nerve system is parasympathetic and sympathetic. and closely resembles the small bowel because it also involves an intrinsic and an extrinsic system. .

Function. The functions of the colon include collection, concentration, transport, and elimination of intestinal waste material . Approximately 1.5 to 2 L of intestinal contents pass through the ileocecal valve to be collected in the cecum daily. Water and electrolytes (sodium and chloride), glucose, and urea are reabsorbed from this material, thereby concentrating the contents so that only 100 to 150 ml of fluid remains to be excreted from the body. Colonic motility transports fecal material through the colon by the synchronous activity of the colonic musculature. Elimination of intestinal waste then occurs at regular intervals.

Colonic motility. The longitudinal and circular muscles work together to propel the fecal material through the colon (peristalsis) and to knead the material into a bolus (segmentation). The longitudinal muscle is primarily responsible for peristalsis, the movement of the fecal bolus through the colon. Several types of peristalsis occur. Receptive relaxation allows the cecum to fill with ileal contents; adaptive relaxation allows the fecal material to accumulate and be stored; pendulum movements (continuous back-and-forth movement of fecal material) aid absorption; and mass movement (the en masse contraction of the left colon) propels the fecal bolus into the rectum to be evacuated.

Segmentation, the alternate contraction and relaxation of the haustra, is created by the circular muscle . Segmentation facilitates the grinding of food masses and fluid and electrolyte reabsorption.

Colonic motility is stimulated by hostility, anger, physical activity, colonic distention, lactose, and medications that absorb water or prevent the movement of water (e.g., bulking agents and saline cathartics). Factors that decrease colonic motility include sleep, anxiety, and fear. The average transit time from the ileocecal valve to the rectum is 24 hours or more.

Elimination . Feces are eliminated from the colon through the defecation process , a complex process that involves the coordination of the anal sphincters, pelvic floor muscles, and voluntary efforts.

Anal Sphincter

The internal and external sphincters compose the anal sphincter Anatomically the internal sphincter is a continuation of the thick circular muscle and encircles the anal canal . The internal sphincter, which is tonically contracted, is not under voluntary control because it is a smooth muscle. The external sphincter is a muscle that surrounds the anal canal.

These two sphincters work together. In response to rectosigmoid distention, the internal sphincter immediately relaxes while the external anal sphincter contracts.

PROCESS.

Fecal material is delivered to the rectum or sigmoid rectum through mass movement. As this material is forced into the rectum and anal canal, the internal sphincter promptly relaxes while the external sphincter automatically contracts. If the individual chooses to proceed with defecation and strains to push the contents into the rectum, intrarectal pressures increase. These pressures are sufficient to overcome the external sphincter contraction. Simultaneously, the external sphincter and pelvic floor muscles relax, thus straightening the rectum and eliminating any resistance presented by Houston’s valves or rectal angles. Reduced angulation is also facilitated by the squatting position. Continued relaxation of the external sphincter occurs as a result of the stimulation of the mucosa of the anal canal by the fecal material.

ACCESSORY ORGANS

The pancreas, the liver, and the biiary system are considered accessory organs to the digestive system . Each of these is discussed with emphasis on its role pertaining to digestion.

Pancreas

Structure . The pancreas is a fish-shaped, lobulated organ that weighs approximately 85 g (3 ounces); it is 10 to 22 cm long and 5 cm wide. The pancreas is divided into the head , the body (which extends horizontally across the abdomen and hides behind the stomach), and the tail (which contacts the spleen at the level of the first and second lumbar vertebrae . The pancreas is described as a lobulated organ because the internal structure is composed of numerous small hollow cavities lined with special secretory cells . These cells secrete digestive enzymes also known as pancreatic secretions .

The pancreas also consists of clusters of cells that form spherical islands embedded within the lobules of acinar tissue known as the islets of Langerhans. These cells also are referred to as endocrine tissue because they have no ductal system and they release their products directly into the bloodstream. Four distinct cell types are present in the islets of Langerhans: A, B, D, and F cells. A cells secrete glucagon, B cells secrete insulin, D cells secrete somatostatin, and F cells secrete pancreatic polypeptide hormone.

Function . The pancreas has both an endocrine and an exocrine function. The islets of Langerhans, produce the endocrine products of insulin, glucagon, somatostatin, and pancreatic polypeptide hormone; these substances are released into surrounding capillaries, empty into the portal vein, and are distributed to target cells in the liver where these hormones enter the general circulation to reach other target tissue.

Exocrine secretions (pancreatic juices), which are produced by the acinar cells, are odorless, colorless, watery, and alkaline . The primary components of pancreatic juice include water (97%) and bicarbonate. Electrolytes such as sodium and potassium in high concentrations and calcium and chloride in smaller concentrations also are present.

Both hormonal and neural signals regulate pancreatic exocrine function. In response to stimulation by chyme in the intestine, the intestinal hormones secretin and cholecystokinin are released and transported to the pancreas via the portal system. In the presence of an acid chyme (pH below 4.5), secretin is released and acts on the pancreas to produce a high volume of fluid low in enzymes but rich in water and bicarbonate. The acid chyme that is delivered to the duodenum is thereby neutralized. Other effects of secretin production include increased bile secretion from the liver, decreased gastrin-induced gastric secretion and emptying, stimulation of insulin release, increased pancreatic flood flow, and potentiation of cholecystokinin.

The release of the intestinal hormone cholecystokinin is stimulated when the chyme present in the duodenum is rich in protein and fat . Cholecystokinin causes the pancreas to produce secretions that are rich in enzymes, low in volume, and high in bicarbonate and amylase. A Neural influences on the pancreas are inhibitory stimulation by the sympathetic system and increased exocrine secretions and blood flow by the parasympathetic system. In addition, anticholinergic agents decrease enzyme production, whereas alcohol and histamines increase pancreatic secretions by stimulating gastric hydrochloric acid production.

Liver

Structure. The liver, the largest organ in the body , weighs 1200 to 1600 g (3 to 4 pounds); it is located in the right upper abdominal quadrant below the right diaphragmatic dome .

The liver is held in place by intraabdominal pressure and by the falciform ligament . This ligament attaches the liver to the anterior portion of the abdomen between the diaphragm and the umbilicus and also divides the liver into the right and left lobes.

Six times larger than the left lobe, the right lobe has three sections: right lobe proper, caudate lobe, and quadrate lobe. The left lobe has two sections.The functional unit of the liver is the lobule; each lobe contains numerous lobules. Approximately 1 million lobules form the liver tissue Each lobule comprises rows of hepatic cells held together with connective tissue. Between the rows of hepatic cells are capillaries or sinusoids that are branches of the portal vein and hepatic artery; central to each lobule is an intralobular vein that drains the lobule.

Of interest is the liver’s ability to regenerate, which is thought to result from the volume of blood supplied to the liver and the proximity of the blood flow to the individual cells. Although factors such as age, hormone levels, and diet may affect the rate, regeneration is believed to occur within 3 weeks; normal function returns within 4 months. Generally, 70% of the liver can be destroyed without manifestation of symptoms.

Blood Supply. The liver is unusual in that it receives blood from one artery and one vein . The portal vein supplies nutrient-rich blood from the intestines, pancreas, spleen, stomach, and mesentery. Approximately one third of the total cardiac output (i.e., 800 to 1500 ml) is carried to the liver every minute through these two blood vessels.

The portal vein carries the majority of the blood flow (75%) in the liver and branches into the sinusoids to supply each of the lobules. Lymphatic vessels surround the hepatic veins and bile ducts. The lymph drains into these vessels from lymphatic spaces that are located between parenchymal cells. Carcinoma frequently metastasizes to the liver because of the considerable lymphatic and vascular supply.

Function.

The liver is a complex organ that performs many metabolic and digestive functions . Briefly, the functions of the liver include bile formation, carbohydrate metabolism, protein metabolism, fat metabolism, steroid metabolism, vitamin storage, coagulation, mineral and water metabolism, and detoxification.

Bile production and secretion are continuous processes within the liver. Components of bile include water, bile salts, bilirubin, cholesterol, fatty acids, and lecithin.

Bile salts function primarily
(1) to emulsify fat globules into minute sizes to facilitate digestion and
(2) to promote the absorption of lipids (cholesterol and fatty acids) across the intestinal mucosa. Fat absorption is important because when fats are not absorbed ade quately, fat-soluble vitamins (A, D, E, and K) are not absorbed adequately. Bile salts also serve as a route to excrete biirubin, cholesterol, and various hormones (sex, thyroid, and adrenal).

Reabsorption of approximately 94% of the bile salts occurs in the terminal ileum; the reabsorbed salts are returned to the liver through the portal blood. In the liver the bile salts are absorbed into the hepatic cells and then resecreted into the bile. The small amounts of bile salts lost are eliminated in the feces. This recirculation of bile salts, known as the enterohepatic circulation , is essential for maintaining normal daily flow of bile; otherwise the liver would not be able to synthesize enough of the salts to produce the normal amount of bile. The daily volume of bile production averages 600 to 1000 ml.

The liver serves an important role in carbohydrate metabolism and storage by synthesizing glycogen (the stored form of glucose) from glucose, protein, fat, or lactic acid. Glycogen is then broken down as necessary and released by the liver to maintain normal blood glucose levels. The liver has a vital role in protein synthesis and amino acid metabolism. Through a complex process amino acids can be formed from the metabolites of carbohydrates and fat. The liver breaks down amino acids, thus producing ketoacids and ammonia. From these substances urea is formed, which is excreted in the urine. Fat metabolism continues in the liver and involves conversion of triglycerides to glycerol and fatty acids, a process called ketogenesis. Synthesis of fat substances such as cholesterol, phospholipids, and lipoproteins also occurs in the liver. Steroid metabolism is another function of the liver. Adrenocorticosteroids, glucocorticosteroids, testosterone, and aldosterone are metabolized and catabolized by the liver.

Vitamins such as B12, B1, riboflavin, B2, nicotinic acid, pyridoxine, D, E, K, and A are stored in the liver . Bile and bile salts are required for vitamin D and K absorption to occur.

Coagulation is an extremely complex process that is influenced and regulated by the liver. The liver also disposes of disintegrating red blood cells. The liver functions to metabolize minerals and water. Ferritin, the storage form of iron, is stored primarily in the liver. Copper, calcium, magnesium, zinc, manganese, and cobalt also are stored in the liver.

Finally, the liver serves a unique function of detoxification of foreign and toxic substances. For example, certain medications are detoxified ard excreted in the urine, whereas other medications (such as morphine and atropine) are stored to be released at a later time into the circulation.

Biliary System

Structure. The biliary system consists of the gallbladder and the biliary ductal system , which provide a passageway for bile from the liver to the intestine . On a daily basis the liver produces an average of 600 to 1000 ml of fluid with a pH of 7.5. This liver bile, when produced, is golden or orange-yellow.

The duct system consists of two hepatic ducts that drain the liver and the cystic duct that drains the gallbladder. These converge to form the common bile duct. The common bile duct empties into the duodenum at Vater’s ampulla. The sphincter of Oddi surrounds Vater’s ampulla to control the flow of bile into the duodenum. The gallbladder is a pear-shaped organ that lies on the underside of the liver . It is approximately 7 to 10 cm long and 2.5 to 3.5 cm wide, with a capacity of 60 ml of bile.

Blood Supply and Innervation. The hepatic artery and cystic artery supply blood to the gallbladder. Venous drainage occurs via the cystic vein. Lymphatic drainage is abundant. Innervation of the gallbladder is provided by the sympathetic (splanchnic) system and the parasympathetic (vagus) system.

Function. The primary function of the gallbladder is to collect, concentrate, acidify, and store bile until it is needed for digestion. Normally the sphincter of Oddi is closed during fasting and between meals. During this time the continuous bile production by the liver increases pressure within this closed system so that as bile is passed into the hepatic ducts, it is forced into the cystic duct to be stored in the relaxed gallbladder Once in the gallbladder, concentration of the bile begins to occur.

Emptying of the gallbladder and the passage of bile into the duodenum t herefore depend on the tone of Oddi’s sphincter. This tone is influenced by the hormone cholecystokinin, which is released in response to the presence of fats in chyme. Sight, smell, and taste of food may stimulate the gallbladder; however, emotional states such as fear or excitement may decrease the flow of bile.

Submaxillary Glands and Sublingual Glands are Salivary glands that provide saliva in the mouth, which helps break down food and aids digestion. The glands are located near the ear and in the submaxillary area at the angle of the jaw.

Trachea

The trachea or windpipe, is a cartilaginous and membranous cylindrical tube, which extends from the lower part of the larynx, where it divides into two bronchi, one for each lung.

Larynx

The Larynx is the cartilaginous box felt in the front of the neck as the Adam's apple. The vocal cords of the larynx are two muscular bands that contract during swallowing to prevent solids or liquids from entering the lungs. The larynx also has a respiratory role. It separates the vocal cords to allow inhaled air to enter the trachea and lungs

Pharynx

The throat portion of the ear-nose-throat unit is known as the pharynx. The pharynx is a long muscular tube that extends from the back of the nose into the swallowing tube (esophagus). Generally , it is divided into the nasopharynx, oropharynx and hypopharynx.

Gastric Ulcer

Ulcers are minature excavations in the mucous membrane. They most often occur in the stomach (gastric ulcer) or duodenum (duodenum ulcer). About 80 percent of all ulcers are duodenal. Agents incriminated in the formation of gastric ulcers include aspirin, alcohol, indomethacin, phenylbutazone.

Polyp

Colonic Polyps - Two types of polyps that may develop in the colon are called pedunculated if they have a stalk and sessile if they have no stalk. Differences in tissue seen under the microscope further divide them into adenomatous and villous. Some colonic polyps apparently lead to the development of cancer. Larger polyps are more dangerous than small ones, and villous polyps are more dangerous than adenomatous.

Hemorrhoid

Hemorrhoids are dilated veins that occur in the rectum. Hemorroids usually cause no symptoms and require no treatment. Occasionally, a blood clot or thrombosis develops in the hemorrhoid, but this is usually treated by a sitz bath. Surgical removal of the thrombosed hemorrhoid occasionally is necessary.

Ulcerative Colitis

Inflamatory bowel disease, IBD, is a general term for inflammation of the lining cells of the colon. Causes include infection, types of dysentary, tuberculosis, gonorrhea and certain parasites.

However the most common forms of inflammatory bowel disease are those whose cause in unknown. There are two distinct types, Crohn's colitis and ulcerative colitis. Crohn's colitis is the same basic disease as reginal enteritis , except that it occurs in the colon.

Diverticulitis
Diverticuli

Diverticulitis and Diverticulosis .
Diverticulosis means the presence of one or more diverticuli in the colon. A diverticulum is an outpouching through a weakened area in the wall of the bowel.
The infected diverticulum is referred to as diverticulitis