The digestive system, also called the alimentary canal or gastrointestinal tract, begins with the mouth, where food enters the body, and ends with the anus, where solid waste material leaves the body. The primary functions of the organs of the digestive systems are threefold.

First, complex food material taken into the mouth must be digested, or broken down, mechanically and chemically, as it travels through the gastrointestinal tract (passageway). Digestive enzymes speed up chemical reactions and help in the breakdown (digestion) of complex nutrients. Complex proteins are digested to simpler amino acids; complicated sugars are reduced to simple sugars, such as glucose; and large fat molecules (triglycerides) are broken down to fatty acids and glycerol.

Second, the digested food must be absorbed into the blood stream by passing through the walls of the small intestine. In this way, valuable nutrients, such as sugar and amino acids, can travel to all the cells of the body. Cells then catabolize (burn) nutrients in the presence of oxygen to release energy stored with the food. Cells also use amino acids to anabolite (build) large protein molecules needed for growth and development. Although the walls of the small intestine also absorb fatty acids and glycerol, these nutrients enter lymphatic vessels rather than blood vessels. Digested fats eventually enter the blood stream as lymph vessels join with blood vessels in the upper chest region.

The third function of the digestive system is the elimination of solid waste materials that cannot be absorbed into the blood stream. The large intestine concentrates these solid wastes, called feces, and the wastes finally pass out of the body through the anus.

Anatomy & Physiology

Oral Cavity
The alimentary canal begins with the oral cavity, or mouth.

The cheeks form the walls of the oral-shaped cavity and the lips surround the opening to the cavity.

The hard palate forms the anterior portion of the roof of the mouth and the muscular soft palate lies posterior to it. Rugae are the irregular ridges in the mucous membrane covering the anterior portion of the hard palate. Hanging from the soft palate is a small soft tissue called the uvula. The word uvula means little grape. The structure functions to aid in producing sound and speech.

The tongue extends across the floor of the oral cavity, and muscles attach it to the lower jaw bone. It moves the food around during mastication (chewing) and deglutination (swallowing). Papillae, small raised areas on the tongue, contain taste buds that are sensitive to chemical nature of foods and allow discrimination of different tastes as food moves across the tongue.

The tonsils are masses of lymphatic tissue located in depressions of the mucous membranes on both sides if the oropharynx (part of the throat near the mouth). They act as filters to protect the body from invasion of microorganisms and produce lymphocytes, which are white blood cells able to fight disease.

The gums are made of fleshy tissue and surround the sockets of the teeth. Each dental arch has 16 permanent teeth. There are 32 permanent teeth in the entire oral cavity.

Dentists use special terms to describe the surfaces of teeth. The labial surface (labio/o means lip), for incisor and canine teeth, is nearest the lips. The buccal surface (bucc/o means cheek), for premolar and molar teeth, is adjacent to the cheek. Some dentists refer to both the labial and buccal surfaces as the facial surface (faci/o means face). Opposite to the facial surface, all teeth have a lingual surface (lingu/o means tongue). Mesial surface of a tooth lies nearest the median line and the distal surface, farthest from the medial line. Premolars and molars have an additional occlusal surface (occlusion means to close) that comes in contact with a corresponding tooth in the opposing arch. The incisors and cuspids have a sharp incisal edge.

A tooth consists of a crown, which shows above the gumline and a root, which lies within the bony tooth socket. The outermost protective layer of the crown, the enamel, protects the tooth. Enamel is a dense, hard, white substance – the hardest substance in the body. Dentin, the main substance of the tooth, lies beneath the enamel and extends throughout the crown. Yellow in color, dentin is composed of bony tissue that is softer than enamel. The cementum covers, protects and holds the tooth in place in the tooth socket.

The pulp lies underneath the dentin. It is soft and delicate tissue and fills the center of the tooth. Blood vessels, nerve endings, connective tissue, and lymphatic vessels are within the pulp canal (also called the root canal). Root canal therapy is often necessary when disease or abscess (pus collection) occurs in the pulp canal. A dentist opens the tooth from above and cleans the canal of infection, nerves, and blood vessels. The canal is then disinfected and filled with material to prevent the entrance of microorganisms and decay.

Three pairs of salivary glands surround the oral cavity. These exocrine glands produce a fluid called saliva that contains important digestive enzymes. Saliva is released from the parotid gland, submandibular gland, and sublingual gland on each side of the mouth. Narrow ducts carry the saliva into the oral cavity.

Pharynx The pharynx or throat is a muscular tube, about five inches long, lined with a mucous membrane. It serves as a common passage for air traveling from the nose (nasal cavity) to the windpipe (trachea) and food traveling from the oral cavity to the esophagus. When swallowing (deglutination) occurs, a flap of tissue, the epiglottis, covers the trachea so that food cannot enter and become lodge there.

Esophagus The esophagus, meaning swallowing (phag/o) inward (eso-), is a 9- to 10-inch muscular tube extending from the pharynx to the stomach. Rhythmic contractions of muscles in the wall of the esophagus propel food toward the stomach. Peristalsis, meaning constriction (-stalsis) surrounding (peri-), is this involuntary, progressive, rhythm-like contraction of the esophagus and the other gastrointestinal tubes. The process is like squeezing a marble (the bolus, or semi-solid mass of food) through a rubber tube.

Stomach Food passes from the esophagus into the stomach. The stomach is composed of an upper portion called the fundus, a middle section known as the body, and a lower portion, the antrum. Rings of muscles called sphincters control the openings into and leading out of the stomach. The lower esophageal sphincter (cardiac sphincter) relaxes and contracts to move food from the esophagus into the stomach, whereas the pyloric sphincter allows food to leave the stomach when it is ready. Folds in the mucous membrane (mucosa) lining the stomach are called rugae. The rugae contain digestive glands that produce the enzyme pepsin (to begin digestion of proteins and hydrochloric acid.

The role of the stomach is to prepare the food chemically and mechanically so that it can be received in the small intestine for digestion and absorption into the bloodstream. Food does not enter the bloodstream through the walls of the stomach. The stomach controls the passing of foods into the first part of the small intestine so that it proceeds only when it is chemically ready and in small amounts. Food leaves the stomach in 1 to 4 hours or longer depending upon the amount and type of food eaten.

Small intestine (small bowel) The small intestine (small bowel) extends for 20 feet from the pyloric sphincter to the first part of the large intestine. It has three parts. The first section is the duodenum, only 1 foot in length, which receives food from the stomach as well as bile from the liver and gallbladder and pancreatic juices from the pancreas. Enzymes and bile help to digest food before it passes into the second part of the small intestine. The jejunum, about 8 feet long. The jejunum connects with the third section, the ileum, about 11 feet long. The ileum attaches to the first part of the large intestine.

Millions of tiny, microscopic projections called villi line the walls of the small intestine. The tiny capillaries (microscopic blood vessels) in the villi absorb the digested nutrients into the bloodstream and lymph vessels.

Large Intestine (Large Bowel) The large intestine extends from the end of the ileum to the anus. It is divided into 6 parts: ascending colon, transverse colon, descending colon, sigmoid colon and rectum. The cecum is a pouch on the right side that connects to the ileum at the ileocecal valve (sphincter). The appendix hangs from the cecum. The appendix has no clear function and can become inflamed and infected when it is clogged or blocked. The colon (large intestine), about 5 feet long, has three divisions. The ascending colon extends from the cecum to the undersurface of the liver, where it turns to the left (hepatic flexure or bend) to become the transverse colon. The transverse colon passes horizontally to the left toward the spleen, and turns downward, splenic flexure into the descending colon. The sigmoid colon, shaped like an S (resembling the Greek letter sigma, which curves like an S), lies at the distal end of the descending colon and leads into the rectum. The rectum terminates in the lower opening of the gastrointestinal tract, the anus.

The large intestine receives the fluid waste products of digestion (the material unable to pass into the bloodstream) and stores these wastes until they can be released from the body. Because the large intestine absorbs most of the water within the waste material, the body can expel solid feces (stools). Defecation is the expulsion or passage of feces from the body through the anus. Diarrhea, watery stools, can result from lack of absorption of the water through the walls of the large intestine.

Liver, Gallbladder, and Pancreas
Three important additional organs of the digestive system – the liver, gallbladder, and pancreas – play crucial roles in the proper digestion and absorption of nutrients.

The liver, located in the right upper quadrant (RUQ) of the abdomen, manufactures a thick, yellowish-brown, sometimes greenish, fluid called bile. Bile contains cholesterol (a fatty substance), bile acids, and several bile pigments. One of these pigments called, bilirubin, is produced from the breakdown of hemoglobin during normal red blood cell destruction. Bilirubin then travels via the blood stream to the livers where it is conjugated (combined) with another substance so that it can be added to bile. Thus, conjugated bilirubin enters the intestine with bile. In the colon, bilirubin is degraded by bacteria into a variety of pigments that give feces a brownish color. Bilirubin and bile leave the body in feces.

If bilirubin cannot leave the body, it remains in the bloodstream, causing jaundice (hyperbilirubinemia), yellow discoloration of the skin, whites of the eyes, and mucous membranes.

The liver continuously release bile, which then travels through the hepatic duct to the cystic duct. The cystic duct leads to the gallbladder, a pear-shaped sac under the liver, which stores and concentrates the bile for later use. After meals, in response to the presence of food in the stomach and duodenum, the gallbladder contracts, forcing the bile out the cystic duct into the common bile duct. Meanwhile, the pancreas secretes pancreatic juices (enzymes) that travel vial the pancreatic duct to join with the common bile duct just as it enters the duodenum. The duodenum thus receives a mixture of bile and pancreatic juices.

Bile has detergent-like effect on fats in the duodenum. It breaks apart large fat globules, creating more surface area so that enzymes from the pancreas can digest the fats. This is called emulsification. Without bile, most of the fat taken into the body would remain undigested.

The liver, besides producing bile, has several other vital and important functions. Some of these are:
1. Keeping the amount of glucose (sugar) in the blood at normal level. The liver removes excess glucose from the bloodstream and stores it as glycogen (starch) in liver cells. When the blood sugar levels become dangerously low, the liver can convert stored glycogen back into glucose via a process called glycogenolysis. In addition, the liver can also convert proteins and fats into glucose, when the body needs sugar, by a process called gluconeogenesis.
2. Manufacture of some blood proteins, particularly those necessary for blood clotting.
3. Release of bilirubin, a pigment in bile.
4. Removal of poisons (detoxification) from the blood.

The portal vein brings blood to the liver from the intestines. Digested foods pass into the portal vein directly after being absorbed into the bloodstream from the small intestine, thus giving the liver the first chance to use the nutrients.

The pancreas is both an exocrine and an endocrine organ as an exocrine gland, it produces enzymes to digest starch, such as amylase (amyl/o = starch, -ase = enzyme), to digest fat, such as lipase (lip/o = fat), and to digest proteins, such as protease (prote/o = protein). These pass into the duodenum through the pancreatic duct.

As an endocrine gland (secreting into the bloodstream) the pancreas secretes insulin. This hormone, needed to help release sugar from the blood, acts as a carrier to bring glucose into cells of the body to be used for energy.

Pathology of the Digestive System

This section is divided into terms that describe symptoms (signs of illness) and terms that describe pathological conditions. The sentences following the definitions use medical words that are familiar to you and often describe the etiology (etio/o = cause) of the illness and its treatment. If the etiology is neither known nor understood, it is called idiopathic (idi/o = unknown).

Symptoms

anorexia – Lack of appetite (-orexia = appetite). Often a sign of malignancy or liver disease. Anorexia nervosa is loss of appetite owing to emotional problems such as anger, anxiety, and fear. It is an eating disorder. It can be related to a similar disorder, bulimia nervosa.

ascites – Abnormal accumulation of fluid in the abdomen. This condition, previously called dropsy, occurs when fluid seeps out of the bloodstream and collects in the peritoneal cavity. It can be a symptom of neoplasm or inflammatory disorders in the abdomen, venous hypertension (high blood pressure) caused by liver disease (cirrhosis), and heart failure.

borborygmus – Rumbling or gurgling noises produced by the movement of gas, fluid, or both in the gastrointestinal tract. These noises are often audible from a distance.

constipation – Difficult, delayed elimination of feces. Stools (feces) are dry and hard when peristalsis is slow. A medication that encourages movement of feces from the colon is called a laxative. A cathartic is a strong laxative.

diarrhea – Frequent, loose, watery stools. Rapid onset of diarrhea soon after eating suggests an acute infection or toxin. However, certain infections may not cause diarrhea for several hours. Watery or bloody stools are a symptom of inflammation or disease in the GI tract.

dysphagia – Difficulty in swallowing. This sensation occurs when a swallowed bolus fails to process, either because of a physical obstruction (obstructive dysphagia) or because of a motor disorder in which the esophageal peristalsis is not properly coordinated (motor dysphagia). Odynophagia is painful swallowing.

eructation – Gas expelled from the stomach through the mouth (belching, burping)

flatus – Gas expelled through the anus. Flatulence is the presence of excessive gas in both the stomach and the intestines. Flatulent refers to a person experiencing flatulence.

hematochezia – Bright, fresh, red blood discharged from the rectum. Hematochezia is associated with rapid bleeding, as from a duodenal ulcer, ulcerative colitis, or hemorrhoids.

jaundice – Yellow-orange coloration of the skin and other tissues due to high levels of bilirubin in the blood (hyperbilirubinemia). Jaundice (icterus) can occur in three major ways: (1) excessive destruction of erythrocytes, as in hemolysis, causes excess bilirubin in the blood; (2) malfunction of liver cells (hepatocytes) because of liver disease prevents the liver from excreting bilirubin with bile; (3) obstruction of bile flow, such as from choledocholithiasis or tumor, prevents bilirubin in bile from being excreted into the duodenum.

melena – Black, tarry stools; feces containing blood. This symptom usually reflects a condition in which blood has had time to be digested (acted on by intestinal juices).

nausea – Unpleasant sensation in the stomach and a tendency to vomit. This term comes from a Greek word meaning seasickness. Irritation of nerve endings in the stomach or other parts of the body sends a message to the vomiting reflex center in the brain. Nausea and vomiting may be symptomatic of a perforation (hole in the wall) of an abdominal organ; obstruction of a bile duct, stomach, or intestine; or toxins (poisons)

steatorrhea – Fat in the feces. Improper digestion or absorption of fat can cause fat to remain in the intestine. This may occur with disease of the pancreas (pancreatitis) when pancreatic enzymes are not excreted. It is also a symptom of disease of the small intestine that involves malabsorption of fat.

Pathological Conditions

Oral Cavity and Teeth
aphthous stomatitis – Inflammation of the mouth with small ulcers. This idiopathic condition is also known as canker sores. Aphth/o means ulcer.

dental caries – Tooth decay (caries means decay). Dental plaque is the accumulation of the foods, proteins from saliva, and necrotic debris on the tooth enamel. Bacteria grow in the plaque and cause the production of acid that dissolves the tooth enamel, resulting in a cavity (area of decay). If the bacteria infection reaches the pulp of the tooth (causing pulpitis), root canal therapy may be necessary.

herpetic stomatitis – Inflammation of the mouth (gingiva, lips, palate, and tongue) by infection with the herpesvirus. Commonly called fever blisters or cold sores.

oral leukoplakia – White plaques or patches (-plakia means plaque) on the mucosa of the mouth. A precancerous condition; major etiological factors are chronic tobacco and alcohol use.

periodontal disease – Inflammation and degeneration of gums, teeth, and surrounding bone; also called pyorrhea (py/o means pus). Chronic inflammation of gums (gingivitis) occurs as a result of accumulation of dental plaque (noncalcified collection of oral microorganisms and their products) and dental calculus or tartar (a white, brown, or yellow-brown calcified deposit at or below the gingival margin of teeth). Gingivectomy (a metal instrument is used to scrape away plaque and tartar from teeth) may be necessary to remove pockets of pus and allow new tissue to form. Infected areas are treated with targeted antibiotics.

Gastrointestinal Tract
achalasia – Failure of the lower esophagus sphincter (LES) muscle to relax (-chalasia means relaxation). In achalasia there is also loss of peristalsis so that food cannot pass easily through the esophagus. Both failure of the LES to relax and the loss of peristalsis cause dilation and widening of the esophagus. Physicians often recommend a bland diet low in bulk, and dilation of the LES to relieve symptoms.

anal fistula – Abnormal tube-like passageway near the anus. The fistula usually, but not always, opens into the rectum. An anal fissure is a narrow crack or slit in the mucous membrane of the anus.

colonic polyposis – Polyps (small benign growths) protrude from the mucous membrane of the colon. Two types of polyps: pedunculated – attached to the membrane by a stalk or peduncle; sessile – sitting directly on the mucous membrane. The polyps protrude into the lumen of the colon.

colorectal cancer – Adenocarcinoma of the colon or rectum, or both. Colorectal cancer can arise from polyps in the wall of the colon. Their removal by colonoscopy prevents progression to colorectal cancer. Approximately 50 percent of colorectal carcinomas develop in the rectosigmoid region. Individuals with strong family history of colorectal cancer are advised to have routine colonoscopies for screening. Prognosis depends on the stage (extent of spread) of the tumor, including size, depth of invasion of the colon, and involvement of lymph nodes.

Crohn disease – Chronic inflammation of the intestinal tract (most often the terminal ileum and colon). This chronic relapsing disease can affect any part of the GI tract from the mouth to the anus. It causes inflammation with diarrhea, cramping, and fever. Similar to ulcerative colitis, both are forms of inflammatory bowel disease (IBD). Etiology is unknown and the disease is treated by drugs or surgical removal of the diseased portion of the intestine, with anastomosis of remaining parts.

diverticula (singular: diverticulum) – Abnormal side pockets (outpouchings) in the intestinal wall (diverticulosis). A common location for diverticula to occur is in the sigmoid colon. When undigested food or bacteria becomes trapped in a diverticulum, diverticulitis occurs. Diverticula (di – aside, verti – to turn, cul – small)

dysentery – Painful, inflamed intestines. Commonly occurring in the colon, the etiology of dysentery is usually ingested food or water containing bacteria (salmonellae or shigellae), amebae (one-celled organisms), or viruses. Colitis, diarrhea, and abdominal cramps occur.

esophageal varices – Swollen, tortuous veins in the distal portion of the esophagus or upper part of the stomach. Liver disease (such as cirrhosis) can cause increased pressure in blood vessels (portal vein hypertension). This pressure can lead to enlarged, tortuous esophageal veins with danger of hemorrhage (bleeding). Treatment includes drug therapy to lower portal hypertension or using sclerosing (hardening) agents to close off veins.

gastric carcinoma – Malignant tumor of the stomach. Chronic gastritis associated with H pylori (bacterial) infection is a major risk factor for gastric carcinoma. Tumor is usually diagnosed by endoscopic examinations in high-risk patients. Cure is dependent on early detection and surgical removal.

gastroesophageal reflux disease (GERD) – Solids and fluids return to the mouth from the stomach. Heartburn is the burning sensation caused by the regurgitation of hydrochloric acid from the stomach to the esophagus. Chronic exposure of the esophageal mucosa to gastric acid and pepsin (a proteolytic enzyme) causes reflux esophagitis. Drug treatment for GERD includes acid-suppressive agents and those that increase the tone of the lower esophageal sphincter.

hemorrhoids – Swollen, tortuous veins in the rectal region. Also known as piles, hemorrhoids can be internal (within the wall of the rectum) or external (in the anal area), and they are often caused by chronic constipation and straining, which increases pressure on anal veins.

hernia – protrusion of an organ or part through the muscle normally containing it. A hiatal hernia occurs when the upper part of the stomach protrudes upward through the esophageal opening in the diaphragm. An inguinal hernia occurs when a small loop of bowel protrudes through a weak place in the lower abdominal muscle wall (groin). Muscle weakness can be inherited or acquired as part of the aging process, pregnancy, obesity, or heavy lifting and coughing.

ileus – Failure of peristalsis. This may be caused by mechanical obstruction of the bowel (adhesions, tumor, or stones) or poor neural stimulation (as in peritonitis and overuse of medications). Paralytic ileus is acute transient (brief) loss of peristalsis. Common causes are surgical, traumatic, or bacterial injury to the peritoneum.

intussusception – Telescoping of the intestines. This is a cause of intestinal obstruction and occurs most commonly in children and in the ileocecal region. Treatment involves resection of the intussusception and anastomosis.

irritable bowel syndrome – A group of GI symptoms (diarrhea and constipation, lower abdominal pain, and bloating) associated with stress and tension, also called spastic colon. No pathological lesions are found in the intestines. Treatment includes psychotherapy to manage stress and medications (antidiarrheals and bulk-forming laxatives) to relieve symptoms. A diet high in bran and fiber also helps soften stools and establish regular bowel movements.

ulcer – Open sore or lesion (wound) of skin (epithelial) tissue. Gastric or duodenal ulcers (both are peptic ulcers) are examples. Duodenal ulcers are now thought to be caused by a bacterium, Helicobacter pylori (H pylori). The combination of bacteria, hyperacidity, and gastric juice (particularly pepsin) damages epithelial linings. Treatment includes drugs to reduce the production of hydrochloric acid and protect the lining of the stomach and intestine. Antibiotics are commonly used against H pylori.

ulcerative colitis – Chronic inflammation of the colon with the presence of ulcers. This is an idiopathic, chronic, recurrent diarrheal disease (inflammatory bowel disease) with rectal bleeding and pain. Often beginning in the rectum, the inflammation spreads proximally, involving the entire colon. Resection of diseased bowel with ileostomy may be necessary as treatment. Ulcerative colitis is associated with a higher risk of colon cancer.

volvulus – Twisting of the intestine upon itself. A surgical emergency, the volvulus must be released immediately.

Liver, Gallbladder, and Pancreas

cirrhosis – Chronic disease of the liver with degeneration of liver cells. Alcoholism combined with malnutrition is a common etiological factor, but infection and poisons can affect the liver cells as well. Alcohol has a toxic effect on hepatocytes, causing fat cells to accumulate, followed by necrosis and fibrous scarring, and discoloration (cirrh/o refers to the liver’s orage-yellow color). As with other liver diseases, jaundice results when the liver cells fail to function and bilirubin is not eliminated from the body.

gallstones (cholelithiasis and choledocholithiasis) Crystallization of cholesterol and other materials to form stones in the gallbladder or bile ducts. Calculi (stones) can prevent bile from leaving the gallbladder and bile ducts to enter the duodenum. The majority of patients with gallstones remain asymptomatic and do not require treatment. However, if a patient experiences episodes of biliary colic (pain from blocked cystic or common bile duct), treatment is required. Conventional treatment has been cholecystectomy or choledocholithotomy. Currently, more cholecystectomies are performed using a laparoscopic technique (laparoscopic cholecystectomy) similar to that used to tie off the fallopian tubes.

pancreatitis – Inflammation of the pancreas. Digestive enzymes attack pancreatic tissue, leading to damage to the gland. Alcoholism and gallstones may be causative factors, but chronic or acute inflammation can develop from abdominal trauma or chemical injury. Acute pancreatitis (marked by massive swelling, bleeding, and necrosis of the pancreas) may be complicated by sacs of fluid called pseudocysts (in the pancreas) and systemic problems such as shock, renal failure, and respiratory collapse. Treatment includes medications to relieve epigastric pain, intravenous fluids, and rarely, surgery to remove portions of the pancreas.
viral hepatitis – Inflammation of the liver caused by virus. There are three major types of viral hepatitis: Hepatitis A (caused by type A virus and previously called infectious hepatitis) is a benign, acute, self-limited disorder transmitted by infected water and food (the virus is also excreted in feces). Hepatitis B (caused by type B virus and called serum hepatitis) is acquired parenterally, through blood (transfusions, hypodermic needles, and dental and surgical instruments) and via body fluids, such as tears, saliva, and semen. A vaccine that provides immunity to hepatitis B is recommended for hospital personnel, dentists, laboratory technicians, and persons requiring frequent transfusions. Hepatitis C is transmitted by blood or blood products; transmission through sexual contact and from mother to infant is rare. About 10 percent of patients develop hepatic fibrosis and cirrhosis. In all types, liver enzymes may be elevated, indicating damage to liver tissue. Symptoms include malaise, anorexia, occasional joint pain, and in severe cases, nausea and jaundice.