Disorders and Diseases of the Human Digestive System

Disorders and Diseases of the Human Digestive System
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The primary purpose of the gastrointestinal tract is to break food down into nutrients, which can be absorbed into the body to provide energy.

Food must be ingested into the mouth to be mechanically processed and moistened. Secondly, digestion occurs mainly in the stomach and small intestine where proteins, fats, and carbohydrates are chemically broken down into their basic building blocks.

Smaller molecules are then absorbed across the epithelium of the small intestine and subsequently enter the circulation.

The large intestine plays a key role in reabsorbing excess water. Finally, undigested material and secreted waste products are excreted from the body via defecation (passing of feces).

In the case of gastrointestinal disease or disorders, these functions of the gastrointestinal tract are not achieved successfully.

Patients in such cases may develop symptoms of nausea, vomiting, diarrhea, malabsorption, constipation or obstruction.

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Diseases of the Mouth

Physical damage

The injury may be caused to tissues in and around the mouth by foods and other substances taken into the mouth if they are: excessively hot or cold, abrasive, or corrosive. Corrosive chemicals are the most likely to cause serious tissue damage and acute inflammation. The outcome depends on the extent and depth of the injury.

Thrush (oral candidiasis)

This acute fungal infection of the epithelium of the mouth is caused by the yeast Candida albicans. In adults it causes infection mainly in debilitated people and in those whose immunity is suppressed by steroids, antibiotics or cytotoxic drugs. In babies it may be a severe infection, sometimes causing epidemics in nurseries by cross-infection. The fungus survives in the fine grooves on the upper surface of the denture and repeatedly reinfects the epithelium.

Angular cheilitis

Caused by Candida albicans and Staphylococcus aureus, painful cracks develop in folds of tissue at the corners of the mouth, usually occurring in elderly debilitated people.

Acute gingivitis (Vincent’s infection)

This is an acute infection with severe ulceration of the lips, gums, mouth, throat, and the palatine tonsil caused by two commensal organisms acting together, Borrelia Vincenti and a fusiform bacillus.

Acute herpetic gingivostomatitis

This is caused by the Herpes simplex virus and is the commonest oral virus infection. It is characterized by extensive and very painful ulceration.

Squamous cell carcinoma

This is the most common type of malignant tumor in the mouth. The usual sites are the lower lip and the edge of the tongue. Ulceration occurs frequently and there is early spread to surrounding tissues and cervical lymph nodes.

Diseases of the Salivary Glands


This is an acute inflammatory condition of the salivary glands, especially the parotids. It is caused by the mumps virus, one of the parainfluenza group.

Calculus formation

Calculi (stones) are formed in the salivary glands by the crystallization of mineral salts in saliva. They may partially or completely block the ducts, leading to swelling of the gland, a predisposition to infection, and, in time, atrophy.

Mixed tumors (pleomorphic salivary adenoma)

This benign tumor consists of epithelial and connective tissue cells and occurs mainly in the parotid gland.


Malignant tumors may occur in any salivary gland or duct. Some forms have a tendency to infiltrate nerves in the surrounding tissues, causing severe pain.

Diseases of the Pharynx


Tonsillitis is inflammation of the tonsils, two oval-shaped pads of tissue at the back of the throat — one tonsil on each side. It is a type of pharyngitis.  Signs and symptoms of tonsillitis include swollen tonsils, sore throat, difficulty swallowing and tender lymph nodes on the sides of the neck. Complications include peritonsillar abscess.


Diphtheria is an infectious disease caused by the bacterium Corynebacterium diphtheria, which primarily infects the throat and upper airways, and produces a toxin affecting other organs. The main characteristics are sore throat, low fever and swollen glands in the neck, and the toxin may, in severe cases, cause myocarditis or peripheral neuropathy. The diphtheria toxin causes a membrane of dead tissue to build up over the throat and tonsils, making breathing and swallowing difficult. 

Diseases of the Oesophagus

Peptic reflux oesophagitis

This condition, the commonest cause of indigestion, is caused by persistent regurgitation of acid gastric juice into the esophagus, causing irritation and painful ulceration. Hemorrhage occurs when blood vessels are eroded.

Swallowing caustic materials

When swallowed, caustic materials burn the walls of the esophagus causing an inflammatory reaction. The extent of the damage depends on the concentration and amount swallowed. Following a severe injury, healing causes fibrosis, and there is a risk of oesophageal stricture developing later, as the fibrous tissue shrinks.


This problem tends to occur in young adults. The cardiac sphincter is constricted and, because of this obstruction blocking the passage of ingested materials into the stomach, the esophagus becomes dilated and the muscle layer hypertrophies. The condition may lead to dysphagia, regurgitation of gastric contents, and possibly aspiration pneumonia.

Oesophageal rupture

This may occur, usually at the distal end, if the esophagus is suddenly distended: during a vomiting attack, by ingestion of foreign bodies or by the passage of an instrument. Gastric contents pass into the mediastinum, causing acute inflammation.


Scirrhous (fibrous) tumors. These spread around the circumference and along the esophagus. They cause thickening of the wall and loss of elasticity.

Soft tissue tumors. These grow into the lumen and spread along the wall.

Diseases of the Stomach


This is a common condition that occurs when an imbalance between the corrosive action of gastric juice and the protective effect of mucus on the gastric mucosa develops. The amount of mucus in the stomach is insufficient to protect the surface epithelium from the destructive effects of hydrochloric acid. It may be acute or chronic.

Peptic ulceration

Ulceration of the gastrointestinal may be viewed as an extension of the cell damage found in acute gastritis. The most common sites for ulcers are the stomach and the first few centimeters of the duodenum. More rarely they occur in the esophagus, following reflux of gastric juice, and round the anastomosis of the stomach and small intestine, following gastrectomy.

Helicobacter-associated gastritis

The microbe Helicobacter pylori are known to be associated with gastric conditions, especially chronic gastritis, and peptic ulcer disease.

Autoimmune chronic gastritis

This is a progressive form of the disease. Destructive inflammatory changes that begin on the surface of the mucous membrane may extend to affect its whole thickness, including the gastric glands. When this stage is reached, the secretion of digestive enzymes, hydrochloric acid, and intrinsic factor are markedly reduced.

Acute peptic ulcers and Chronic peptic ulcers

Acute lesions involve tissue to the depth of the submucosa and the lesions may be single or multiple. They are found in many sites in the stomach and in the first few centimeters of the duodenum.  Chronic ulcers penetrate through the epithelial and muscle layers of the stomach wall and may include the adjacent pancreas or liver. These ulcers may lead to hemorrhage, perforation, pyloric stenosis, and development of a malignant tumor.

Diseases of the Intestines (Small and Large Intestines)


The lumen of the appendix is very small and there is little room for swelling when it becomes inflamed. Microbial infection is commonly superimposed on obstruction by, e.g., hard fecal matter (faecoliths), kinking, or a foreign body. Inflammatory exudate, with fibrin and phagocytes, causes swelling and ulceration of the mucous membrane lining. The rising pressure inside the appendix occludes first the veins, then the arteries and ischemia develops, followed by gangrene and rupture.

Typhoid fever

This type of enteritis is caused by the microbe Salmonella typhi, ingested in food and water. In certain cases, the typhoid fever becomes a chronic, asymptomatic infection of the biliary and urinary tracts. Microbes continue to be excreted indefinitely in urine and feces.

Paratyphoid fever

This disease is caused by Salmonella paratyphi A or B spread in the same way as typhoid fever, i.e. in food and drink contaminated by infected urine or feces. The infection, causing inflammation of the intestinal mucosa, is usually confined to the ileum.

Food poisoning

Food poisoning syndrome results from the ingestion of water and a wide variety of food contaminated with pathogenic microorganisms (bacteria, viruses, protozoa, fungi), their toxins, and chemicals. Most types of food poisoning cause one or more of the following signs and symptoms: nausea, vomiting, watery or bloody diarrhea, abdominal pain and cramps, fever, etc caused by its interaction with the digestive system.


Cholera is caused by Vibrio cholerae and is spread by contaminated water, food, hands, and fomites. This leads to persistent diarrhea, severe dehydration, and electrolyte imbalance, and may cause death due to hypovolaemic shock. The microbes occasionally spread to the gall bladder where they multiply. They are then excreted in bile and feces.

Amoebic dysentery

This disease is caused by Entamoeba histolytica. The disease may progress in a number of ways. Healing may produce fibrous adhesions, causing a partial or complete obstruction. The amoebae may spread to the liver, causing amoebic hepatitis and abscesses.  Chronic dysentery may develop with intermittent diarrhea and amoebae in the feces.

Crohn’s disease (regional ileitis)

This chronic inflammatory condition of the alimentary tract usually occurs in young adults. The terminal ileum and the rectum are most commonly affected but the disease may be more widespread. There is chronic patchy inflammation with edema of the full thickness of the intestinal wall, causing partial obstruction of the lumen There are periods of remission of varying duration.

Ulcerative colitis

This is a chronic inflammatory disease of the mucosa of the colon and rectum which may ulcerate and become infected. It usually occurs in young adults and begins in the rectum and sigmoid colon. From there it may spread to involve a variable proportion of the colon and, sometimes, the entire colon.

Tumors of the small and large intestines

Benign and malignant tumors of the small intestine are rare, compared with their occurrence in the stomach and colon.


A hernia is a protrusion of bowel through a weak point in the musculature of the anterior abdominal wall or an existing opening. It occurs when there are intermittent increases in intra-abdominal pressure, most commonly in men who lift heavy loads at work.


This occurs when a loop of bowel twists through 180°, cutting off its blood supply, causing gangrene and obstruction. It occurs in parts of the intestine that are attached to the posterior abdominal wall by a long double fold of visceral peritoneum, the mesentery.

Intestinal Obstruction

It may be caused by any of the following causes:

  • Stenosis and thickening of the intestinal wall, e.g. in diverticulosis, Crohn’s disease and malignant tumors; there is usually a gradual progression from partial to complete obstruction
  • Obstruction by, e.g., a large gallstone or a tumor growing into the lumen
  • Pressure on the intestine from outside, e.g. a large tumor in any pelvic or abdominal organ, such as a uterine fibroid; this type is most likely to occur inside the confined space of the bony pelvis.


Impaired absorption of nutrient materials and water from the intestines is not a disease in itself. It is the result of diseases causing one or more of the following changes:

  • atrophy of the villi of the mucosa of the small intestine incomplete digestion of food
  • interference with the transport of absorbed nutrients from the small intestine to the blood.

Diseases of the Pancreas

Acute pancreatitis

Proteolytic enzymes produced by the pancreas are secreted in inactive forms, which are not activated until they reach the intestine; this protects the pancreas from digestion by its own enzymes. If these precursor enzymes are activated while still in the pancreas, pancreatitis results. The severity of the disease is directly related to the amount of pancreatic tissue destroyed. Common complications include infection, suppuration, and local venous thrombosis.

Chronic pancreatitis

This is due to repeated attacks of acute pancreatitis or may arise gradually without evidence of pancreatic disease. There is obstruction of the tiny acinar ducts by protein material secreted by the acinar cells. This eventually leads to the formation of cysts which may rupture into the peritoneal cavity. Intact cysts may cause obstruction of the common bile duct, causing jaundice and portal vein, causing venous congestion in the organs drained by its tributaries.

Malignant tumors

These are relatively common and affect men more than women. They occur most frequently in the head of the pancreas, obstructing the flow of bile and pancreatic juice into the duodenum. Jaundice and acute pancreatitis usually develop.

Diseases of the Liver

Acute hepatitis

Areas of necrosis develop as groups of hepatocytes die and the eventual outcome depends on the size and number of these areas. Causes of the damage may be a variety of conditions, including viral infections, toxic substances, or circulatory disturbances.

Viral Hepatitis

Viral hepatitis is most commonly caused by the hepatitis A virus (HAV), hepatitis B virus (HBV), and hepatitis C virus (HCV). These three viruses can all result in acute disease with symptoms of nausea, abdominal pain, fatigue, malaise, and jaundice. Additionally, acute infection with HBV and HCV can lead to chronic infection. Patients who are chronically infected may go on to develop cirrhosis and hepatocellular carcinoma (HCC).

Chronic hepatitis

This is defined as any form of hepatitis which persists for more than 6 months.

Chronic persistent hepatitis: This is a mild, persistent inflammation following acute viral hepatitis. There is usually little or no fibrosis.

Chronic active hepatitis: This is a continuing progressive inflammation with cell necrosis and the formation of fibrous tissue that may lead to cirrhosis of the liver. There is a distortion of the liver blood vessels and hypoxia, leading to further hepatocyte damage.

Ascending cholangitis

Infection, usually by Escherichia coli, may spread from the biliary tract. The most common predisposing factor is obstruction of the common bile duct by gallstones.

Liver abscess

Septic emboli from septic foci in the abdomen and pelvis may lodge in branches of the portal vein and cause multiple abscesses or infect the vein, causing portal pylephlebitis.

Liver failure

This occurs when liver function is reduced to such an extent that other body activities are impaired. It may be acute or chronic and may be the outcome of a wide variety of disorders.

Diseases of the Gall Bladder and Bile Ducts

Gallstones (cholelithiasis)

Gallstones consist of deposits of the constituents of bile, most commonly cholesterol. Many small stones or one large stone may form. It may lead to:

Biliary colic. If a gallstone gets stuck in the cystic or common bile duct there is a strong peristaltic contraction of the smooth muscle in the wall of the duct (spasm) in an effort to move the stone onwards.

Inflammation. Gallstones cause irritation and inflammation of the walls of the gall bladder and the cystic and common bile ducts.

Impaction. The blockage of the cystic duct by a gallstone leads to distension of the gall bladder and cholecystitis. This does not cause jaundice because bile from the liver can still pass directly into the duodenum. Obstruction of the common bile duct leads to retention of bile, jaundice, and cholangitis (infection of the bile ducts).


Jaundice develops when there is an abnormality at some stage in the metabolic sequence caused by one or more factors, e.g.:

  • excess hemolysis of red blood cells with the production of more bilirubin than the liver can deal with
  • an abnormal liver function that may cause:

— incomplete uptake of unconjugated bilirubin by hepatocytes

— ineffective conjugation of bilirubin

— interference with bilirubin secretion into the bile

  • obstruction to the flow of bile from the liver to the duodenum.


  1. Waugh, A., & Grant, A. (2009). Ross and Wilson: Anatomy and Physiology in Health and Illness. (11th edition). Churchill Livingstone
  2. Chung, K. W., Chung, H. M., & Halliday, N. L. (2015). BRS Gross anatomy (Eighth edition.). Philadelphia: Wolters Kluwer Health. 
  3. https://www.myvmc.com/anatomy/gastrointestinal-system/
  4. https://emedicine.medscape.com/article/775507-overview
  5. https://www.cdc.gov/diphtheria/index.html
  6. https://www.who.int/immunization/diseases/diphtheria/en/
  7. https://maflekumen-institute.org/wp-content/uploads/2013/05/food_poisoning-bacteriology.pdf

Disorders and Diseases of the Digestive System

About Author

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Sagar Aryal

Sagar Aryal is a microbiologist and a scientific blogger. He is doing his Ph.D. at the Central Department of Microbiology, Tribhuvan University, Kathmandu, Nepal. He was awarded the DAAD Research Grant to conduct part of his Ph.D. research work for two years (2019-2021) at Helmholtz-Institute for Pharmaceutical Research Saarland (HIPS), Saarbrucken, Germany. Sagar is interested in research on actinobacteria, myxobacteria, and natural products. He is the Research Head of the Department of Natural Products, Kathmandu Research Institute for Biological Sciences (KRIBS), Lalitpur, Nepal. Sagar has more than ten years of experience in blogging, content writing, and SEO. Sagar was awarded the SfAM Communications Award 2015: Professional Communicator Category from the Society for Applied Microbiology (Now: Applied Microbiology International), Cambridge, United Kingdom (UK). Sagar is also the ASM Young Ambassador to Nepal for the American Society for Microbiology since 2023 onwards.

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