Intestinal failure is decreased intestinal function such that nutrients, water, and electrolytes are not sufficiently absorbed. Short bowel syndrome is when there is less than of working bowel and is the most common cause of intestinal failure.

The symptoms of short bowel syndrome can include:

- Abdominal pain

- Diarrhea and steatorrhea (oily, bulky stool, which can be malodorous)

- Fluid depletion

- Weight loss and malnutrition

- Fatigue

Persons with short bowel syndrome may have complications caused by malabsorption of vitamins and minerals, such as deficiencies in vitamins A, D, E, K, B (folic acid), and B, calcium, magnesium, iron, and zinc. These may appear as anemia, hyperkeratosis (scaling of the skin), easy bruising, muscle spasms, poor blood clotting, and bone pain.

Depending on the level of obstruction, bowel obstruction can present with abdominal pain, swollen abdomen, abdominal distension, vomiting, fecal vomiting, and constipation.

Bowel obstruction may be complicated by dehydration and electrolyte abnormalities due to vomiting; respiratory compromise from pressure on the diaphragm by a distended abdomen, or aspiration of vomitus; bowel ischemia or perforation from prolonged distension or pressure from a foreign body.

In small bowel obstruction, the pain tends to be colicky (cramping and intermittent) in nature, with spasms lasting a few minutes. The pain tends to be central and mid-abdominal. Vomiting may occur before constipation.

In large bowel obstruction, the pain is felt lower in the abdomen and the spasms last longer. Constipation occurs earlier and vomiting may be less prominent. Proximal obstruction of the large bowel may present as small bowel obstruction.

The small intestine consists of the duodenum, jejunum and ileum. Inflammation of the small intestine is called enteritis, which if localised to just part is called duodenitis, jejunitis and ileitis, respectively. Peptic ulcers are also common in the duodenum.

Chronic diseases of malabsorption may affect the small intestine, including the autoimmune coeliac disease, infective Tropical sprue, and congenital or surgical short bowel syndrome. Other rarer diseases affecting the small intestine include Curling's ulcer, blind loop syndrome, Milroy disease and Whipple's disease. Tumours of the small intestine include gastrointestinal stromal tumours, lipomas, hamartomas and carcinoid syndromes.

Diseases of the small intestine may present with symptoms such as diarrhoea, malnutrition, fatigue and weight loss. Investigations pursued may include blood tests to monitor nutrition, such as iron levels, folate and calcium, endoscopy and biopsy of the duodenum, and barium swallow. Treatments may include renutrition, and antibiotics for infections.

Symptoms of ileus include, but are not limited to:

- moderate, diffuse abdominal discomfort

- constipation

- abdominal distension

- nausea/vomiting, especially after meals

- vomiting of bilious fluid

- lack of bowel movement and/or flatulence

- excessive belching

Decreased propulsive ability may be broadly classified as caused either by bowel obstruction or intestinal atony or paralysis. However, instances with symptoms and signs of a bowel obstruction occur, but with the absence of a mechanical obstruction, mainly in acute colonic pseudo-obstruction, Ogilvie's syndrome.

Terms such as "functional colonic disease" (or "functional bowel disorder") refer in medicine to a group of bowel disorders which are characterised by chronic abdominal complaints without a structural or biochemical cause that could explain symptoms. Other "functional" disorders relate to other aspects of the process of digestion.

The consensus review process of meetings and publications organised by the Rome Foundation, known as the Rome process, has helped to define the functional gastrointestinal disorders. Successively, the Rome I, Rome II, Rome III and Rome IV proposed consensual classification system and terminology, as recommended by the Rome Coordinating Committee. These now include classifications appropriate for adults, children and neonates / toddlers.

The current Rome IV classification, published in 2016, is as follows:

A. Esophageal Disorders

- A1. Functional chest pain

- A2. Functional heartburn

- A3. Reflux hypersensitivity

- A4. Globus

- A5. Functional dysphagia

B. Gastroduodenal Disorders

- B1. Functional dyspepsia

- B1a. Postprandial distress syndrome (PDS)

- B1b. Epigastric pain syndrome (EPS)

- B2. Belching disorders

- B2a. Excessive supragastric belching

- B2b. Excessive gastric belching

- B3. Nausea and vomiting disorders

- B3a. Chronic nausea vomiting syndrome (CNVS}

- B3b. Cyclic vomiting syndrome (CVS)

- B3c. Cannabinoid hyperemesis syndrome (CHS)

- B4. Rumination syndrome

C. Bowel Disorders

- C1. Irritable bowel syndrome (IBS)

- IBS with predominant constipation (IBS-C)

- IBS with predominant diarrhea (IBS-D)

- IBS with mixed bowel habits (IBS-M)

- IBS unclassified (IBS-U)

- C2. Functional constipation

- C3. Functional diarrhea

- C4. Functional abdominal bloating/distension

- C5. Unspecified functional bowel disorder

- C6. Opioid-induced constipation

D. Centrally Mediated Disorders of Gastrointestinal Pain

- D1. Centrally mediated abdominal pain syndrome (CAPS)

- D2. Narcotic bowel syndrome (NBS)/ Opioid-induced GI hyperalgesia

E. Gallbladder and Sphincter of Oddi disorders

- E1. Biliary pain

- E1a. Functional gallbladder disorder

- E1b. Functional biliary sphincter of Oddi disorder

- E2. Functional pancreatic sphincter of Oddi disorder

F. Anorectal Disorders

- F1. Fecal incontinence

- F2. Functional anorectal pain

- F2a. Levator ani syndrome

- F2b. Unspecified functional anorectal pain

- F2c. Proctalgia fugax

- F3. Functional defecation disorders

- F3a. Inadequate defecatory propulsion

- F3b. Dyssynergic defecation

G. Childhood Functional GI Disorders: Neonate/Toddler

- G1. Infant regurgitation

- G2. Rumination syndrome

- G3. Cyclic vomiting syndrome (CVS)

- G4. Infant colic

- G5. Functional diarrhea

- G6. Infant dyschezia

- G7. Functional constipation

H. Childhood Functional GI Disorders: Child/Adolescent

- H1. Functional nausea and vomiting disorders

- H1a. Cyclic vomiting syndrome (CVS)

- H1b. Functional nausea and functional vomiting

- H1b1. Functional nausea

- H1b2. Functional vomiting

- H1c. Rumination syndrome

- H1d. Aerophagia

- H2. Functional abdominal pain disorders

- H2a. Functional dyspepsia

- H2a1. Postprandial distress syndrome

- H2a2. Epigastric pain syndrome

- H2b. Irritable bowel syndrome (IBS)

- H2c. Abdominal migraine

- H2d. Functional abdominal pain ‒ NOS

- H3. Functional defecation disorders

- H3a. Functional constipation

- H3b. Nonretentive fecal incontinence

Causes of small bowel obstruction include:

- Adhesions from previous abdominal surgery (most common cause)

- Barbed sutures.

- Pseudoobstruction

- Hernias containing bowel

- Crohn's disease causing adhesions or inflammatory strictures

- Neoplasms, benign or malignant

- Intussusception

- Volvulus

- Superior mesenteric artery syndrome, a compression of the duodenum by the superior mesenteric artery and the abdominal aorta

- Ischemic strictures

- Foreign bodies (e.g. gallstones in gallstone ileus, swallowed objects)

- Intestinal atresia

After abdominal surgery, the incidence of small bowel obstruction from any cause is 9%. In those where the cause of the obstruction was clear, adhesions are the single most common cause (more than half).

Diseases that affect the large intestine may affect it in whole or in part. Appendicitis is one such disease, caused by inflammation of the appendix. Generalised inflammation of the large intestine is referred to as colitis, which when caused be the bacteria "Clostridium difficile" is referred to as pseudomembranous colitis. Diverticulitis is a common cause of abdominal pain resulting from outpouchings that particularly affects the colon. Functional colonic diseases refer to disorders without a known cause, and include irritable bowel syndrome and intestinal pseudoobstruction. Constipation may result from lifestyle factors, impaction of a rigid stool in the rectum, or in neonates, Hirschprung's disease.

Diseases affecting the large intestine may cause blood to be passed with stool, may cause constipation, or may result in abdominal pain or a fever. Tests that specifically examine the function of the large intestine include barium swallows, abdominal x-rays, and colonoscopy.

External signs and symptoms are constipation of very long duration, abdominal bloating, abdominal tenderness and tympany, abdominal pain, palpation of hard fecal masses and, in toxic megacolon, fever, low blood potassium, tachycardia and shock. Stercoral ulcers are sometimes observed in chronic megacolon, which may lead to perforation of the intestinal wall in approximately 3% of the cases, leading to sepsis and risk of death.

In addition to the extent of involvement, people may also be characterized by the severity of their disease.

- "Mild disease" correlates with fewer than four stools daily, with or without blood, no systemic signs of toxicity, and a normal erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP). Mild abdominal pain or cramping may occur. Patients may believe they are constipated when in fact they are experiencing tenesmus, which is a constant feeling of the need to empty the bowel accompanied by involuntary straining efforts, pain, and cramping with little or no fecal output. Rectal pain is uncommon.

- "Moderate disease" correlates with more than four stools daily, but with minimal signs of toxicity. Patients may display anemia (not requiring transfusions), moderate abdominal pain, and low grade fever, .

- "Severe disease", correlates with more than six bloody stools a day or observable massive and significant bloody bowel movement, and evidence of toxicity as demonstrated by fever, tachycardia, anemia or an elevated ESR or CRP.

- "Fulminant disease" correlates with more than ten bowel movements daily, continuous bleeding, toxicity, abdominal tenderness and distension, blood transfusion requirement and colonic dilation (expansion). Patients in this category may have inflammation extending beyond just the mucosal layer, causing impaired colonic motility and leading to toxic megacolon. If the serous membrane is involved, a colonic perforation may ensue. Unless treated, the fulminant disease will soon lead to death.

Functional gastrointestinal disorders are very common. Globally, irritable bowel syndrome and functional dyspepsia alone may affect 16–26% of the population.

Ulcerative colitis is normally continuous from the rectum up the colon. The disease is classified by the extent of involvement, depending on how far the disease extends:

- "Distal colitis", potentially treatable with enemas:

- "Proctitis": Involvement limited to the rectum.

- "Proctosigmoiditis": Involvement of the rectosigmoid colon, the portion of the colon adjacent to the rectum.

- "Left-sided colitis": Involvement of the descending colon, which runs along the patient's left side, up to the splenic flexure and the beginning of the transverse colon.

- "Extensive colitis", inflammation extending beyond the reach of enemas:

- "Pancolitis": Involvement of the entire colon, extending from the rectum to the cecum, beyond which the small intestine begins.

Megacolon is an abnormal dilation of the colon (also called the large intestine). The dilation is often accompanied by a paralysis of the peristaltic movements of the bowel. In more extreme cases, the feces consolidate into hard masses inside the colon, called fecalomas (literally, "fecal tumor"), which can require surgery to be removed.

A human colon is considered abnormally enlarged if it has a diameter greater than 12 cm in the cecum (it is usually less than 9 cm), greater than 6.5 cm in the rectosigmoid region and greater than 8 cm for the ascending colon. The transverse colon is usually less than 6 cm in diameter.

A megacolon can be either acute or chronic. It can also be classified according to cause.

The condition is typically seen in premature infants, and the timing of its onset is generally inversely proportional to the gestational age of the baby at birth (i.e. the earlier a baby is born, the later signs of NEC are typically seen). Initial symptoms include feeding intolerance, increased gastric residuals, abdominal distension and bloody stools. Symptoms may progress rapidly to abdominal discoloration with intestinal perforation and peritonitis and systemic hypotension requiring intensive medical support.

Bacterial overgrowth can cause a variety of symptoms, many of which are also found in other conditions, making the diagnosis challenging at times. Many of the symptoms are due to malabsorption of nutrients due to the effects of bacteria which either metabolize nutrients or cause inflammation of the small bowel, impairing absorption. The symptoms of bacterial overgrowth include nausea, flatus, constipation, bloating, abdominal distension, abdominal pain or discomfort, diarrhea, fatigue, and weakness. SIBO also causes an increased permeability of the small intestine. Some patients may lose weight. Children with bacterial overgrowth may develop malnutrition and have difficulty attaining proper growth. Steatorrhea, a sticky type of diarrhea where fats are not properly absorbed and spill into the stool, may also occur.

Patients with bacterial overgrowth that is longstanding can develop complications of their illness as a result of malabsorption of nutrients. Anemia may occur from a variety of mechanisms, as many of the nutrients involved in production of red blood cells are absorbed in the affected small bowel. Iron is absorbed in the more proximal parts of the small bowel, the duodenum and jejunum, and patients with malabsorption of iron can develop a microcytic anemia, with small red blood cells. Vitamin B is absorbed in the last part of the small bowel, the ileum, and patients who malabsorb vitamin B can develop a megaloblastic anemia with large red blood cells.

In older adults, small bowel bacterial overgrowth is associated with a higher frequency of diarrhea, a lower body mass index, and a significantly lower serum albumin concentration.

Constipation is a symptom, not a disease. Most commonly, constipation is thought of as infrequent bowel movements, usually less than 3 stools per week. However, people may have other complaints as well including:

- Straining with bowel movements

- Excessive time needed to pass a bowel movement

- Hard stools

- Pain with bowel movements secondary to straining

- Abdominal pain

- Abdominal bloating.

- the sensation of incomplete bowel evacuation.

The Rome Criteria are a set of symptoms that help standardize the diagnosis of constipation in various age groups. These criteria help physicians to better define constipation in a standardized manner.

The diagnosis is usually suspected clinically but often requires the aid of diagnostic imaging modalities, most commonly radiography. Specific radiographic signs of NEC are associated with specific Bell's stages of the disease:

Bell's stage 1/Suspected disease:

- Mild systemic disease (apnea, lethargy, bradycardia, temperature instability)

- Mild intestinal signs (abdominal distention, increased gastric residuals, bloody stools)

- Non-specific or normal radiological signs

Bell's stage 2/Definite disease:

- Mild to moderate systemic signs

- Additional intestinal signs (absent bowel sounds, abdominal tenderness)

- Specific radiologic signs (pneumatosis intestinalis or portal venous air)

- Laboratory changes (metabolic acidosis, thrombocytopaenia)

Bell's stage 3/Advanced disease:

- Severe systemic illness (hypotension)

- Additional intestinal signs (striking abdominal distention, peritonitis)

- Severe radiologic signs (pneumoperitoneum)

- Additional laboratory changes (metabolic and respiratory acidosis, disseminated intravascular coagulation)

More recently ultrasonography has proven to be useful as it may detect signs and complications of NEC before they are evident on radiographs, specifically in cases that involve a paucity of bowel gas, a gasless abdomen, or a sentinel loop. Diagnosis is ultimately made in 5–10% of very low-birth-weight infants (<1,500g).

Many people with Crohn's disease have symptoms for years before the diagnosis. The usual onset is between 15 and 30 years of age, but can occur at any age. Because of the 'patchy' nature of the gastrointestinal disease and the depth of tissue involvement, initial symptoms can be more subtle than those of ulcerative colitis. People with Crohn's disease experience chronic recurring periods of flare-ups and remission.

Abdominal pain may be the initial symptom of Crohn's disease usually in the lower right area. It is often accompanied by diarrhea, especially in those who have had surgery. The diarrhea may or may not be bloody. The nature of the diarrhea in Crohn's disease depends on the part of the small intestine or colon involved. Ileitis typically results in large-volume, watery feces. Colitis may result in a smaller volume of feces of higher frequency. Fecal consistency may range from solid to watery. In severe cases, an individual may have more than 20 bowel movements per day and may need to awaken at night to defecate. Visible bleeding in the feces is less common in Crohn's disease than in ulcerative colitis, but may be seen in the setting of Crohn's colitis. Bloody bowel movements typically come and go, and may be bright or dark red in color. In the setting of severe Crohn's colitis, bleeding may be copious. Flatulence and bloating may also add to the intestinal discomfort.

Symptoms caused by intestinal stenosis are also common in Crohn's disease. Abdominal pain is often most severe in areas of the bowel with stenoses. Persistent vomiting and nausea may indicate stenosis from small bowel obstruction or disease involving the stomach, pylorus, or duodenum. Although the association is greater in the context of ulcerative colitis, Crohn's disease may also be associated with primary sclerosing cholangitis, a type of inflammation of the bile ducts.

Perianal discomfort may also be prominent in Crohn's disease. Itchiness or pain around the anus may be suggestive of inflammation, fistulization or abscess around the anal area or anal fissure. Perianal skin tags are also common in Crohn's disease and may appear with or without the presence of colorectal polyps. Fecal incontinence may accompany perianal Crohn's disease. At the opposite end of the gastrointestinal tract, the mouth may be affected by recurrent sores (aphthous ulcers). Rarely, the esophagus, and stomach may be involved in Crohn's disease. These can cause symptoms including difficulty swallowing (dysphagia), upper abdominal pain, and vomiting.

Intestinal Connective tissue abnormality may cause Intestinal Desmosis The absence of the tendinous plexus layer was first described in 1998 by Meier-Ruge. Desmosis is implicated in disturbed gut motility.

Normal peristalsis depends upon the interaction between muscles, nerve cells and tendinous connective tissue. A malfunction of any of these leads to intestinal motility disorders.

Desmosis may be congenital (aplastic form) or acquired (atrophic form).

The "aplastic" form is rare. Typical clinical findings are hypoperistalsis, and pseudo-obstruction. These are found in premature infants, associated with low birth weight. The "atrophic" form is more frequent. Inflammation of the muscularis propria releases enzymes including collagenases which destroy the connective tissue of the bowel wall. Primarily newborns and small children are affected, although this manifestation can also be found in adults. The most common location is the colon with a necrotizing enterocolitis as well as Crohn Disease and diverticulitis. If the taenia are also affected, the disease is defined as complete atrophic desmosis, all other forms without involvement of the taenia are referred to as incomplete. Clinically, patients demonstrate chronic constipation.

As proposed by Giuseppe Martucciello, microscopic diagnosis requires laparoscopic intestinal full-thickness biopsies from colon. Histological findings are absence of the tendinous plexus layer and connective tissue fibers in longitudinal and circular muscle layer.

Intestinal neuronal dysplasia (or neuronal intestinal dysplasia or NID) is an inherited disease of the intestine that affects one in 3000 children and adults. The intestine uses peristalsis to push its contents toward the anus; IND sufferers have a problem with the motor neurons that lead to the intestine, inhibiting this process and thus preventing digestion.

It can often be confused for Hirschsprung's disease, as both have similar symptoms.

Although some cases present with black, tarry stool (melena), the blood loss can be subtle, with the anemia symptoms predominating. Fecal occult blood testing is positive when bleeding is active. If bleeding is intermittent the test may be negative at times.

Small intestine bacterial overgrowth (SIBO), also termed bacterial overgrowths, or small bowel bacterial overgrowth syndrome (SBBOS), is a disorder of excessive bacterial growth in the small intestine. Unlike the colon (or large bowel), which is rich with bacteria, the small bowel usually has fewer than 10,000 organisms per millilitre. Patients with bacterial overgrowth typically develop symptoms including nausea, bloating, vomiting, diarrhea, malnutrition, weight loss and malabsorption, which is caused by a number of mechanisms.

The diagnosis of bacterial overgrowth is made by a number of techniques, with the gold standard being an aspirate from the jejunum that grows in excess of 10 bacteria per millilitre. Risk factors for the development of bacterial overgrowth include dysmotility; anatomical disturbances in the bowel, including fistulae, diverticula and blind loops created after surgery, and resection of the ileo-cecal valve; gastroenteritis-induced alterations to the small intestine; and the use of certain medications, including proton pump inhibitors.

Small bowel bacterial overgrowth syndrome is treated with an elemental diet or antibiotics, which may be given in a cyclic fashion to prevent tolerance to the antibiotics, sometimes followed by prokinetic drugs to prevent recurrence if dysmotility is a suspected cause.

Constipation refers to bowel movements that are infrequent or hard to pass. The stool is often hard and dry. Other symptoms may include abdominal pain, bloating, and feeling as if one has not completely passed the bowel movement. Complications from constipation may include hemorrhoids, anal fissure or fecal impaction. The normal frequency of bowel movements in adults is between three per day and three per week. Babies often have three to four bowel movements per day while young children typically have two to three per day.

Constipation has many causes. Common causes include slow movement of stool within the colon, irritable bowel syndrome, and pelvic floor disorders. Underlying associated diseases include hypothyroidism, diabetes, Parkinson's disease, celiac disease, non-celiac gluten sensitivity, colon cancer, diverticulitis, and inflammatory bowel disease. Medications associated with constipation include opioids, certain antacids, calcium channel blockers, and anticholinergics. Of those taking opioids about 90% develop constipation. Constipation is more concerning when there is weight loss or anemia, blood is present in the stool, there is a history of inflammatory bowel disease or colon cancer in a person's family, or it is of new onset in someone who is older.

Treatment of constipation depends on the underlying cause and the duration that it has been present. Measures that may help include drinking enough fluids, eating more fiber, and exercise. If this is not effective, laxatives of the bulk forming agent, osmotic agent, stool softener, or lubricant type may be recommended. Stimulant laxatives are generally reserved for when other types are not effective. Other treatments may include biofeedback or in rare cases surgery.

In the general population rates of constipation are 2–30 percent. Among elderly people living in a care home the rate of constipation is 50–75 percent. People spend, in the United States, more than on medications for constipation a year.

Crohn's disease, like many other chronic, inflammatory diseases, can cause a variety of systemic symptoms. Among children, growth failure is common. Many children are first diagnosed with Crohn's disease based on inability to maintain growth. As it may manifest at the time of the growth spurt in puberty, up to 30% of children with Crohn's disease may have retardation of growth. Fever may also be present, though fevers greater than 38.5 °C (101.3 °F) are uncommon unless there is a complication such as an abscess. Among older individuals, Crohn's disease may manifest as weight loss, usually related to decreased food intake, since individuals with intestinal symptoms from Crohn's disease often feel better when they do not eat and might lose their appetite. People with extensive small intestine disease may also have malabsorption of carbohydrates or lipids, which can further exacerbate weight loss.