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.

Clinical features of intestinal pseudo-obstruction can include abdominal pain, nausea, severe distension, vomiting, dysphagia, diarrhea and constipation, depending upon the part of the gastrointestinal tract involved. In addition, in the moments in which abdominal colic occurs, an abdominal x-ray shows intestinal air fluid level. All of these features are also similar in true mechanical obstruction of the bowel.

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.

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.

Intestinal pseudo-obstruction is a clinical syndrome caused by severe impairment in the ability of the intestines to push food through. It is characterized by the signs and symptoms of intestinal obstruction without any lesion in the intestinal lumen. Clinical features can include abdominal pain, nausea, severe distension, vomiting, dysphagia, diarrhea and constipation, depending upon the part of the gastrointestinal tract involved. The condition can begin at any age and it can be a primary condition (idiopathic or inherited) or caused by another disease (secondary).

It can be chronic or acute.

Early symptoms can include periodic abdominal pain, nausea, vomiting (sometimes green in color from bile), pulling legs to the chest area, and intermittent moderate to severe cramping abdominal pain. Pain is intermittent—not because the intussusception temporarily resolves, but because the intussuscepted bowel segment transiently stops contracting. Later signs include rectal bleeding, often with "red currant jelly" stool (stool mixed with blood and mucus), and lethargy. Physical examination may reveal a "sausage-shaped" mass, felt upon palpating the abdomen. Children, or those unable to communicate symptoms verbally, may cry, draw their knees up to their chest, or experience dyspnea (difficult or painful breathing) with paroxysms of pain.

Fever is not a symptom of intussusception. However, intussusception can cause a loop of bowel to become necrotic, secondary to ischemia due to compression to arterial blood supply. This leads to perforation and sepsis, which causes fever.

In rare cases, intussusception may be a complication of Henoch-Schönlein purpura (HSP), an immune-mediated vasculitis disease in children. Such patients who develop intussusception often present with severe abdominal pain in addition to the classic signs and symptoms of HSP.

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).

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

Intussusception is a medical condition in which a part of the intestine folds into the section next to it. It typically involves the small bowel and less commonly the large bowel. Symptoms include abdominal pain which may come and go, vomiting, abdominal bloating, and bloody stool. It often results in a small bowel obstruction. Other complications may include peritonitis or bowel perforation.

The cause in children is typically unknown while in adults a lead point is often present. Risk factors in children include certain infections, diseases like cystic fibrosis, and intestinal polyps. Risk factors in adults include endometriosis, bowel adhesions, and intestinal tumors. Diagnosis is often supported by medical imaging. In children, ultrasound is preferred while in adults a CT scan is preferred.

Intussusception is an emergency requiring rapid treatment. Treatment in children is typically by an enema with surgery used if this is not successful. Dexamethasone may decrease the risk of another episode. In adults, surgical removal of part of the bowel is more often required. Intussusception occurs more commonly in children than adults. In children, males are more often affected than females. The usual age of occurrence is six to eighteen months old.

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.

Acute abdomen is occasionally used synonymously with peritonitis. While this is not entirely incorrect, peritonitis is the more specific term, referring to inflammation of the peritoneum. It manifests on physical examination as rebound tenderness, or pain upon "removal" of pressure more than on "application" of pressure to the abdomen. Peritonitis may result from several of the above diseases, notably appendicitis and pancreatitis. While rebound tenderness is commonly associated with peritonitis, the most specific finding is rigidity.

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

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.

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.

Colic can be divided broadly into several categories:

1. excessive gas accumulation in the intestine (gas colic)

2. simple obstruction

3. strangulating obstruction

4. non-strangulating infarction

5. inflammation of the gastrointestinal tract (enteritis, colitis) or the peritoneum (peritonitis)

6. ulceration of the gastrointestinal mucosa

These categories can be further differentiated based on location of the lesion and underlying cause (See Types of colic).

On rare occasions, a piece of small intestine (or rarely colon) can become trapped through the epiploic foramen into the omental bursa. The blood supply to this piece of intestine is immediately occluded and surgery is the only available treatment. This type of colic has been associated with cribbers, possibly due to changes in abdominal pressure, and in older horses, possibly because the foramen enlarges as the right lobe of the liver atrophies with age, although it has been seen in horses as young as 4 months old. Horses usually present with colic signs referable to small intestinal obstruction. During surgery, the foramen can not be enlarged due to the risk of rupture of the vena cava or aorta, which would result in fatal hemorrhage. Survival is 74–79%, and survival is consistently correlated with abdominocentesis findings prior to surgery.

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.

Vascular disorders are more likely to affect the small bowel than the large bowel. Arterial supply to the intestines is provided by the superior and inferior mesenteric arteries (SMA and IMA respectively), both of which are direct branches of the aorta.

The superior mesenteric artery supplies:

1. Small bowel

2. Ascending and proximal two-thirds of the transverse colon

The inferior mesenteric artery supplies:

1. Distal one-third of the transverse colon

2. Descending colon

3. Sigmoid colon

Of note, the splenic flexure, or the junction between the transverse and descending colon, is supplied by the most distal portions of both the inferior mesenteric artery and superior mesenteric artery, and is thus referred to medically as a watershed area, or an area especially vulnerable to ischemia during periods of systemic hypoperfusion, such as in shock.

Acute abdomen of the ischemic variety is usually due to:

1. A thromboembolism from the left side of the heart, such as may be generated during atrial fibrillation, occluding the SMA.

2. Nonocclusive ischemia, such as that seen in hypotension secondary to heart failure, may also contribute, but usually results in a mucosal or mural infarct, as contrasted with the typically transmural infarct seen in thromboembolus of the SMA.

3. Primary mesenteric vein thromboses may also cause ischemic acute abdomen, usually precipitated by hypercoagulable states such as polycythemia vera.

Clinically, patients present with diffuse abdominal pain, bowel distention, and bloody diarrhea. On physical exam, bowel sounds will be absent. Laboratory tests reveal a neutrophilic leukocytosis, sometimes with a left shift, and increased serum amylase. Abdominal radiography will show many air-fluid levels, as well as widespread edema.

Acute ischemic abdomen is a surgical emergency. Typically, treatment involves removal of the region of the bowel that has undergone infarction, and subsequent anastomosis of the remaining healthy tissue.

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.

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.

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.

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.

Obstructed defecation, is "difficulty in evacuation or emptying the rectum [which] may occur even with frequent visits to the toilet and even with passing soft motions". The conditions that can create the symptom are sometimes grouped together as defecation disorders. The symptom tenesmus is a closely related topic.

Another source defines evacuatory dysfunction as "a constellation of symptoms such as prolonged repeated straining at bowel movements, sensation of incomplete evacuation, and the need for digital manipulation".

Some describe an "obstructed defecation syndrome", defining it loosely as "difficulty in evacuation, which may or may not be associated with constipation"

Others inappropriately equate obstructed defecation with anismus. Although anismus is a type of obstructed defecation, obstructed defecation has many other possible causes other than anismus.