Small Intestine

The small intestine is that part of the gastrointestinal tract that occurs after the stomach and culminates or ends into the large intestine. It is that place, where most of food that is consumed is digested and the nutrients from food are absorbed.

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The size of small intestine

An adult male has a small intestine that has an average length of 22 feet and 6 inches or 6.9 meters, whereas the average length of the small intestine in an adult female is 23 feet and 4 inches or 7.1 meters. The length of the small intestine does tend to a lot and can range from around 15 feet or 4.6 meters to as long as 9.8 meters or 32 feet. The diameter of the small intestine is approximately 2.5 to 3 cm.

The functions of small intestine

The small intestine consists of three distinct parts, which are as follows:

  • Duodenum
  • Jejunum
  • Ileum

The food that is present in the stomach is pumped into the duodenum with the help of a muscle known as the pylorus, or the pyloricistalsis.

There are two main functions of the small intestine, i.e. digestion and absorption of food. They are discussed below in detail.


Most of the chemical digestion of food occurs in the small intestine. The digestive enzymes that that work in the small intestine are mostly secreted by the pancreas and passed on via the pancreatic duct. The presence of nutrients in the small intestine causes the secretion of the hormone cholecystokinin which causes the digestive enzymes to enter the small intestine. The hormone secretin leads to the release of bicarbonate into the small intestine by the pancreas to offset the effects of harmful acids sent by the stomach.

There are three types of nutrients that are subjected to digestion. They are proteins, lipids or fats and carbohydrates.

  • The proteins and peptides are broken down into amino acids. Such a process of chemical degradation begins in the stomach and persists in the large intestine. Proteolytic enzymes are split into smaller peptides; Carboxypeptidase from the pancreas breaks down one amino acid at a time, and the end amino acid produce are freed by dipeptidase and aminopeptidase.
  • Fats or lipids are split into glycerol and fatty acids. Pancreatic lipase uses salts secreted by the gall bladder and liver to emulsify triglycerides and then break them down into monoglycerides and free fatty acids. Such emulsification is required due to the fact that the lipase is water-soluble and the fatty triglycerides tend to drift away from the watery environment of the small intestine. The triglycerides are thus split into smaller products that go into the villi for absorption.
  • Some carbohydrates are split into monosaccharides or simple sugars in the small intestine, while pancreatic amylase degrades some other carbohydrates into oligosaccharides. Still other forms of carbohydrates such as lactose and cellulose are not acted upon by the small intestine enzymes. They pass onto the large intestine to be acted upon by the intestinal bacteria and brush border enzymes.


Most of the nutrients from the digested food are absorbed in the small intestine. Absorption of nutrients happens through a process called diffusion which allows the nutrients to pass onto the blood vessels present on the wall of the small intestine. The inner wall or mucosa consists of columnar epithelial tissues, permanent plicae circulares and non-permanent rugae which allows temporary contraction and distention. Plicae circulars have finger like projections called villi and epithelial tissue has similar projections called microvilli. All of these tissues and structures are used for absorption of nutrients

The nutrients thus absorbed are passed onto other organs of the body to form complex products such as proteins. The unabsorbed food passes onto the large intestine.

Small intestine problems

There are a number of problems that affect the small intestine. An individual may be affected by any one of these problems in their life span. As per Wikipedia, some of the conditions that affect the small intestine are listed below:

Obstructive disorders or obstruction of the small intestine

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  • Paralytic ileus
  • Hernia
  • Volvulus
  • Adhesions
  • Obstruction of the lumen by masses which can be bezoar, foreign bodies, gallstones, etc
  • Obstruction from external pressure

Infectious diseases

  • Giardiasis
  • Tropical sprue
  • Ascariasis
  • Tapeworm infestation
  • Presence of nematodes such as Ascaris lumbricoides
  • Hookworm infestation
  • Other Protozoa such as cryptosporidium parvum, cyclospora, Isopora belli, entamoeba histolytica, microsporidia, etc

Bacterial Infections

  • Enterotoxigenic E. coli
  • Campylobacter
  • Salmonella enterica
  • Shigella
  • Clostridium difficile such as antibiotic-associated colitis, pseudomembranous colitis
  • Yersinia
  • Mycobacterium, for e.g. disseminated Mycobacterium tuberculosis
  • Vibrio, Cholera
  • Whipple’s Disease
  • Enteric (Typhoid) Fever [Salmonella enterica var. typhii] and Paratyphoid fever
  • Clostridium perfringens [Gas Gangrene]
  • Bacillus cereus

Viral Infections

  • Rotavirus
  • Calicivirus
  • Astrovirus
  • Norovirus
  • Adenovirus

Neoplasms or Cancers

  • Adenocarcinoma
  • Gastrointestinal Stromal Tumor or GIST
  • Carcinoid
  • Lymphoma
  • Leiomyoma
  • Sarcoma
  • Metastatic tumors, particularly Melanoma or SCLC

Congenital, developmental, or genetic disorders

  • Duodenal (Intestinal) Atresia
  • Meckel’s Diverticulum
  • Hirschsprung’s Disease
  • Pyloric Stenosis
  • Ectopic Pancreas
  • Pancreas Divisum
  • Enteric duplication cyst
  • Cystic Fibrosis
  • Situs Inversus
  • Malrotation
  • Omphalocele
  • Familial Adenomatous Polyposis Syndrome (FAP)
  • Persistent Urachus
  • Gastroschisis
  • Primary Bile Acid Malsorption
  • Disachharidase (lactase) deficiencies
  • Gardner Syndrome

Other Conditions

  • Crohn’s disease, and the more general Inflammatory Bowel Disease
  • Celiac disease (Sprue or Non-Tropical Sprue)
  • Typhlitis (neutropenic colitis in the immunosuppressed individuals)
  • Mesenteric ischemia
  • Arteriovenous malformation
  • Embolus or Thrombus of the Superior Mesenteric Artery or the Superior Mesenteric Vein
  • Gastric dumping syndrome
  • Duodenal (Peptic) Ulcers
  • Irritable Bowel Syndrome
  • Gastrointestinal Perforation
  • Hyperthyroidism
  • Lymphatic Obstruction due to various causes
  • Diabetic Neuropathy
  • Radiation Enterocolitis
  • Sclerosing Retroperitonitis
  • Diverticula
  • Drug Induced Injury
  • Mesenteric Cysts
  • Diversion Colitis
  • Peritoneal Infection

Small intestinal bacterial overgrowth or SIBO

The small intestine, just like the entire gastrointestinal tract, generally contains bacteria. The number and type of bacteria are lower and different than those present in the colon. SIBO is a condition, which results in an unusually large number of bacteria in the small intestine and which resemble the bacteria found in the colon. The condition is also known as small bowel bacterial overgrowth or SBBO.

Symptoms of SIBO

  • Excessive gas
  • Diarrhea
  • Abdominal distension and bloating
  • Pain in the abdomen
  • Some individuals may experience chronic constipation instead of diarrhea
  • Interference in digestion and absorption of nutrients can result in loss of weight
  • There may be other unrelated symptoms such as fatigue and/or body aches

SIBO may be present just by itself or it may be accompanied by IBS

Causes of SIBO

There is muscular activity in the small intestine that allows the digestion of food and the passing of undigested food to the colon. Such muscular activity within the small intestine also propels the bacteria to be swept into the colon. When there is lack of such muscular activity, the bacteria tend to stay in the colon and multiply, which causes SIBO. Also, the lack of muscular activity in the small intestine also allows the bacteria in the colon to come to the small intestine.

Treatment of SIBO

  • Classic SIBO can be treated with the help of antibiotics
  • SIBO with IBS is treated using a combination of oral antibiotics and probiotics. Probiotics are live bacteria that can be consumed to produce a health benefit. It’s most common form is lactobacilli that are also used in making yogurt.

SIBO is prone to reoccurrence and hence the dosage, treatment period, etc are decided by the health care provider as per individual basis.

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