Small Intestine Study Guide


The small intestine is the longest part of the alimentary canal, extending approximately 6 meters! It is a helical structure extending from the stomach to the large intestine, and is the main organ that carries out the digestive process.

Anatomy of the Small Intestine

The small intestine consists of three parts:

  • Duodenum
  • Jejunum
  • Ileum

Small Intestine PartsSource

Duodenum: It is the starting point of the small intestine. It begins at the stomach’s pyloric sphincter, arcs in a C-shape around the pancreatic head, and finishes at the duodenojejunal flexure. It is also where the hepatopancreatic sphincter (sphincter of Oddi) opens, which carries the bile juice.

Jejunum: The jejunum is the second part of the small intestine. It is present in the upper left quadrant of the abdomen.

Ileum: The ileum is the last and the longest part of the small intestine. It is present in the lower right part of the abdomen. The ileum terminates where the cecum of the large intestine begins. Although there is no apparent distinction between the jejunum and the ileum, there are several anatomical and histological differences:

  • The jejunum represents the proximal two-fifths of the jejunum-ileum continuum.
  • The jejunum has a thicker wall and a broader lumen than the ileum.
  • Kecking’s circular folds are more apparent in the jejunum.

Histology of Small Intestine

History of Small IntestineSource

The small intestine is divided into four layers histologically. The layers in order of internal to external layers are:

  • Mucosa
  • Submucosa
  • Muscularis externa
  • Serosa

Nerves go to the small intestine from two divisions of the autonomic nervous system. Parasympathetic nerves trigger muscle contractions that transport food down the tract (peristalsis), whereas sympathetic nerves repress intestinal motions.

The small intestine has numerous unique characteristics that considerably increase its absorptive surface:

  • Circular folds
  • Intestinal villi are finger-like projections that project into the lumen of the small intestine. They are extensions of the mucosa.
  • Microvilli.
  • Peyer’s patches are a part of gastrointestinal-associated lymphoid tissue (GALT).
  • Brunner’s gland: found in the submucosa, they secrete mucus which is of alkaline medium which protects the duodenum from the gastric juices, which are highly acidic and can be corrosive

Functions of the small intestine:

  • The small intestine’s primary functions are secretion and absorption.
  • The small intestine’s epithelial cells release enzymes that break down chyme into the tiniest particles, allowing it to be absorbed.
  • The duodenum mixes food with bile and pancreatic enzymes to continue the digestion of carbs, lipids, and proteins at the same time.
  • The duodenum and jejunum absorb carbs and proteins, respectively.
  • The jejunum is also responsible for fat absorption.
  • Vitamin B12, bile salts, and all digestive products that were not absorbed in the duodenum and jejunum are absorbed in the ileum.
  • Water and electrolytes are absorbed by all three segments of the small intestine.
  • The undigested and unabsorbed food then enters the large intestine.

Common diseases of the small intestine:

Duodenal ulcer:

  • An ulcer is a defect in the intestine’s mucosa.
  • Ulcers in the stomach or duodenum can occur due to a bacterial infection of the stomach’s pylorus with Helicobacter pylori and the corrosive effects of gastric acid and pepsin.
  • Duodenal peptic ulcers are most commonly located in the duodenal ampulla. They are more common in the younger population than stomach ulcers, more common in the elderly.
  • The presence of a burning sensation in the epigastric area of the abdomen about 2-3 hours after a meal is the most prevalent sign of duodenal ulcers. Pain may be accompanied by nausea and vomiting, depending on the degree of ulceration.


It is the passing of unformed feces regularly. Microorganisms such as E. coli, Salmonella, and Shigella are the most common causes of diarrhea.

Obstructive diseases, including paralytic ileus, hernias, and volvulus, are typical, but they can get complicated.

Infectious disorders including tapeworm, tropical sprue, and giardiasis are rare, yet they can be fatal if left untreated.

Crohn’s disease, celiac disease, gastric dumping syndrome, and irritable bowel syndrome, to mention a few, are other disorders or a combination of diseases that can damage the small intestine.


  • The small intestine is the longest part of the alimentary canal, extending approximately 6 meters.
  • It consists of 3 parts- duodenum, jejunum, ileum.
  • The small intestine’s primary functions are complete digestion of chyme and absorption.
  • It also contains microvilli that increase the surface area of the small intestine for absorption.


1. What are the functions of the 3 parts of the small intestine?

The primary function of the small intestine is to break down the partially digested food in its smallest equivalent and then absorb them.

2. What are the symptoms of small intestine dysfunction?

Pain in the abdomen, nausea, vomiting, undigested fecal matter, excessive blood or mucus in stool, lack of hunger can be common symptoms of small intestine dysfunction.

3. What happens when the small intestine does not work properly?

The normal absorption and digestion of food will not occur if the small intestine is not working properly, leading to many deficiency disorders.

4. How can I heal my small intestine?

Eating properly cooked foods, washing fruits and vegetables before consumption, drinking clean water free from any contamination, eating natural probiotic-rich foods like curd and yogurt is good for small intestine health.

5. What will cause an infection in the small intestine?

Eating unhealthy food contaminated with disease-causing bacteria or protozoans can cause an infection in the small intestine.

6. Can a person survive without the small intestine?

Although most people can live without a stomach or large intestine, but it is nearly impossible to live without a small intestine.

We hope you enjoyed studying this lesson and learned something cool about the Small Intestine! Join our Discord community to get any questions you may have answered and to engage with other students just like you! Don’t forget to download our App to experience our fun, VR classrooms – we promise, it makes studying much more fun!😎


  1. Small intestine. Accessed on 25 Nov, 2021.
  2. Small Intestine Disorders. Accessed on 25 Nov, 2021.
  3. Small Intestine. Accessed on 25 Nov, 2021.
  4. Small Intestine. Accessed on 25 Nov, 2021.

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