Functions of the small intestine
In addition to its main role — digestion and absorption, the small intestine performs a number of additional, yet equally important tasks:
- Motor (mechanical): due to peristalsis (coordinated contraction of the muscular layer), undigested chyme residues are moved toward the large intestine.
- Endocrine: solitary endocrine cells of the epithelium secrete hormones for local and systemic regulation.
- Protective: the lymphoid tissue of the mucous membrane provides an immune response to antigens entering with food.
Digestion features and enzymes
Absolutely all classes of nutrients are broken down in the small intestine. The process occurs in two stages:
- Cavitary digestion: pancreatic juice enzymes function freely in the intestinal lumen.
- Membrane (contact) digestion: intermediate products are broken down by intestinal juice enzymes fixed directly on the surface of enterocytes.
Various groups of enzymes are responsible for chemical processing: proteins are broken down by trypsin, chymotrypsin, peptidases, and enterokinase; carbohydrates by amylase, sucrase, maltase, and lactase. Lipids, pre-processed by bile acids, are hydrolyzed by lipases and phospholipases, while nucleic acids are broken down by nucleases and nucleotidases.
Histological structure
The wall of the small intestine classically consists of four layers:
- Mucous membrane: lined with simple columnar striated epithelium. The lamina propria made of loose fibrous connective tissue forms the stroma of the villi and contains lymphoid follicles (solitary or aggregated into Peyer's patches, especially in the ileum). The muscularis mucosae includes an inner circular and an outer longitudinal layer.
- Submucosa: formed by loose connective tissue, including the submucosal nerve plexus, blood vessels, and lymphoid tissue. In the duodenum, duodenal (Brunner's) glands are located here.
- Muscular layer: two powerful layers of smooth muscle — an inner circular and an outer longitudinal, between which lies the myenteric nerve plexus.
- Serous membrane: loose connective tissue covered with mesothelium; adipocytes often accumulate in the subserosal layer.
Relief structures: villi and crypts
For efficient absorption, the surface area of the intestine is significantly increased due to circular folds, crypts, villi, and microvilli.
- Villi: projections 0.5–1.0 mm high, covered with striated epithelium. Their stroma contains blood and lymphatic capillaries, immune cells, and smooth myocytes. Contracting 4–6 times per minute, myocytes "pump" absorbed substances (amino acids and sugars into the blood, lipids into the lymph).
- Crypts: tubular depressions (intestinal glands). They house 5 cell types: striated (enterocytes and M cells), goblet, endocrinocytes, undifferentiated (stem) cells, and Paneth cells (located at the bottom of the crypts). Cells migrate from the crypts (except stem and Paneth cells) onto the villi.
Briefly: answers to topical questions
Where are Paneth cells located and what function do they perform?
Paneth cells (exocrinocytes with acidophilic granules) are located exclusively at the bottom of the small intestine crypts and are absent in the epithelium of the villi. The main functions of these cells are digestive and protective (antimicrobial).
Substances secreted by Paneth cells:
- Dipeptidases — digestive enzymes that ensure the breakdown of dipeptides into amino acids.
- Lysozyme — an antibacterial protein that destroys the cell walls of bacteria and protozoa.
What specific hormones do the endocrinocytes of the small intestine secrete?
The endocrinocytes of the small intestine secrete serotonin, motilin, secretin, and cholecystokinin. Hormones accumulate in the basal part of the cells and are released into blood capillaries or act paracrine.
Depending on the secreted hormones, the following cell types are distinguished:
- EC cells — secrete serotonin and motilin (stimulate motility and intestinal juice production).
- S cells — secrete secretin (inhibits gastric secretion, stimulates pancreatic juice and bile secretion).
- I cells — produce cholecystokinin or pancreozymin (stimulate pancreatic and biliary secretion).
Class and exam questions
Why are smooth myocytes inside the villi needed?
Smooth myocytes derived from the muscularis mucosae provide a pumping function. They contract rhythmically, pushing substances absorbed into the villus vessels further along the bloodstream.
What is the difference between crypt epithelium and villus epithelium?
The epithelium of the villi contains only three cell types: columnar, goblet, and endocrinocytes. In the crypts, in addition to these, there are undifferentiated stem cells and Paneth cells with acidophilic granules.
Where are duodenal glands located and what is their function?
Duodenal (Brunner's) glands are located in the submucosa of the duodenum. They are complex alveolar-tubular glands and secrete a mucous fluid.
What else to study on this topic
- Detailed ultramicroscopic structure of the enterocyte and microvilli
- Mechanisms of lipid absorption and their transport into the lymph
- Functions and localization of Paneth cells
- The role of M cells in local immunity formation
- Innervation of the small intestine: myenteric and submucosal plexuses
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on the "Small intestine" topic and 3,485 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in the section "Digestive system: stomach and small intestine"
Free video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.