Digestive system

Digestive system

• Provides absorption and mechanical processing of food, secretion of digestive juices and chemical treatment (hydrolysis) of food substrates, absorption of nutrients, excretion of metabolic products and their evacuation together with undigested food residues from the body.
• Includes digestive canal (digestive tube) and associated organs: large digestive glands and gallbladder.

Digestive canal

• It is divided into anterior (upper), middle and posterior (lower) divisions:
  • anterior section:
    • includes the organs of the oral cavity, pharynx and esophagus;
    • also includes large salivary glands;
    • mechanical and chemical processing of food takes place in it;
  • middle section:
    • includes the stomach, small intestine and large intestine (except the anal canal of the rectum);
    • it also includes the pancreas, liver and gallbladder;
    • performs mainly chemical processing of food, absorption of its cleavage products and formation of feces;
  • posterior section:
    • corresponds to the anal canal of the rectum;
    • provides evacuation of feces.

Large digestive glands

• Are located outside the wall of the digestive tube (extramurally), are connected to it by ducts and produce their secret into the organ cavity.
• Are represented by large salivary glands, pancreas and liver.

General outline of the structure of the digestive tube wall

• The wall consists of four membranes:
  • mucosa:
    • internal, has three layers (plates):
      • epithelial layer is formed by a stratified squamous epithelium (anterior and posterior sections of the digestive tube) or a single-layer columnar epithelium (middle section of the digestive tube);
      • lamina propria consists of loose connective tissue;
      • lamina muscularis contains smooth muscle tissue;
    • its microrelief is heterogeneous throughout the digestive canal;
  • submucosa:
    • is formed by loose connective tissue;
    • оensures the mobility of mucosa and participates in the formation of folds forming the macrorelief of the surface of organs;
  • musclaris externa:
    • is formed by layers of visceral striated muscle tissue (anterior and posterior sections of the digestive tube) or smooth muscle tissue (partially anterior and middle sections of the digestive tube);
    • forms sphincters, located on the border of the departments and organs of the digestive tube;
    • its rhythmic contractions promote mixing and promotion of food during digestion;
  • the outer membrane can be adventitious or serous:
    • adventitia:
      • is formed by loose connective tissue;
      • is present in the anterior and posterior sections of the digestive tube;
    • serosa:
      • formed by the visceral sheet of the peritoneum, consisting of a connective tissue plate and mesothelium;
      • covers the organs of the middle part of the digestive tube and provides free peristaltic movement of the intestinal loops in the abdominal cavity.
• The wall contains:
  • intramural (located in the wall) glands:
    • endoepithelial — goblet cells of the small and large intestine;
    • exoepithelial, located in:
      • lamina propria mucosae (oral cavity, esophagus, stomach);
      • submucosa (oral cavity, esophagus, duodenum, anal canal);
      • musclaris externa (lips, cheeks, tongue);
  • intramural ganglia, which are part of:
    • intermuscular nerve plexus (Auerbach) — regulates the motility of the organs of the digestive tube;
    • submucosal nerve plexus (Meissner) — regulates the contraction of myocytes of the lamina muscularis mucosae, secretion of glands of the mucosa and submucosa;
  • lymphoid tissue, associated with the surface in the form of diffuse clusters, single or aggregated lymphoid nodules;
  • single hormone-producing cells, нthe most numerous in the organs of the middle part of the digestive canal and forming a unit gastroenteropancreatic (GEP) endocrine system.

Along with the main functions that provide the appropriate stages of the digestive process, the organs of the digestive tube, containing a significant amount of lymphoid tissue and GEP cells, also have immune and endocrine functions.

Lymphoid tissue in the wall of the stomach and intestine

Single hormone-producing cells in the stomach wall

Nomenclature of GEP cells and the hormones they produce