Week 34 - The Gastrointestinal (GI) tract

• Gastrointestinal (GI) Tract


  A long tube made up of the oesophagus, stomach, small and large intestine, rectum and anus
• GI Tract


  • Approximately 4.5m long
  • Surface area is approximately 300m2 (bigger than a doubles tennis court!)
  • Has associated accessory digestive organs (e.g., the pancreas)
  • Has extensive immune cell areas (GALT), which is the gut-associated lymphoid tissue
  • Has a complex microbiome (normally bacterial species) controlling both the levels of infectious bacteria and the immune response locally in the gut and systemically around the body
• Motility


  The movement of food through the digestive system
• Mouth and oesophagus


  • The oesophagus is lined with stratified squamous epithelial cells to protect against injury
  • Salivary glands produce β-amylase enzyme to break down starch
• Bile


  Produced by the liver and stored in the gall bladder
• Stomach (anatomy)


  • Hormones trigger acid secretion, gut mobility and enzyme release
  • Hydrochloric acid in the stomach cuts inactive pepsinogen to form active pepsin, an enzyme that digests proteins into short peptides
  • The pyloric sphincter serves as a kind of gateway between the stomach and the small intestine
  • The rugae (folds) allow the stomach to expand after a large meal has been eaten
• Food movement in the stomach

  1. Gastric motility is stimulated by: a full stomach (distension), increased parasympathetic nervous activity e.g. from the vagus nerve, secretion of gastrin, a hormone made and secreted by G cells of the gastric mucosa

  2. Gastric mobility is inhibited by a sympathetic nervous activity e.g. vigorous exercise
• Small intestine


  • Surface area of the small intestine is increased by foldings & projections
  • Large folds are plicae circulares
  • Microscopic finger-like projections are villi
  • Apical hair-like projections on the villi are called microvilli (not shown)
  • Intestinal crypts increase the surface area of the mucosa of the small intestine and produce new cells for the mucosa
• Layers of the GI tract in the small intestine

  • The Mucosa (absorption, and secretion, home of immune cells within lymphoid areas)
  • The Submucosa (blood vessels and mucus secretion happens here)
  • The Muscularis area causes peristalsis, which is involuntary movements of the longitudinal and circular muscles that occur in progressive wavelike contractions
  • The Serosa (stops tangling of the small intestine)
• Mucosa layer of the small intestine

  • The top layer of epithelium is in contact with the contents of the GI tract. The cells here are continually sloughed off and replaced.
  • The lamina propria: this is a layer that contains blood vessels, lymphocytes, macrophages and dendritic cells within lymphoid areas/nodules.
  • The muscularis mucosae: a thin layer of smooth muscle between the mucosa and submucosa.
• Lymphoid areas in the lamina propria of the small intestine

  • The domed Peyer's patches contain separate areas of T cells (blue) and B cells (yellow). M cells take up antigen here.
  • The lymphatics (white vessels) drain from the lamina propria (pale yellow) to the mesenteric lymph nodes.
• M cells


  Specialised epithelial cells (found directly over the Peyer's patch area) that do not have microvilli, but are designed to continually sample (using endocytosis) small samples of antigen from the lumen of the digestive tract. The antigen is then transported across the cell to a pocket of lymphoid cells clustered below and also to the underlying lamina propria and Peyer's patch.
• Submucosa


  • Connective tissue containing larger blood and lymph vessels that feed into the Mucosa
• Muscularis externa


  • Contains two sheets of smooth muscle
• Serosa


  A thin section consisting of connective tissue and epithelia
• Food movement in the small intestine

  1. Peristalsis used to create forward movement

  2. Segmental contraction (segmentation) to mix the chyme with secreted digestive enzymes and mucus
• Slow waves in the intestine

  Contractions of the intestinal smooth muscle occur automatically in response to endogenous pacemaker activity. These so called pacemaker cells are called the interstitial cells of Cajal.
• Large intestine


  • Absorbs water, electrolytes and vitamins from the chyme it receives from the small intestine
  • Produces antibodies
  • Some vitamins (e.g. vit K) are made by the gut microbes living here
  • Absorption of water occurs passively in the same way as in the kidney with Na+/K+ pumps at the basolateral membrane
  • The caecum varies in size greatly between species!
• Appendix


  • Numerous lymphatic nodules (similar to those in tonsils!) which function in maintaining immunity
  • Prone to inflammation, called appendicitis
• Cardiac sphincter


  Muscle that opens and allows food to exit the esophagus and enter the stomach
• Stomach


  • Flexible pouch/sack
  • Site of mechanical and chemical digestion
  • Digests proteins into smaller polypeptides
• Amount of food ingested and protein content

  Determines how long the stomach stores the food (minutes to hours)
• No nutrient absorption takes place in the stomach
• Exocrine glands


  Glands that produce and release substances directly into a duct that empties into a cavity (like the stomach lumen)
• Types of cells in stomach exocrine glands

  • Mucus cells
  • Chief cells
  • Parietal cells
  • G cells
  • Enterochromaffin-like cells
• Mucus cells


  Secrete mucus to lubricate and protect the stomach lining
• Chief cells


  Secrete pepsinogen, which is converted to the active enzyme pepsin
• Parietal cells


  Secrete hydrochloric acid (HCl) to:

  1. Decrease pH and activate pepsin

  2. Denature proteins

  3. Kill bacteria

  4. Produce intrinsic factor for vitamin B12 absorption
• G cells


  Secrete the hormone gastrin, which stimulates parietal cells to release HCl
• Enterochromaffin-like cells


  Secrete histamine, which also stimulates parietal cells to release HCl
• Stomach function


  1. Mechanical digestion through muscle contractions

  2. Chemical digestion through enzymes and acid to break down food

  3. Kills bacteria

  4. Minimal nutrient absorption (only some small molecules like caffeine, alcohol, aspirin)
• Gastric juice consists of HCl, enzymes, and other molecules that help break down food
• Gastric juice turns food into a fluid-like substance (chyme) that exits the stomach and enters the small intestine
• Small intestine


  Digestive organ where majority of digestion and almost all absorption in the human body takes place
• Digestion and absorption in the small intestine

  1. Digestion occurs in the duodenum

  2. Absorption occurs in the jejunum and ileum
• Duodenum


  • Where digestion of macromolecules into amino acids, fatty acids, and monosaccharides takes place
• Jejunum and ileum


  • Where the digested nutrients are absorbed into the intestinal cells (enterocytes) and then transported into the blood and lymph systems
• Villi


  Projections on the inner surface of the small intestine that greatly increase the surface area for digestion and absorption
• Microvilli


  Tiny hair-like projections on the surface of the enterocytes that further increase the surface area
• Brush border


  The border of the villi containing the microvilli where digestion and absorption take place