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3. the small intestine
- 1. THE SMALL INTESTINE PHYSIOLOGY: GASTROINTESTINAL
- 2. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE
- 3. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE OVERVIEW ā£ The small intestine is an organ located in the gastrointestinal tract, between the stomach and the large intestine ā£ It is, on average, 23ft long and is comprised of three structural parts; the duodenum, jejunum and ileum ā£ Functionally, the small intestine is chiefly involved in the digestion and absorption of nutrients ā£ It receives pancreatic secretions and bile through the hepatopancreatic duct which aid with its functions ā£ Learning Goal ā£ To discuss the histology, structure and secretions of the small intestine
- 4. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE HISTOLOGY ā£ The histological structure of the small intestine is similar to the other organs in the digestive tract ā£ There areĀ fourĀ main layers: ā£ MucosaĀ (Innermost layer) ā Contains the epithelium, lamina propria and muscularis mucosae ā£ SubmucosaĀ ā Connective tissue layer, which contains blood vessels, lymphatics and the submucosal plexus ā£ Muscularis externaĀ ā Consists of two smooth muscle layers; the outer longitudinal layer and inner circular layer and the myenteric plexus lies between them ā£ AdventitiaĀ (Outermost layer) ā Comprised of loosely arranged fibroblasts and collagen, with the vessels and nerves passing through it while the majority of the small intestine adventitia is covered by mesothelium and is commonly called the serosa
- 5. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE HISTOLOGY ā£ The small intestine is the major absorptive site in the gastrointestinal tract, and therefore has a number of modifications to aid its function ā£ The mucosa and submucosa form large numbers of folds (orĀ plicae) arranged in a circular fashion in the lumen (therefore called plicae circulares) ā£ Additionally, the plicae contain microvilli to further increase the surface area, which increases absorption
- 6. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE CELLS OF THE EPITHELIUM ā£ The epithelium of the small intestine lines the luminal surface ā£ There are a number of components to the epithelium: ā£ EnterocytesĀ ā Tall columnar cells having an absorptive function and containing brush border enzymes on the surface which have an important digestive function ā£ Goblet cellsĀ ā Exocrine glands which secrete mucin ā£ Crypts of Lieberkuhn
- 7. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE CELLS OF THE EPITHELIUM ā£ The Crypts of Lieberkuhn are glands found in the epithelial lining ā£ They contain numerous cells such as stem cells to produce new cells to replenish the cells lost due to abrasion, as well asĀ enteroendocrine cellsĀ to synthesize and secrete hormones ā£ To protect from pathogens, there areĀ Paneth cellsĀ which secrete protective agents (such as defensins and lyzozymes) and Peyerās patches which are only found in the ileum ā£ Peyerās patches contain mucosal-associated lymphatic tissue (MALT) which house white blood cells and lymphocytes ā£ These cells can produce antibodies to further protect the small intestine from infection
- 9. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE ENTEROENDOCRINE CELLS ā£ The enteroendocrine cells are located within the Crypts of Lieberkuhn ā£ They secrete hormones in response to various stimuli ā£ There are four main classes of enteroendocrine cell, each with a different secretory product ā£ These areĀ I cells, S cells, K cells and enterochromaffin cells
- 10. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE ENTEROENDOCRINE CELLS ā£ I Cells secrete CholecystokininĀ (CCK)Ā in response to the presence of fat in the small intestine ā£ CCK stimulates the contraction of the gallbladder (which pushes bile out into the cystic duct) and the release of pancreatic enzymes ā£ Both bile and pancreatic enzymes have a key role in lipid digestion ā£ S Cells secrete Secretin in response to the low pH of chyme in the small intestine ā£ Secretin induces HCO3āĀ secretion from the pancreas and inhibits gastric emptying
- 12. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE ENTEROENDOCRINE CELLS ā£ K Cells secrete Gastric-Inhibitory PeptideĀ (GIP),Ā in response to chyme entering the small intestine ā£ GIP has a misleading name as it actually stimulates insulin release, ready to put the newly digested carbohydrates into cells for storage ā£ Finally,Ā Enterochromaffin CellsĀ are mechanically stimulated by the presence of chyme in the small intestine ā£ They release serotonin, which acts on the enteric nervous system to activate the cystic fibrosis transmembrane regulators (CFTR) ā£ This ion channel secretes ClāĀ ions into the intestinal lumen, with Na+Ā ions and H2O following ā£ Na+Ā is required in the lumen for the absorption of a number of nutrients
- 13. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE SECRETIONS ā£ The small intestine receives secretory products from other abdominal viscera which aid in intestinal function ā£ There are three main substances that the small intestine receives ā£ bile, pancreatic enzymes and alkaline juice (HCO3ā) ā£ BileĀ serves an important role in lipid digestion, and is secreted from the gallbladder into the common bile duct, in response to CCK ā£ Pancreatic enzymes and alkaline juice are secreted from the pancreas into the pancreatic duct, in response to CCK and secretin
- 14. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE SECRETIONS ā£ The common bile duct and pancreatic duct unite to form theĀ hepatopancreaticĀ ampullaĀ (also known as the Ampulla of Vater) ā£ This opens out onto the internal surface of the duodenum, and is marked by the major duodenal papilla ā£ The major duodenal papilla serves as an important anatomical landmark, as it is where the embryonic foregut becomes midgut ā£ The secretions into the small intestine are regulated by theĀ sphincter of Oddi
- 16. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE PANCREATIC ENZYMES ā£ The pancreas plays an important role in digestion, as it produces many of the enzymes needed in digestion ā£ These enzymes come in two forms ā£ Some enzymes are already active, such as pancreatic lipase and pancreatic amylase, whilst some are secreted as inactive enzymes such asĀ trypsinogen ā£ The inactive enzymes are released asĀ zymogensĀ (precursors of the active form) ā this prevent the enzymes digesting the pancreas itself ā£ Trypsinogen is converted to trypsin (active form) in the small intestine by coming into contact with the brush border enzyme enterokinase ā£ Trypsin then converts the remaining inactive proteases to their active form
- 20. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE REVIEW QUESTIONS ā£ Which of the following is not an anatomical section of the small intestine? ā£ Pylorus ā£ Duodenum ā£ Jejunum ā£ Ileum
- 21. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE REVIEW QUESTIONS ā£ Which of the following is not an anatomical section of the small intestine? ā£ Pylorus ā£ Duodenum ā£ Jejunum ā£ Ileum
- 22. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE REVIEW QUESTIONS ā£ Which cellular adaptation do enterocyte cells possess? ā£ Cilia ā£ Absent mitochondria ā£ Microvilli ā£ Eosinophilic granules
- 23. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE REVIEW QUESTIONS ā£ Which cellular adaptation do enterocyte cells possess? ā£ Cilia ā£ Absent mitochondria ā£ Microvilli ā£ Eosinophilic granules ā£ Microvilli increase the surface area of the cell, enhancing its ability to absorb nutrients from the small intestine.
- 24. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE REVIEW QUESTIONS ā£ Which part of the duodenum does the Ampulla of Vater open into? ā£ 1st ā£ 2nd ā£ 3rd ā£ 4th
- 25. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE REVIEW QUESTIONS ā£ Which part of the duodenum does the Ampulla of Vater open into? ā£ 1st ā£ 2nd ā£ 3rd ā£ 4th
- 26. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE REVIEW QUESTIONS ā£ Which cells is cholecystokinin released from? ā£ I Cells ā£ S Cells ā£ K Cells ā£ Enterochromaffin Cells
- 27. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE REVIEW QUESTIONS ā£ Which cells is cholecystokinin released from? ā£ I Cells ā£ S Cells ā£ K Cells ā£ Enterochromaffin Cells
- 28. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE REVIEW QUESTIONS ā£ Which histological layer of the small intestine is the myenteric plexus located? ā£ Mucosa ā£ Submucosa ā£ Muscularis Externa ā£ Adventitia
- 29. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE REVIEW QUESTIONS ā£ Which histological layer of the small intestine is the myenteric plexus located? ā£ Mucosa ā£ Submucosa ā£ Muscularis Externa ā£ Adventitia ā£ Muscularis Externa consists of two smooth muscle layers; the outer longitudinal layer and inner circular layer. The myenteric plexus lies between them.
- 30. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE REVIEW QUESTIONS ā£ What is the role of the Crypts of Lieberkuhn in the small intestine? ā£ Help to replenish cells lost to abrasion and well as hormone synthesis ā£ Immune function ā£ Erythropoiesis ā£ Tumour markers
- 31. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE REVIEW QUESTIONS ā£ What is the role of the Crypts of Lieberkuhn in the small intestine? ā£ Help to replenish cells lost to abrasion and well as hormone synthesis ā£ Immune function ā£ Erythropoiesis ā£ Tumour markers ā£ The Crypts of Lieberkuhn are glands found in the epithelial lining. They contain numerous cells such as stem cells to produce new cells to replenish the cells lost due to abrasion, as well as enteroendocrine cells to synthesize and secrete hormones.
- 32. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE REVIEW QUESTIONS ā£ Which of the following histological changes are seen in biopsy in coeliac disease? ā£ Villous atrophy, flattening of the villous, intraepithelial lymphocytes and crypt hyperplasia ā£ Granuloma formation ā£ Crypt abscess and infiltration ā£ Variable neutrophils, modest villus blunting of small bowel
- 33. HISTOLOGY AND CELLULAR FUNCTION OF THE SMALL INTESTINE REVIEW QUESTIONS ā£ Which of the following histological changes are seen in biopsy in coeliac disease? ā£ Villous atrophy, flattening of the villous, intraepithelial lymphocytes and crypt hyperplasia ā£ Granuloma formation ā£ Crypt abscess and infiltration ā£ Variable neutrophils, modest villus blunting of small bowel ā£ Villous atrophy, flattening of the villous, intraepithelial lymphocytes and crypt hyperplasia - Coeliac Disease. Option 2 describes Crohns Disease, Option 3- ulcerative colitis and Option 4- small bowel bacterial enterocolitis
- 35. DIGESTION AND ABSORPTION OVERVIEW ā£ Digestion is the chemical breakdown of the ingested food into absorbable molecules ā£ Absorption refers to the movement of nutrients, water and electrolytes from the lumen of the small intestine into the cell, then into the blood ā£ Learning Goal ā£ To look at the digestion and absorption of carbohydrates, protein and lipids
- 36. DIGESTION AND ABSORPTION CARBOHYDRATES - DIGESTION ā£ There are threeĀ carbohydrateĀ products which are absorbed by the small intestine ā£ glucose,Ā galactoseĀ andĀ fructose ā£ Digestion of starch is initiated in the mouth, facilitated by salivary amylase ā£ The majority of carbohydrate digestion occurs in the small intestine ā£ The main enzyme isĀ pancreatic amylase, which yields disaccharides from starch by digesting the alpha 1-4 glycosidic bonds ā£ The disaccharides produced (maltose, maltotriose, and Ī±-dextrins) are all converted to glucose by brush border enzymes ā£ Disaccharides occurring naturally in food do not require amylase to break them down ā£ Brush border enzymes (lactase, sucrase, trehalase) hydrolyse these compounds into molecules of glucose, galactose and fructose
- 37. DIGESTION AND ABSORPTION CARBOHYDRATES - ABSORPTION ā£ Glucose and galactose are absorbed across the apical membrane by secondary active transport (along with Na+) through the Sodium-Glucose cotransporter (SGLT1) ā£ Both glucose and galactose exit the cell via GLUT2 receptors across the basolateral membrane into the blood ā£ Fructose enters the cell by facilitated diffusion via GLUT5 and is transported into the blood via GLUT2 receptors
- 38. DIGESTION AND ABSORPTION PROTEIN - DIGESTION ā£ Protein digestion begins in the stomach with the action ofĀ pepsin, which breaks protein into amino acids and oligopeptides ā£ The process of digestion is completed in the small intestine with brush border and pancreatic enzymes ā£ They split the oligopeptides into amino acids, dipeptides and tripeptides
- 39. DIGESTION AND ABSORPTION PROTEIN - ABSORPTION ā£ Amino acids are absorbed via a Sodium cotransporter, in a similar mechanism to the monosaccharides ā£ They are then transported across the basolateral membrane viaĀ facilitated diffusion ā£ Di and tripeptides are absorbed via separate H+ dependent cotransporters and once inside the cell are hydrolysed to amino acids
- 40. DIGESTION AND ABSORPTION LIPID - DIGESTION ā£ Lipids are hydrophobic, and so are poorly soluble in the aqueous environment of the digestive tract ā£ Their digestion is started by lingual and gastricĀ lipases, but this only digests 10% of ingested lipids ā£ The remainder of the lipids are digested in the small intestine ā£ In the small intestine, bile aids digestion byĀ emulsifyingĀ the fat goblets into smaller chunks, called micelles, which have a much larger surface area ā£ 3 major enzymes involved in lipid digestion hydrolyse theĀ micelles ā£ Pancreatic lipase, phospholipase A2 and cholesterol ester hydrolase ā£ Breaking them down into fatty acids, monoglycerides, cholesterol and lysolecithin
- 41. DIGESTION AND ABSORPTION LIPID - ABSORPTION ā£ The products from digestion are released at the apical membrane and diffuse into the enterocyte ā£ Inside the cell, the products are re-esterified to form the original lipids, triglycerides, cholesterol and phospholipids ā£ The lipids are then packaged inside apoproteins to form aĀ chylomicron ā£ The chylomicrons are too large to enter circulation, so they enter lymphatic system via lacteals
- 43. DIGESTION AND ABSORPTION WATER ABSORPTION ā£ The average adult usually ingests 1-2 L of water each day, but the fluid load to the small intestine is 9 to 10 L (8 to 9 L being added by secretions of the GI system) ā£ Most absorption of water and electrolytes occurs in theĀ small intestine, with some water absorbed in theĀ colonĀ as well ā£ Water absorption is dependent on the absorption of solutes such asĀ (Na+Ā and Clā) ā£ Na+Ā is absorbed from the intestinal lumen, most use the cotransport with glucose and amino acids and the Na+/H exchange, which allow Na+Ā ions to move from the lumen into the enterocyte
- 44. DIGESTION AND ABSORPTION WATER ABSORPTION ā£ Na+Ā is rapidly removed from the enterocyte via the Na+Ā pumps ā£ Allowing water to follow via osmosis either transcellularly or paracellularly (between the tight junctions of the enterocytes) ā£ Water as well as Na+Ā ions can then diffuse into the capillaries
- 48. DIGESTION AND ABSORPTION REVIEW QUESTIONS ā£ Which enzymes digest disaccharides? ā£ Salivary amylase ā£ Pancreatic amylase ā£ Brush border enzymes ā£ Trypsin
- 49. DIGESTION AND ABSORPTION REVIEW QUESTIONS ā£ Which enzymes digest disaccharides? ā£ Salivary amylase ā£ Pancreatic amylase ā£ Brush border enzymes ā£ Trypsin
- 50. DIGESTION AND ABSORPTION REVIEW QUESTIONS ā£ What transporters does glucose exit the cell through? ā£ GLUT1 ā£ GLUT2 ā£ GLUT4 ā£ GLUT5
- 51. DIGESTION AND ABSORPTION REVIEW QUESTIONS ā£ What transporters does glucose exit the cell through? ā£ GLUT1 ā£ GLUT2 ā£ GLUT4 ā£ GLUT5
- 52. DIGESTION AND ABSORPTION REVIEW QUESTIONS ā£ What ions are dipipetides co-transported with? ā£ Sodium ā£ Hydrogen ā£ Potassium ā£ Chloride
- 53. DIGESTION AND ABSORPTION REVIEW QUESTIONS ā£ What ions are dipipetides co-transported with? ā£ Sodium ā£ Hydrogen ā£ Potassium ā£ Chloride
- 54. DIGESTION AND ABSORPTION REVIEW QUESTIONS ā£ Which of the following is not a cause of steatorrhoea? ā£ Cystic fibrosis ā£ Pancreatic cancer ā£ Inflammatory Bowel Disease ā£ Diabetes Mellitus
- 55. DIGESTION AND ABSORPTION REVIEW QUESTIONS ā£ Which of the following is not a cause of steatorrhoea? ā£ Cystic fibrosis ā£ Pancreatic cancer ā£ Inflammatory Bowel Disease ā£ Diabetes Mellitus
- 56. References These slide reflect a summary of the contents of TeachMePhysiology.com and are to be used for educational purposes only in compliance with the terms of use policy. Specific portions referenced in this summary are as follows: ā£ https://teachmephysiology.com/gastrointestinal-system/small-intestine/ digestion-and-absorption/ ā£ https://teachmephysiology.com/gastrointestinal-system/small-intestine/ histology-and-cellular-function/ Additional sources are referenced on the slide containing that specific content.