4. Small intestine Flashcards
What separates the small intestine from the stomach and large intestine?
- Stomach - pyloric sphincter
* Large intestine - ileocecal valve
What are the 3 parts of the small intestine?
- Duodenum - (0.25m) digestion and gut regulation
- Jejunum - (2.5m) absorption
- Ileum - (3.75m) absorption
Outline the structure of the lining of the small intestine
- Epithelial layers - villi to increase SA, motile and rich blood supply/lymphoid drainage
- Crypts of Lieberkühn
What 5 cell types are the mucosa made up of?
- Enterocytes
- Goblet cells
- Enter-endocrine cells
- Paneth cells
- Stem cells
Describe the Enterocytes
• Most abundant
• Absorption
• Columnar epithelia with 2000 microvilli on apical surface (brush border)
• Nucleus and mitochondria at basolateral membrane
• RER at apical region
• Surface has a rich carbohydrate containing glycocalyx
- helps trap water, mucus and enzymes for protection
- regulates digestion and absorption
• Connected by tight junctions
• Life span = 1-6 days
Describe the Goblet cells
• 2nd most abundant
• Apical region - mucin granules that distort cell shape (goblet)
• Mucus = water + glycoproteins
- lubricant to facilitate gut passage
• Number increases down the lumen due to water being absorbed (dehydration)
• Life span < 36 hours
Describe the Enter-endocrine cells
• Hormone secreting epithelial cells • Bottom of crypts • Columnar • Sensory apparatus in apical portion • Hormones kept near basolateral membrane for secretion into blood e.g. - G-cells: gastrin - I-cells: cholecystokinin - S-cells: selectin - D-cells: somatostatin
Describe the Paneth cells
• Immunological cells
• Bottom of crypts
• Lots of acidophilic granules with:
- lysozyme
- glycoproteins - protect local cells from enzymes
- zinc - lysozyme cofactor
• Located near stem cells to protect progenitor cells
• Engulf bacteria and regulate intestinal flora
Describe the stem cells
- Pluripotent
- Proliferate in crypts and move up villus to the tip
- Continuous escalator
- Energy intensive, more energy efficient - lesions are rapidly repaired
What are the distinguishing features of the duodenum?
- Coiled mucus secreting submucosal gland
- Secretes bicarbonate-rich alkaline solution that open up into base of crypts
- Help neutralise acid chyme
- Protect lining of proximal small intestine
- Provide optimum pH for enzymes
- Contains specific Brunner’s glands in sub-mucosa - secretes alkaline substances
What are the distinguishing features of the jejunum?
- Large submucosal folds = plicae circulares
- Similar to folds of the rest of the small intestine
- Larger and closer together
What are the distinguishing features of the ileum?
- Peyer’s patches
- Can initiate leucocyte and immunoglobulin responses to pathogens
- Patches without microvilli contain M-cells
What are the 3 functions of ‘motility’?
- Mix ingested food with digestive secretions and enzymes
- Facilitate contact between content and mucosal surface
- Propel content along
How is motility achieved (3 ways)?
• Segmentation - circular muscle - alternate contraction of non-adjacent segments - food pushed forwards and backwards • Peristalsis - circular and longitudinal muscle - sequential contraction - propulsion • Migrating motor complexes - periodic contractions from stomach to distal ileum during fasted state - prevents flora travelling backwards - helps cleanse of food
Describe carbohydrate digestion in the GI tract
• Begins slightly in the mouth - amylase • None in the stomach • Resumes in duodenum - pancreatic amylase & brush border enzymes - complex => basic/branched chain • Pancreatic amylase (and disaccharide enzymes) break polysaccharides into - sucrose (to glucose & fructose) - maltose (to x2 glucose) - lactose (to glucose & galactose)
How are the monosaccharides absorbed?
- Glucose and galactose - Sodium Glucose Linked Transporter 1 on apical surface (secondary active transport)
- Fructose - Glucose transporter 5/GLUT-5 (facilitated diffusion)
- All leave the basolateral membrane via GLUT-2
Describe protein digestion in the small intestines
• Continues in duodenal lume post stomach
• Protease-rich pancreatic juice
- longer peptides => tri/dipeptides
- contains trypsin, chymotrypsin and carboxypeptidase
• Final stage at brush borders
- enzymes secreted from epithelial cells
- usually tri/dipeptidases: endopeptidase, dipeptidase, aminopolypeptidase, carboxypeptidase
How are proteins absorbed?
• Amino acids
- transporters on enterocytes (secondary active transport)
• Short peptides (2-3 a.a.)
- absorbed via apical surface
- broken down via cytoplasmic peptidase enzymes
- move across basolateral membrane into blood (facilitated diffusion)
Describe lipid digestion in the GI tract
• Begins in mouth via lingual lipase - hydrolyses triglycerides
• Stomach
- ingested lingual lipase + secreted gastric lipase
- free triglycerides => single fatty acid chains
- mechanical stomach churning - fat emulsion
• Duodenum - bile emulsifies undigested fat => fat droplets
• Pancreatic juice - triglycerides => monoglycerides + free fatty acids (luminal digestion)
• Digested products + bile salts (micelles) reach brush border
How are fats absorbed?
• Lipolytic products diffuse through apical membrane
• Bile salts remain in lumen, travel through small intestines and reabsorbed in terminal ileum
• Monoglycerides and free fatty acids are resynthesized:
- primary mechanism: monoglyceride acylation pathway
- secondary mechanism: phosphatidic pathway
• Once resynthesized, packaged into chylomicrons (proteins, phospholipids and cholesterols)
• Exocytosed => lymphatic system
