Alimentary Systems 3 - Small Intestine Flashcards
What is the function of the small intestine?
To absorb nutrients salt and water
List the dimentions of the small intestine
- 6m long, 3.5cm diameter
- Duodenum 25cm
- Jejunum 2.5m
- Ileum 3.75m
- No sudden transition between the 3 parts
What is the function of mesentry in the small intestine?
- Throws the small intestine into folds
- Supports the blood supply
Describe the layout of the epithelium of the small intestine
- External wall has longitudinal and circular muscles
- Internal mucosa arranged in circular folds
- Mucosa covered in villi
- Invaginations (crypts of Lieberkuhn)
List the characteristics of villi
- Only present in the small intestine to increase the SA
- Motile with rich blood supply and lymph drainage for absorption of digested nutrients
- Good innervation from the submucosal plexus.
- Simple epitelium dominated by enterocytes
List the cell types in the mucosa
- Simple columnar epithelium
- Primarily enterocytes (absorptive)
- Scattered goblet cells
- Enteroendocrine cells
List the cell types in the crypts of Lieberkuhn
- Paneth cells
- Stem cells
Where are villi located on the enterocytes?
- Apical membrane
- Blood at the basolateral membrane
What are microvilli
- They make up the brush border, present on villi.
- Have glycocalyx, a carboydrate layer
What are the functions of glycocalyx
- Allows for absorption while protecting from the digestional lumen.
- Also traps a layer of water and mucous called the unstirred layer which regulates the rate of absorption from the intestinal lumen
What is the function of goblet cells?
- Secrete mucous containing granules
- There are more the further down the digestive tract
What is the function of enteroendocrine cells in the small intestine? Give examples
- Found in the lower parts of the crypts of Lieberkuhn
- Secrete hormones to influence gut motility
- G-cells secrete gastrin
- I-cells secrete cholecystokinin
- S-cells secrete secretin
- D-cells secrete somatostatin
What is the function of paneth cells in the SI?
- Found only in the bases of the crypts
- Contain acidophilic granules containing lysozyme and glycoproteins
- Engulf some bacteria and protozoa
Why is the cell turnover in the epithelium of the gut so high?
- Due to damage by toxins
- Lesions will be sort lived
- Allows people to recover from cholera if hydrated for 1-2 days
How is the duodenum distinguished?
- The presence of brunners glands
- These glands are submucosal coiled tubular mucous glands that secrete alkaline fluid
- Open into the base of the crypts
- Alkaline secretions neutralise chyme from the stomach, and optimise pH for pancreatic digestion enzymes
How is the jejunum distinguished?
- Presence of numerous large folds in the submucosa called plicae circulares (valves of Kerckring)
- Though present in all the small intestine, they are taller, thinner and more frequent here
How is the ileum distinguished?
- Lots of peyers patches (lymph nodules in the submucosa)
- Prevents bacteria from the colon migrating into the SI
How are things moved through the small intestine?
- Segmentation mixes contents of the lumen by contraction of circular muscles
- Peristalsis moves chyme towards the colon
- Migrating motor complex
What is the migrating motor complex?
- Seen in fasting
- Cycles of smooth muscle contraction to clean the intestine and move bacteria away from the ileum
Where does digestion occur in the duodenum?
- In the lumen and in contact with the membrane
What is the difference between primary and secondary active transport?
- Primary involves hydrolysis of ATP
- Secondary involves electrochemical gradients
Describe the process of carbohydrate digestion
- Starts with salivary amylase in the mouth, destroyed by the stomach pH
- In the duodenum pancreatic a-amylase is secreted in response to a meal. Needs an alkaline pH and Cl-.
- a-amylase acts on the lumen to break down starch and glycogen to smaller carbohydrates
- Amylase products and simple carbohydrates are digested at the membrane
How does absorption of carbohydrates occur in the SI?
- Secondary active transport of glucose and galactose by SGLT-1
- Fructose by facilitated diffusion using GLUT-5
- GLUT-2 facilitates exit at the basolateral membrane
Describe the digestion of proteins in the SI
- Begins in the stomach by pepsin which is inactivated in the duodenum due to pH
- Trypsin is activated by enterokinase at the duodenal brush border, and then activates other proteases
- Brush border peptides break down larger peptides
How are amino acids and di/tri peptides absorbed at the brush border?
- Absorption of amino acids occurs by facilitated diffusion and secondary active transport
- Di/tri-peptides require specific carrier proteins
- Cytoplasmic peptidases break down di/tri-peptides before they cross the basolateral membrane
How are lipids digested?
- Secretion of bile and lipases
- Emulsification (increase SA for digestion)
- Enzymatic hydrolysis and ester linkages (lipase + colipase)
- Solubilization of lipolytic products in bile salt micelles
What is the function of colipase?
Prevents bile salts from displacing lipase from the fat droplet
What are the other enzymes, not icluding lipase, important for lipid digestion?
- Phospholipase A2 hydrolyses fatty acids at the 2 position in many phospholipids, resulting in lyso-phospholipids and free fatty acids.
- Pancreatic cholesterol esterase hydrolyses cholesterol ester to free cholesterol and fatty acid.
Describe the structure of a bile salt
- Steroid nucleus planar- two faces.
- Amphipathic.
- Hydrophobic (nucleus and methyl) face dissolves in fat
- Hydrophilic (hydroxyl and carboxyl) face dissolves in water
What are bile salt micelles?
- Hydrophilic head in contact with solvent, hydropobic tails in the middle with a lipid core
- Water insolble in SI
How are lipids absorbed?
- Micelles important in absorption
- Allow transport across the unstirred layer, present the fatty acids and monoglycerides to the brush border
- Bile absorbed at the ileum, lipids by the middle of the jejunum
How are lipids resynthesised into triglycerides?
- Monoglyceride acylation (major),
- Phosphatidic acid pathway (minor)
Describe the process of monoglyceride acylation
- Fatty acids bind to the apical membrane.
- Fatty acid binding proteins (FABP) facilitate transfer of fatty acids from apical membrane to the smooth ER.
- In the smooth ER - fatty acids esterified into diglycerides and triglycerides.
Describe the process that occurs in the phosphatidic acid pathway
Triglycerides synthesised from CoA fatty acid and a-glycerophosphate
Describe the pathway taken by chylomicrons
- Chylomicrons transported to the golgi and secreted across the basement membrane by exocytosis
- Too big to enter blood capillaries of villi, so travel through lymph channels instead
What is the function of the ileocaecal sphincter
- Separates the ileum from the colon
- Relaxation and contraction controls the passage of material into the colon
- Prevents backflow of bacteria into the ileum
