Duodenum, jejunum and ileum Flashcards
What are the lengths of the duodenum, jejunum and ileum?
25 cm
- 5 m
- 75 m
How tall are villi?
1 mm
What are the invaginations formed by the folding of the submucosa called?
Crypts of Lieberkuhn
What are the three types of cells that constitute the mucosa (excluding the crypts of Lieberkuhn)?
Enterocytes
Goblet Cells
Enteroendocrine Cells
What are the two main cells types found within the crypts of Lieberkuhn?
Stem Cells and Paneth Cells
What is the role of enterocytes and what is their lifespan?
Enterocytes are primary involved in the absorption of substances - they have a life span of around 36 hours (1-6 days)
What is the length of a microvillus?
0.5-1.5 micrometers
What is the layer that covers the microvilli?
Glycocalyx
What are the properties of the glycocalyx?
Rich in carbohydrates and protects the epithelia from the digestional lumen but is still allows absorption. It traps a layer of water and mucus called the ‘unstirred layer’, which regulates the rate of absorption from the intestinal lumen.
The folds, villi and microvilli increase the surface area of the small intestine by what factor?
500 fold
How does goblet cell distribution change across the intestines?
Goblet cells increase in number as you go down the small intestine - this is because the intestinal contents become more and more solid meaning that you require more lubrication to facilitate movement.
What is the role of enteroendocrine cells?
Hormone secreting cells - most often found in the lower parts of the crypts (they are columnar epithelial cells)
They produce hormones that influence gut motility
What do paneth cells produce and what is their role?
They are ONLY found in the bases of crypts and they contain large acidophilic granules containing antibacterial enzyme (LYSOZYME), glycoproteins and zinc. They regulate intestinal flora and protect stem cells.
Describe the arrangement of paneth cells and stem cells in the crypts.
Paneth cells are found right at the bottom of the crypts of Lieberkuhn and on either side of them you find stem cells.
Why is there such a rapid turnover of enterocytes?
Enterocytes are the first line of defence against GI pathogens and they may be directly affected by toxic substances in the diet. Rapid turnover means that any lesions are short lived and that any interference with host cell function will be diminished.
How does the cholera endotoxin affect the gut?
Results in prolonged opening of the chloride channels in the small intestine allowing uncontrolled secretion of water.
Describe a characteristic feature of the duodenum.
Brunner’s Glands - submucosal coiled tubular mucus glands secreting alkaline fluid.
Alkaline fluid neutralises acidic chyme from the stomach, protecting the proximal small intestine and it helps optimise the pH for pancreatic enzymes.
Describe a characteristic feature of the jejunum.
Plicae circulares (valves of Kerckring) - numerous large folds in the submucosa These are present in the duodenum and ileum but in the jejunum they are taller, thinner and more frequent.
Describe a characteristic feature of the ileum.
Peyer’s patches - primes the immune response against intestinal bacteria
They are well positioned to prevent bacteria from the colon
migrating into the small intestine
Describe segmentation and the point of it.
Occurs by the stationary contraction of circular muscles at intervals.
It mixes the contents of the lumen (pancreatic enzymes and bile mix with the chyme)
Describe peristalsis.
Involves sequential contraction of adjacent circular rings of smooth muscle. It propels chyme towards the colon.
What is the migrating motor cortex?
Cycles of smooth muscle contraction. Begins in the stomach, through the small intestine and stops at the start of the colon.
Purpose: SCREEN the gut and keep it clear
More ordered and frequent when fasting
Describe the digestion of carbohydrates.
Digestion begins in the mouth with the action of salivary alpha-amylase
Most digestion takes place in the duodenum by the action of pancreatic alpha-amylase
Digestion of amylase products and simple carbohydrates occurs at the membrane
What conditions does pancreatic alpha-amylase need to function properly?
Needs CHLORIDE for optimum activity
Also needs SLIGHTLY ALKALINE pH
Once the pancreatic enzymes have acted on the carbohydrates, where does the rest of the digestion take place?
Digestion of amylase products and simple carbohydrates takes place at the membrane
Describe the absorption of carbohydrates.
Absorption of glucose and galactose is by secondary active transport via SGLT-1. Glucose and galactose then move into the plasma by facilitated diffusion through GLUT-2.
How does fructose absorption differ from glucose and galactose absorption?
Fructose has a lower concentration inside cells so it can move in by facilitated diffusion via GLUT-5
What is the main pancreatic protease that is required to activate other proteases and how is it activated?
TRYPSIN
Trypsin is secreted by the pancreas as an inactive precursor.
It is activated by ENTEROKINASE (an enzyme found in the duodenal brush border).
Trypsin can then activate other enzymes.
How are amino acids absorbed by the cells?
Brush border peptidases break down the larger peptides before absorption.
Amino acids are then absorbed by facilitated diffusion and secondary active transport.
Cytoplasmic peptidases will break down the di and tripeptides before they cross the basolateral membrane
State the four step procedure by which lipids are digested.
Secretion of bile and lipases
Emulsification
Enzymatic hydrolysis of ester linkages
Solubilisation of lipid products in bile salt micelles
What is the point of emulsification?
Increases the surface area for digestion
What is the point of bile salt micelles?
Allows pancreatic lipase to break down the triglycerides
How does lipase break down triglycerides?
Triglycerides are broken down to 2 free fatty acids and a monoglyceride
How are trypsin and lipase different in the way that they are activated?
Trypsin is secreted by the pancreas as an inactive precursor that is converted to its active form by enterokinase.
Pancreatic lipase is secreted in its active form but it must bind to a cofactor (Colipase - which is secreted in an inactive form) to become active.
State two important enzymes involved in lipid hydrolysis.
Phospholipase A2 - hydrolyses fatty acids in 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
What is the role of micelles?
They are shuttles that take lipids to the surface of the epithelium (across the unstirred layer) and present the fatty acids and monoglycerides to the brush border.
Where are bile salts reabsorbed?
Ileum
NOTE: lipid absorption is usually complete by the middle of the jejunum
What are the two pathways of resynthesis of triglycerides? Describe them.
Monoglyeride Acylation (MAIN one) - acylates the monoglyceride to produce a triglyceride Phosphatidic Acid Pathway - triglycerides are synthesised from CoA fatty acid and alpha-glycerophosphate
How are TAGs packaged and released from the basolateral membrane?
TAGs are packaged into chylomicrons which then enter the lacteals (they are too big to enter the capillaries)
