Transport Across Alimentary Tract Flashcards
What nervous systems are present in the gut?
The gut has an intrinsic and enteric nervous system
- large neuronal connections in the gut
Name the 2 intrinsic neural plexus of the gut
Submucosal Meissner Plexus
- regulates digestive glands
Myenteric Auerbach’s Plexus
- connected to gut motility
What are the intrinsic neural controls of the GI tract?
Nerve plexus’ near GI tract initiate short reflexes mediated by local ENS plexus
What are the extrinsic neural controls of the GI tract?
Long reflexes arising within/outside GI tract
Involve CNS and extrinsic Parasympathetic reflexes
What type of neurons are within the plexi?
The nerve plexi contain local sensory + motor neurones as well as inter-neurons for communication
What stimulates the motor neurons in the myenteric plexus?
Motor neurons stimulated by Ach (m receptors) and substance P release
What do inhibitory Motor neurons release?
Cause the release of VIP/NO
What is the Serosa?
tissue of the outer serous membrane of the gut
Is continuous with the mesentery containing blood vessels, lymph vessels and nerve fibres
Where are sensory neurons found in the gut?
Connected to mucosal chemoreceptors and stretch receptors
What is the function of sensory neurons in the GI tract?
Respond to gut wall tension caused by food and chyme
What type of cells is the small intestine composed of?
epithelial folds
villi + microvilli
- to provide a large SA of 200 m²
What is the fate of the small intestine contents?
all dietary nutrients, water and electrolytes that enter the upper small intestine are absorbed
What are crypt cells?
Cells located where the epithelia of villi extend down into the lamina propria to form crypts
Where us the lamina propria located?
Beneath the epithelium
Which cells are located within the crypts?
Many important cells reside in the crypts including host defence + signalling cells, and stem cells for replacement
What is transcellular transport (give an example)?
Transport of solutes by a cell through a cell
e.g. glucose transport from intestinal lumen →ECF by epithelial cells
What is paracellular transport?
Passage of solutes between cells; passive but selective, variable and regulated
e.g. Na+, Cl-, H₂O
How are carbohydrates absorbed?
Can only be absorbed as Monosaccharides
complex CHO need to be reduced to disaccharides by amylase
How are disaccharides converted to monosaccharides?
Specific brush border enzymes convert disaccharides → monosaccharides (e.g. glucose and galactose)
Give examples of disaccharides conversions to monosaccharides via their specific enzymes
Sucrose → Glucose + fructose
- via sucrase
Starch →glucose oligomers
- via salivary + pancreatic amylase
Glucose oligomers →glucose
- via glucoamylase
Lactose→glucose + galactose
- via lactase
How are glucose and galactose transported?
Rapidly absorbed by a secondary active transport mechanism
Explain how glucose and galactose are transported
- Na generates an electrochemical gradient
- Na+ flux out of cell
- K+ moves into cell
- Glucose moves out along with Na+ as it’s pumped out
due to SGLT1 transporter binding site - Galactose also joins due to a similar binding site
- GLUT2 transporters transport glucose → bloodstream
- Fructose →blood via passive diffusion through GLUT5
transporter
Which molecules does SGLT transport?
Glcose and fructose
Which enzyme facilitates protein production?
Pepsin action produces polypeptides
Which enzymes aid digestion and absorption of polypeptides?
Pancreatic proteases
Dipeptidases in the brush border cause complete digestion into amino acids
How are amino acids transported within the small intestine?
On a sodium coupled carrier system (similar to glucose)
Separate carriers for different amino acids
How are some di/tri-peptides transported in the small intestine?
On a carrier system using inwardly directed H+ gradient
What dietary lipids do we obtain?
90% TGs
Phospholipids
Cholesterol
Fat soluble vitamins A,D,E,K
How is lipid absorption facilitated?
Dietary TGs are broken down into simpler units to facilitate absorption
Where along the GI tract are lipids digested?
Small fraction digested by salivary lipases (in mouth)
Most TGs digested in small intestine
Gastric lipase hydrolyses 10-30%
remainder digested by pancreatic lipases
Explain why TGs are harder to dissolve
Water insoluble
Chyme (emulsion of fat particles in water
Lipase is water soluble
TGs must be dissolved in the aqueous phase before they can be digested
Which two processes facilitate lipid digestion + absorption?
- emulsification
2. micelle formation
Describe the composition of TGs
1 glycerol + 3 fatty acids
Explain how TG emulsification enables digestion by lipases
Lipase action requires emulsification of TGs by bile salts which dissolve TG in water
Pancreatic lipase then binds to the surface of the small emulsion particles
What is the role of bile salts in emulsification of lipids?
Break large fat globules → smaller globules
SA for lipase to act on is increased
What are micelles?
lipid molecules that arrange themselves in spherical formation in aqueous solutions
How are micelles transported?
small enough to be taken up by enterocytes → lacteals→ lymphatics before travelling to blood via thoracis duct → subclavian vein
Explain why TG formation occurs?
to form chylomicrons
What are chylomicrons?
Largest lipoprotein
What is a lipoprotein?
Protein and lipid components
What is an apo?
coenzyme/cofactor attached to protein
Explain the milky appearance of lymph
Lymph has a milky appearance during drainage from small intestine due to chylomicrons
Explain the route chylomicrons take to reach the blood
Chylomicrons enter lymphatic system via lacteals and then travel via lymph vessels and enter blood
What are the disorders of fat digestion?
Gallstones
Pancreatitis
Crohn’s disease
Liver disease
What is the consequence of fat digestion disorders?
Can lead to fat malabsorption (steatorrhoea)
; excess fat in faeces
