lecture 27 - absorption Flashcards
What is GI absorption?
The net passage of substances from the GI lumen across the lining of the intestine into the interstitial fluid and then into the blood or lymph
What absorption occurs in the mouth, oesophagus and stomach?
minimal absorption - lipid soluble substances
What absorption occurs in the small intestine?
All nutrients, and 90% of water and sodium
What absorption occurs in the large intestine?
9% of water and sodium - no nutrients
What part of the GI tract does the majority of absorption occur in?
Small intestine
What are the 4 factors affecting GI absorption?
Motility, surface area, transport across epithelium, removal from interstitial fluid
How does motility affect absorption in the GI tract?
The correct rate of propulsion needs to ensure sufficient time for digestion. Segmentation allows products of digestion to be exposed to the absorptive surfaces
What are the 2 types of transport across epithelium in absportion in the GI tract?
Paracellular and transcellular pathways
What is required to efficiently remove the products of digestion from interstitial fluid after absorption across the epithelium?
A large blood supply with villi arranged with blood vessels and lacteals
What is the mechanism of water absorption?
Osmosis - the passive movement of water from lumen into the blood - from low solute to high solute concentration.
What creates the osmotic gradient that allows for water to absorbed in the intestines?
The absorption of salts and nutrients generates a high solute concentration within the blood
What are the 2 possible pathways for sodium absorption?
Passive movement via the paracellular pathway or active transport via cells
What are the 3 transcellular mechanisms in sodium absorption from the lumen to the epithelial cells and then out in the SI/LI?
Na+ transporters alone, Na+ transport coupled to monosaccharides, Na+ transport coupled to amino acids
How is Na+ absorbed (on its own) from the lumen into epithelial cells of the SI, as part of a transcellular pathway ?
Na+ moves down its concentration gradient into cells via transport proteins such as the Na+ channel and Na+/H+ exchanger
How is Na+ released from SI epithelial cells into the interstitial fluid in a transcellular absorption pathway?
Uses active transport to go from low to high concentration via Na+,K+-ATPase, which pumps Na+ out of the cell while bringing K+ in.
What are the 2 absorption mechanisms for carbohydrates?
Passive via paracellular pathways, and actively via transcellular pathways
What carbohydrate molecules can be passively absorbed via the paracellular pathway in the SI?
Monosaccharides - glucose, galactose and fructose - which diffuse down their concentration gradient
What monosaccharides can be passively absorbed in the paracellular pathway in the SI?
glucose, galactose, fructose
What transcellular mechanism is used for carbohydrate absorption in the SI?
Co-transport with Na+
What monosaccharides are absorbed in the SI via the transcellular pathway?
Glucose, galactose
What cotransporter is required to absorb monosaccharides from the lumen into the SI epithelial cells?
Na+ monosaccharide contrasporter, which brings Na+ and a monosaccharide into the cell, with Na+ down its concentration gradient
What transporter is used to release monosaccharides from an SI epithelial into the interstitial fluid?
Monosaccharide carrier - passive diffusion
What is used to ensure that there is a Na+ concentration gradient across the apical membrane so that the Na+ monosaccharide cotransporter has a driving force?
A Na+,K+ - ATPase into the serosal membrane pumps Na+ from the interstitial fluid to maintain a low concentration of sodium within the cell.
What are the 2 mechanisms of SI absorption of amino acids (protein digestion products)?
passive via paracellular pathway, and active via transcellular pathway
What cotransporter is required to absorb amino acids from the lumen into the SI epithelial cells?
Na+ amino acid contransporter
What transporter is used to release absorbed amino acids from an SI epithelial cell into the interstitial fluid?
Amino acid carrier - passive diffusion
What is used to ensure that there is a Na+ concentration gradient across the apical membrane so that the Na+ amino acid cotransporter has a driving force?
A Na+,K+ - ATPase into the serosal membrane pumps Na+ from the interstitial fluid to maintain a low concentration of sodium within the cell.
What is the mechanism of absorption of larger products of protein digestion - di and tri peptides?
Active transcellular absorption via contransport with H+
What cotransporter is required to absorb di and tri peptides from the lumen into the SI epithelial cells?
H+ peptide cotransporter on the apical membrane
What is used to break di and tri peptides down into amino acids when they have been absorbed into SI epithelial cells?
Intracellular peptidases - the amino acids can then be transported to the interstitial fluid via amino acid carriers in the basolateral membrane
How are fats absorbed across the SI epithelial apical membrane into the epithelial cells?
Bile salts dissociate from free fatty acids and monoglycerides so they are no longer bound in a micelles and can pass though apical membrane by simple diffusion.
What happens to fats when they have diffused into SI epithelial cells?
The are transported to the endoplasmic reticulum where they are resynthesised to triglycerides to maintain a gradient for apical diffusion. They are secreted from the Golgi as chylomicrons that target the basolateral membrane for exocytosis.
What aids in the resynthesis of monoglycerides and free fatty acids in SI epithelial cells?
Triglyceride synthetic enzymes in the Endoplasmic Reticulum
Why must free fatty acids and monoglycerides be resynthesised to triglycerides once absorbed into SI epithelial cells?
To maintain a concentration gradient for further absorption from the apical membrane, and so that they can be packaged into chylomicrons
In what form are triglycerides exocytosed from SI epithelial cells?
Chylomicrons
Where do chylomicrons enter after being exocytosed from SI epithelial cells?
Enter lymph via lacteals of villi
When does bile salt reabsorption occur?
After fat absorption is complete
Where does bile salt reabsorption occur?
In the ileum and then in the colon
What is the mechanism of bile salt reabsorption in the ileum?
Active transport using apical Na+ dependent bile acid cotransporters.
What is the mechanism of bile salt reabsorption in the colon?
Passive absorption
What are the 2 types of vitamin?
Fat soluble vitamins and water soluble vitamins
How are fat soluble vitamins absorbed?
With fats
How are water soluble vitamins absorbed?
Via Na+ dependent absorption
What is the key water soluble vitamin?
Vitamin C
How is Vitamin B12 absorbed?
In the ileum by binding to intrinsic factor and being absorbed via a specific cotransporter.
What is the condition caused by a lack of intrinsic factor?
Pernicious anemia
