Lecture 27 Flashcards
Absorption in GIT
What is absorption?
Movement of solutes and water from the GIT lumen across the epithelium, into the interstitial fluid and into the blood or lymph
How much absorption occurs in the mouth, oesophagus and stomach?
Very minimal
What is the main site of absorption?
The small intestine
What does the small intestine absorb?
About 90% of water and sodium, and all nutrients
How much absorption, and of what, happens in the large intestine?
9% of water and sodium
How does absorption occur in the small intestine?
Bulk absorption in leaky epithelium
How does absorption occur in the large intestine?
Regulated / fine tuning absorption in tight epithelium
What are the factors that affect absorption? (5)
Reduction in nutrient size, motility, transport across epithelium. surface area available and removal from interstitial fluid
Why must food and chyme travel at the correct rate through the GIT?
To allow mechanical and chemical digestion and absorption
What does the rate of food/chyme travel through the GIT depend on?
Storage in stomach and SI and, peristalsis in the stomach, SI and LI
What is segmentation?
Motility breaking down food to affect the exposure of nutrients to absorptive surfaces
What are the two pathways of transport across GIT epithelium absorption can take?
Paracellular or transcellular
What is the transcellular pathway?
Across the cell membrane and through the cytoplasm
What is the paracellular pathway?
Movement through the tight junctions between cells
What does movement across GIT epithelium require?
A driving force
What are the three different driving forces?
ATP, electrochemical gradients, osmotic gradients
Is the movement of nutrients via the paracellular pathway passive or active?
Passive
How selective is the paracellular pathway?
Dependent on type of tight junction - tight / leaky - and size of particle
How do lipid-soluble particles diffuse through the transcellular pathway?
Able to diffuse directly through the membrane if there is a gradient
How do water-soluble particles diffuse through the transcellular pathway?
They require a channel or transporter membrane protein with a driving force
How does surface area affect absorption?
The rate of particle absorption is proportional to the surface area
How do anatomical adaption maximise the surface area of the small intestines?
Long length, circular folds, villi and microvilli
How is absorption maximised within a fixed surface area?
Through reducing nutrients to their smallest unit via chemical digestion and through using specific transport proteins to absorb particles
Why do particles need to be removed from the interstitial fluid after absorption?
To prevent build up
How are particles removed from interstitial fluid after absorption?
Through having high blood supply to the intestines and blood vessels & lacteals close to the basolateral side of epithelial cells
What absorption mechanism do carbohydrates use in the SI?
Passive and active
What is the driving force allowing carbohydrates to absorb passively?
A glucose concentration gradient, allowing them to diffuse down through the leaky tight junctions
How do carbohydrates absorb actively across the apical membrane? (transcellular)
Using secondary active transport via the sodium-glucose cotransporter
How do carbohydrates absorb actively across the basolateral membrane? (transcellular)
Using facilitated diffusion via the glucose carrier
What is the driving force allowing carbohydrates to travel actively through the transcellular pathway?
The sodium gradient generated by Na+ / K+ ATPase
What absorption mechanism do amino acids from proteins use in the SI?
Passively and actively
What is the driving force allowing amino acids to absorb passively?
An amino acid concentration gradient, allowing them to diffuse down through the leaky tight junctions
How do amino acids absorb actively across the apical membrane? (transcellular)
Using secondary active transport via the sodium-amino cotransporter
How do amino acids absorb actively across the basolateral membrane? (transcellular)
Using facilitated diffused via the amino acid carrier
What is the driving force allowing amino acids to travel actively through the transcellular pathway?
The sodium gradient generated by Na+ / K+ ATPase
What absorption mechanism do di- and tri- peptides from proteins use in the SI?
Only active transport via the transcellular pathway
What is the driving force allowing di- and tri- peptides to travel actively through the transcellular pathway?
The negative membrane potential generated by Na+ / K+ ATPase, generates an electrical gradient for H+
How do di- and tri- peptides absorb actively across the apical membrane? (transcellular)
Using secondary active transport via the H+ - peptide cotransporter
What happens to di- and tri- peptides within the cytoplasm of the cell?
Peptidase digests them into amino acids
How do di- and tri- peptides absorb actively across the basolateral membrane? (transcellular)
Using facilitated diffused via the amino acid carrier
What happens to free fatty acids and monoglycerides in the lumen and apical membrane during lipid absorption?
Free fatty acids and monoglycerides move by simple diffusion from micelles into the cell
Are micelles absorbed during lipid absorption?
No
Where in the SI are bile salts absorbed in the digestive process?
Absorbed in the ileum.
What happens to free fatty acids and monoglycerides intracellularly during lipid absorption?
They are transported to the endoplasmic reticulum and resynthesized into triglycerides, which are then packaged into chylomicrons
What happens to triglycerides at the basolateral membrane during lipid absorption?
Triglycerides (TAGs) are exocytosed in chylomicrons and enter the lymph via lacteals of the villi
How is water lost during the day?
Via sweat, urine, faeces, respiration and secretion into the GIT
What forms of secretion are secreted into the GIT?
Salivary, gastric, pancreatic, biliary and small intestinal secretion
How much water in secreted into the GIT per day?
~8L
How much water arrives in the SI per day?
~9-10L
How is water absorbed?
Passively via osmosis
What is the driving force of water absorption?
Osmotic gradient generated by the absorption of nutrients and ions
How is water absorbed in the SI?
Mostly via paracellular pathway through leaky tight junctions and some via transcellular pathways
How is water absorbed in the LI?
All via the transcellular pathways because of the tight tight junctions in the paracellular pathway
How is water absorbed via the transcellular pathways in the small and large intestines?
Through aquaporins
What are the mechanisms of sodium absorption in the SI?
Passive and active
What is the driving force of passive sodium absorption in the SI?
Sodium ions diffuse down their concentration gradient across leaky tight junctions
What is the driving force for active sodium absorption in the SI?
Sodium gradient created by Na+ / K+ ATPase
How is sodium transported actively across the apical membrane of the SI?
Secondary active transport using the sodium-glucose/amino acids cotransporter
How is sodium transported actively across the basolateral membrane of the SI?
Pumped out by the Na+ / K+ ATPase
What are the mechanisms of sodium absorption in the LI?
Only via Active transport
What is the driving force for active sodium absorption in the LI?
Sodium gradient created by Na+ / K+ ATPase
How is sodium transported actively across the apical membrane of the LI?
Via sodium channels, down its concentration gradient
How is sodium transported actively across the basolateral membrane of the LI?
Pumped out by Na+ / K+ ATPase
What are the fat soluble vitamins?
A, D, E, K
How are fat-soluble vitamins absorbed?
Passively via the transcellular pathway
What is the driving force for fat-soluble vitamin absorption?
A chemical gradient
How do fat-soluble vitamins cross the apical membrane?
Dissolved in micelles with fat: via diffusion through the cell membrane
How do fat-soluble vitamins cross the basolateral membrane?
Packages into chylomicrons with fat
What are the water-soluble vitamins?
C and most of B
What is the driving force for water-soluble vitamin absorption?
Sodium gradient created by Na+ / K+ ATPase
How do water-soluble vitamins cross the apical membrane?
Secondary active transport via the sodium cotransporter
How do water-soluble vitamins cross the basolateral membrane?
Via a transporter
How does vitamin B12 absorb in the ileum?
Through binding to intrinsic factor which is then endocytosed and released from the cell to travel in the blood bound to transcobalamin
What happens to the nutrients we do not absorb?
Removed from body via the defecation reflex
