27 Flashcards
Absorption of the products of digestion for all nutrients always requires a driving force : Tr u e or False?
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What is absorbtion
Absorption is 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
Sites of absorption - how much is absorbed in each place and what is absorbed?
Mouth, esophagus, stomach
- Minimal absorption
- Lipid soluble substances
Small intestine
- Main site of absorption
- 90% of water and sodium
- All nutrients
Large intestine
- 9% on water and sodium
Factors affecting absorption
- Motility
- Surface area available for absorption
- Transport across epithelium
- Reduction in size (chemical digestion)
- Membrane transporters
- Removal from interstitial fluid
Affect of motility on absorption
- propulsion
Correct rate of propulsion to allow
- digestion (storage - having enough time to digest)
- absorbtion (peristalsis - having enough time to absorb)
Affect of motility on absorption
- segmentation
- exposure of products of digestion to absorb active surface
Rate of absorption proportional to
Surface area
- Greater the surface area the faster the rate of absorption
Anatomical adaptations maximizes
surface area
- Length of intestine (6m)
- Circular folds (plicae circulares)
- Villi
- Microvilli
Transport of molecules across epithelium - what’s the prob??
- The lumen of the intestine is continuous
with the outside world - The intestinal epithelium is a barrier
Two pathways for Transport of molecules across epithelium
- BLOOD and LYMPH
- Paracellular
- Between cells
- Transcellular
- Across the cell membranes
- Through the cytoplasm
Paracellular pathways – gaps between cells
- FEATURES
- Solutes do not cross cell membranes
- Only barrier is tight junctions binding cells together
- Relatively non-selective
- If the solute is small enough it can get across
- Passive
- Requires a gradient
- (blood vessels on other side of epithelia help with the
driving force)
Transcellular pathway features
- Solutes must cross two cell membranes
- Cell membranes are lipid bilayers
- If solute is not lipid soluble it requires a transport protein
Two ways to maximise absorption across available surface area
- chemical digestion
- reduces nutrients into smallest unit possible
- Specific transport protein (need one in both membranes)
- absorb what is required
- allows active transport
- against a gradient
Removal of substance from interstitial fluid
- how it affects absorbtion
There is a large blood flow to intestine
Arrangement in vili of blood vessels and lacteals prevent the build up in interstitial fluid
How much water do we absorb each day? - kinda YAPPY
Drink ≈1.5 L per day
- Replaces water lost sweat, urine & faeces and as we
breathe
But how much is secreted into the GI tract?
- Salivary secretion ≈1.5 L per day
- Gastric secretion ≈ 3 L per day
- Pancreatic secretion ≈ 1.5 L per day
- Biliary (Liver) secretion ≈ 0.5 L per day
- Small intestinal secretion ≈1.5L per day
Total water delivered to small intestine ≈ 9-10 L per day
- If we do not replace the losses and reabsorb the secreted water – major problem
Mechanism of water absorption
Osmosis
- Passive movement of water from lumen into blood
- Osmotic gradient (driving force) set up by absorption of salts and nutrients
- Mostly by the paracellular pathway
Sodium (Na+) absorption
Passive movement via paracellular pathway
OR
Active transport via the cells
- Transcellular
- Requires transporters to cross the cell membranes
Mechanism:
- Na+ transport alone
- Na+ transport coupled to monosaccharides (e.g.,
glucose, galactose)
- Na+ transport coupled to amino acids
Absorption of Na+ alone
- Na+ moves into cell down its gradient
- High to low concentration
- Transport proteins in apical membrane
- Na+ channels
- Na+/H+ exchangers
-To get out of cell it needs to go from low to high concentration using active transport (Uses energy – ATP - Na+,K+-ATPase)
What are the 2 Mechanisms
Of carbohydrate absorbtion
Passive and active absorption
Carbohydrate Passive absorption via paracellular path
Monosaccharides
- Glucose
- Galactose
- Fructose
Diffuse down concentration gradient via
paracellular pathway
Carbohydrate absorption - active absorbtion
Cotransport with Na+
- Via cellular pathway
Monosaccharides
- E.g. Fructose, Glucose Galactose
Transported across cell membrane
- Driving force = Na+/K+ATPase
- on Apical membrane - Na+ glucose cotransporter
- on Basolateral membrane - Glucose carrier
Absorption of products of protein digestion – Passive
Passive absorption via paracellular path
Amino acids Diffuse down concentration gradient via
paracellular pathway
Absorption of products of protein digestion – active absorption of amino acids
Active absorption
Cotransport with Na+
- Via cellular pathway (Similar to glucose absorption)
Transported across cell membrane
- Driving force from Na+/K+ATPase in basolateral membrane
- In Apical membrane - Na+ amino acid cotransporter
- In Basolateral membrane - Amino acid carrier
Absorption of products of protein digestion – Small Peptides (di and tri peptides)
- Active absorption
- Cotransport with H+
- Via cellular pathway - Transported across cell membrane
- Driving force is Na+/K+ATPase in basolateral membrane (maintains –ve membrane potential for H+ movement)
- in Apical membrane - H+ / Peptide cotransporter
- Cytoplasmic peptidases - Digest small peptides to AA’s
- in Basolateral membrane there is an Amino acid carrier
Absorption of products of fat digestion
Lumen and Apical membrane:
- Free fatty acids and monoglycerides move by simple diffusion from micelle into cell.
- Micelle not absorbed.
- Bile salts absorbed in ileum.
Intracellular:
- Transported to the endoplasmic reticulum.
- Resynthesized to triglycerides to maintain gradient for apical diffusion.
- Secreted from golgi as chylomicrons.
- Targeted to basolateral membrane
Basolateral membrane:
- Exocytosed as chylomicrons.
- Enter lymph via lacteals of villi.
Bile salt absorption
- The bile salts in the micelles are eventually absorbed
- Occurs after fat absorption complete
In the ileum: Active transport process
- Apical Na+ dependent bile acid cotransporter
In the colon - Passive absorption
- Absorbs 95% of bile salts
Vitamin absorption - fat soluble vitamins
V i t a m i n s A , D, E , K - Absorbed with fats
Vitamin absorbtion - Water soluble vitamins
- B group and vitamin C - Na+ dependent
- Very similar to monosaccharide and amino acid absorption.
Vitamin absorbtion - vitamin B12
- Absorbed in the ileum
- Binds to intrinsic factor produced in stomach
- Specific transporter for Intrinsic factor vitamin B12 in the ileum
We do not absorb everything
- ELMINATION
Expulsion of residues of digestion
- Faeces formed in large intestine
- Transferred to rectum via peristaltic waves (Mass movements)
- Elimination from body by defecation reflex
