HUBS192 Lecture 27 - Absorption Flashcards
what is absorption?
the net movement of substances from the GI lumen across the lining of the intestine into the interstitial fluid and then into the blood of lymph
e.g through the villi
why is there minimal absorption in the mouth and the oesophagus?
because food does not spend much time within the mouth, it is quickly chewed and forms a bolus and passes quickly down the oesophagus into the stomach
why is there minimal absorption in the stomach?
not much digestion occurs in the stomach there is some proteolytic digestion but it does not break large molecule down to the smallest size and the stomach is selectively permeable (does not allow for much to cross the epithelial lining)
what is absorbed in the mouth, oesophagus and stomach?
lipid soluble substances
what is the main site for absorption?
the small intestine
what is absorbed in the small intestine?
absorbs 90% of the water and sodium required each day and all other nutrients
what does the large intestine/colon absorb?
9% of the water and sodium required each day
what are the 5 sites of absorption?
1) mouth
2) oesophagus
3) stomach
4) small intestine
5) large intestine
what are the 4 factors affecting absorption?
1) motility
2) surface area available for absorption
3) transport across epithelium
- reduction in size from chemical digestion
- membrane transporters
4) removal from interstitial fluid
what does the correct motility rate of propulsion allow?
allows for digestion and absorption to occur simultaneously
what are the 5 ways that motility affects absorption?
1) correct rate of propulsion to allow digestion and absorption
2) storage
3) peristalsis
4) exposure to products of digestion to absorptive surfaces
5) segmentation
how does the available surface area for absorption affect absorption?
1) the rate of absorption is proportional to surface area (greater the surface area the faster the rate of absorption)
2) anatomical adaptations maximise surface area
what are the 4 anatomical adaptations?
1) length of the intestine (6m)
2) circular folds or plicae circulars
3) villi
4) microvilli
what are the 2 pathways molecules are transported across the epithelium?
1) paracellular pathway
2) cellular pathway
what is the paracellular pathway?
the ‘gap’ between cells which is the tight junction, which is the only barrier that binds cells together
what is the cellular pathway?
transport across the cell membranes through the cytoplasm
what does it mean that the paracellular pathway is relatively non-selective and passive?
- passive = requires a gradient
- relatively non-selective = if the solute is small enough it can get across (solutes do not cross cell membranes)
what are cell membranes?
lipid bilayers
how many membranes must solutes cross in the cellular pathway?
solutes must cross 2 cell membranes
what does a solute require if it is not lipid soluble?
it requires a transport protein
how is absorption maximised across the available surface area?
- reduces nutrients into the smallest possible unit through chemical digestion
- specific transport proteins by absorbing what is required and allowing active transport
how does the removal of substance from the interstitial fluid affect absorption?
-large blood flow to the intestine contributes to the arrangement in villi of the blood vessels and lacteal. This allows the substances to be removed very quickly from the interstitial fluid and absorbed into the villi, preventing build up in the interstitial fluid
what are the 7 specific substances absorbed?
1) water
2) sodium
3) carbohydrates (sugars)
4) proteins
5) lipids (fats)
6) bile salts
7) vitamins
why do we need to drink 1.5 L of water per day?
to replace the water lost in sweat, urine and faeces as we breathe
what is the total volume of water delivered to the small intestine per day?
9-10 L per day
what is the mechanism of water absorption?
osmosis
-passive movement of water from the lumen into the blood
how is the osmotic gradient established from absorption?
osmotic gradient set up by the absorption of salts and nutrients
what are the 2 pathways of absorption of sodium (Na+)?
1) passive movement via the paracellular pathway
2) active transport via the cells
what is the active transport via the cells of sodium absorption?
transcellular absorption that requires transporters to cross the cell membranes
what are the 3 mechanisms of active transport via the cells of sodium absorption?
1) Na+ transport alone
2) Na+ transport coupled to monosaccharides (e.g glucose)
3) Na+ transport coupled to amino acids
what is cellular sodium absorption through the mechanism of sodium transport alone?
sodium moves into the cell down its gradient (from high concentration to low concentration) through the sodium channel and the Na+/H+ exchanger. For sodium to leave the cell it needs to go from a low concentration to a high concentration though active transport (uses ATP) and involves Na+, K+ -ATPase
what are the mechanisms of the absorption of carbohydrates?
1) passive absorption
2) active absorption
what is the passive absorption of carbohydrates?
passive absorption of monosaccharides (glucose, galactose, fructose) down their concentration gradient via a paracellular pathway
what is the active absorption of carbohydrates?
uses the diffusion of sodium down its concentration gradient to provide the energy to transport the monosaccharide (glucose, galactose) across the cell membrane. Sodium is transported out across the basolateral membrane via the Na+/K+ -ATPase
what are the 2 mechanisms of protein absorption?
1) passive absorption
2) active absorption
what is the passive absorption of proteins?
absorption of amino acids that diffuse down their concentration gradient s via a paracellular pathway
what is the active absorption of proteins?
proteins are broken down to amino acids which are cotransported with Na+ amino acid cotransporter via a cellular pathway. Amino acids establish a concentration gradient through the basolateral membrane through specific transporters across the cell membrane
what is the absorption of the products of fat digestion?
delivered to the brush border by micelles where the whole micelle is not absorbed only the fatty acids and monoglycerides diffuse into the cell
-the whole micelle is not absorbed
what are the 4 steps in the formation of chylomicrons?
1) fatty acids and monoglycerides are resynthesises into triglycerides
2) packaged into chylomicrons
3) chylomicrons exits the cell by exocytosis
4) chylomicrons enter the lacteals
