Lecture 27 - Absorption Flashcards
Absorption
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
Once it crosses the epithelial layer it either foes to the blood or to the lymph
Sites of absorption
Mouth, esophagus and stomach
Minimal absorption - lipid soluble substances
Minimal absorption since the food spends very little time in the mouth and esophagus, it’s job is to form a bolus and pass it down
Small intestine (most important site of absorption)
Main site of absorption
90% of water and sodium
All nutrients
Large intestine
9% of water and sodium
Factors that affect the 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
Correct rate of propulsion to allow - digestion and absorption
Storage
Perstalsis
Exposure of products of digestion to absorptive surface
Segmentation - mixes food up and exposes it to absorptive surfaces which accelerates the process of digestion and absorption
Affect of available surface area on absorption
Rate of absorption proportional to surface area - greater the surface area the faster the rate of absorption
Anatomical adaptations maximises surface area Length of intestine 6m Circular folds Villi Microvilli
Transport of molecules across epithelium affect on absorption
The problem
The lumen of the intestine is continuous with the outside via out mouth and anus
The intestinal epithelium is a barrier - epithelium provide us with an absorptive surface but it also creates a barrier to stop material getting in from the body like viruses and bacteria
Two pathways for molecules to get across the epithelial layer
Paracellular - gap between cells
Cellular - across the cell membranes, through the cytoplasm
Paracellular pathways
Gaps between cells
Solutes do not cross cell membranes
Only barrier os tight junctions binding cells together
Relatively non-selective
If the solute is small enough it can get across
Passive
Requires a gradient - higher concentration of it in the lumen than in the interstitial fluid
Cellular pathway
Solutes must cross two cell membranes
Cell membranes are lipid bilayers
If the solute is not lipid soluble (such as sodium) it requires a transport protein
To maximise absorption across available surface area
Reduces nutrients into smallest possible unit - monosaccharides or amino acids for example
This is done through chemical digestion
Specific transport proteins
Absorb what is required
Allows active transport - again a gradient which allows us to virtually absorb all the nutrients we need from a meal
Removal of substance from interstitial fluid affect on absorption
Large blood flow to the intestine
Arrangement in villi of …
Blood vessels
Lacteal
Prevents build up of interstitial fluid
Send capillaries (blood vessels) right up to the tip of the villus and back down again and the lacteal penetrates right to the tip and back down also which means that any substance that is absorbed across the epithelial layer and into the interstitial space only has a short distance to go before it is in the blood and can be transported away and the advantage of this is that it stops material building up on the other side of the absorptive surface so it mains any passive gradients and also minimises the concentration against which the substance has to be transported
How much water do we absorb each day?
Drink approximately 1.5L a day
Replaces water lost in sweat (secrete water onto the surface of the skin and it evaporates) , urine and faeces and as we breathe
But how much do we secrete into the GI tract?
Salivary secretion is approx 1.5 L per day
Gastric secretion is approx 3L per day
Pancreatic secretion is approx 1.5L a day
Biliary (liver) secretion is approx 0.5L per day
Small intestinal secretion is approx 1.5L per day
Total water delivered to the small intestine is approx 9-10L per day
If we do not replace the losses and reabsorb the secreted water there is a major problem
Mechanism of water absorption
Osmosis
Passive movement of water from lumen into the blood
Osmotic gradient set up by absorption of salts and nutrients
Sodium absorption
Passive movement via a paracellular pathway which requires a gradient to occur
OR
Active transport via the cells
Transcellular
Requires transporters to cross the cell membranes (because sodium is not lipid soluble)
Mechanism
-Sodium transport alone
-Sodium transport coupled to monosaccharides such as glucose or galactose (transported across the epithelium along with monosaccharides)
-Sodium transport coupled to amino acids
Absorption of sodium alone
Step 1 - Na+ moves into cell down its concentration gradient
High to low concentration
Transport proteins - Na+ channel and Na+/H+ exchanger
Step 2 - To get out of cell it needs to go from low to high concentration
Utilises ATP for active transport
Na+,K+-ATPase
High conc of Na+ outside the cell and low Na+ inside the cell
Carbohydrate absorption
2 mechanisms
Passive or active absorption
Passive absorption
Monosaccharides - glucose, galactose, fructose
Diffuse down concentration gradient via paracellular pathway (concentration gradient from the lumen to the interstitial fluid and they are small enough to flow through the paracellular pathway from the lumen to the blood/lymph
Active absorption (for both sodium and monosaccharide)
Cotransport with Na+ via cellular pathway
Monosaccharides - glucose, galactose
Transprted across cell membrane - specific transporters, apical membrane - Na+ glucose transporter, basolateral membrane - glucose carrier
Active absorption of carbohydrates summary
Starch is broken down initially within the lumen by pancreatic amylase to produce disaccharides. This disaccharide is then digested and broken down into monosaccharides by membrane bound brush border enzymes. Monosaccharides are absorbed across the luminal membrane of the epithelial cell via a sodium coupled monosaccharide transporter and it uses the diffusion gradient of sodium down its concentration gradient into the cell to provide the energy to transport the monosaccharide across the luminal membrane and into the cell as well. Because there is a big concentration gradient for sodium to move into the cell, quite a lot of monosaccharide can be bought into the cell and the sodium has been bought in is then diffused across the epithelial cell and is transported out through the serousal membrane via the sodium-potassium ATPase which uses energy to transport sodium against its concentration gradient and now monosaccharides can leave the cell via diffusing down its concentration gradient via a monosaccharide transporter (glucose carrier/monosaccharide carrier)
Absorption of products of protein digestion - amino acids
2 mechanisms
Passive or active absorption
Passive absorption
Amino acids
Diffus down concentration gradient via the paracellular pathway
Active absorption
Cotransport with Na+ - via cellular pathway, similar to glucose absorption
Transported across the cell membrane - specific transporters, apical membrane (Na+ amino acid cotransporter), basolateral membrane (amino acid carrier)
Active absorption of amino acids summary
Out of the lumen of the small intestine, we have digestion of proteins by pancreatic proteases which initially breaks down our proteins into polypeptides and these polypeptides are then broken down into individual amino acids by membrane bound enzymes at the surface of the epithelial cells in the small intestine and it is the individual amino acids that are absorbed via the cellular pathway or the paracellular pathway. Absorption of the amino acids via the cellular pathway involves a sodium dependent process, transport protein present in the luminal membrane that can transport both sodium and amino acids across and sodium diffuses across the cell and is transported out the serosal membrane via the sodium-potassium ATPase into the blood and is transported away from the epithelial surface. Sodium is used to accumulate amino acids in the cell at a concentration that is above that in the cell at a concentration that is above that in the interstitial fluid so that there is a concentration gradient so now the amino acids can diffuse across the membrane via the transporter/carrier and get taken up by the blood
Absorption of products of fat digestion
Products of fat digestion are lipid soluble
Can diffuse across the cell membrane
Don’t need a transporter
Delivered to brush border by micelles
Whole micelle is not absorbed - the whole micelle and the bile salts are not absorbed, fatty acids and monoglycerides diffuse into the cell
In cell - resynthesises into triglycerides, packaged into chylomicrons, chylomicrons exit cell by exocytosis, chylomicrons enter the lacteals
Micelles are carried up to the surface of the epithelium by the motility patterns of the intestinal tract and now the fatty acid and monoglycerides within the micelle diffuse out when in close proximity to the epithelial cell and across the lumeninal membrane and into the epithelial cell and the micelle themselves and the bile salts do not enter the cell at this stage. Once the protects of fat digestion get into the epithelial cell what happens is that they are resynthesised into triglycerides and then packaged into chylomicrons which are little packages that contain triglycerides , these chylomicrons are too big to get out across a membrane transporter so they get out via exocytosis and once they are transported into the interstitial fluid they are too big to get into the blood system so they enter the lacteal which is much leaky to than the blood system and are transported away from the absorptive surface via the lacteal
Bile salt absorption
Remember the enterohepatic circulation
The bile salts in the micelles are eventually absorbed
Occurs after fat absorption is complete
In the ileum (this is the location of bile salt absorption) - active transport process, apical Na+ dependent bile acid cotransporter
In the colon - passive absorption
Absorbs 95% of bile salts because it is energy demanding to produce it, it is instead recycled
Bile salts are secreted constantly by the liver and is concentrated in the gallbladder and then when food comes along the gallbladder contracts and the concentrated bile salts get transported to the lumen of the small intestine and in the jujuenum these bile salts are involved in both the emulsification and then the formation of micelles so that the products of fat digestion can remain in solution and the bile salts are transported to the surface of the epithelium by the motility patterns and the bile salts don’t get absorbed at this stage but what actually happens in the jejunum is that the fatty acids and monoglycerides diffuse out of the micelles and into the epithelial cell and these empty micelles then pass on down the small intestine until they reach the terminal region of the small intestine the ileum and then within the ileum there is a specific sodium dependent transport process that absorbs the bile salts and we absorb about 95% of the bile salts
Vitamin absorption
Fat soluble vitamins
Vitamins A,D,E and K
Absorbed with fats
Water soluble vitamins
Na+ dependent absorption - vitamin C
Special case Vitamin B12 (only need a small amount each day top maintain the production of cells, important for cell division) Absorbed in the ileum Binds to intrinsic factor - produced in stomach Specific transporter for intrinsic factor vitamin B12 in the ileum Pernicious anaemia - if you don’t absorb enough then after several month you start to lose the ability to produce red blood cells and get anaemia
We do not absorb everything…
Elimination
Expulsion of residues of digestion
Faeces formed in large intestine
Transferred to rectum via peristaltic waves - mass movements
Elimination from body by defecation reflex
