Lecture 27 - Absorption

Question 1

Absorption

Answer 1

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

Question 2

Sites of absorption

Answer 2

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

Question 3

Factors that affect the absorption

Answer 3

Motility

Surface area available for absorption

Transport across epithelium-
Reduction in size (chemical digestion)
Membrane transporters

Removal from interstitial fluid

Question 4

Affect of motility on absorption

Answer 4

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

Question 5

Affect of available surface area on absorption

Answer 5

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

Question 6

Transport of molecules across epithelium affect on absorption

Answer 6

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

Question 7

Paracellular pathways

Answer 7

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

Question 8

Cellular pathway

Answer 8

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

Question 9

To maximise absorption across available surface area

Answer 9

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

Question 10

Removal of substance from interstitial fluid affect on absorption

Answer 10

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

Question 11

How much water do we absorb each day?

Answer 11

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

Question 12

Mechanism of water absorption

Answer 12

Osmosis
Passive movement of water from lumen into the blood
Osmotic gradient set up by absorption of salts and nutrients

Question 13

Sodium absorption

Answer 13

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

Question 14

Absorption of sodium alone

Answer 14

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

Question 15

Carbohydrate absorption

Answer 15

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

Question 16

Active absorption of carbohydrates summary

Answer 16

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)

Question 17

Absorption of products of protein digestion - amino acids

Answer 17

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)

Question 18

Active absorption of amino acids summary

Answer 18

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

Question 19

Absorption of products of fat digestion

Answer 19

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

Question 20

Bile salt absorption

Answer 20

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

Question 21

Vitamin absorption

Answer 21

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

Question 22

We do not absorb everything…

Answer 22

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