Absorption

Question 1

What is 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
Capillary networks and lacteals help take away nutrients and distribute them across the body

Question 2

Sites of absorption:

Answer 2

Mouth, oesophagus and stomach is where minimal absorption occurs of soluble substances
Small intestine is the main site of absorption and is where 90% of water and sodium is absorbed and all nutrients are absorbed
Large intestine is where 9% of the water and sodium is absorbed, this is regulated absorption as it is where the body decides to absorb or excrete the water

Question 3

Factors affecting absorption:

Answer 3

Motility, surface area, chemical digestion, membrane transporters and removal of interstitial fluid are factors that affect absorption

Question 4

What is the affect of motility on absorption?

Answer 4

Segmentation affects absorption as it is the circular muscle pinching in alternating segments, mixing chyme with enzymes and exposing chyme to absorptive epithelia
Peristalsis - having chyme travel through your GI tract at the right rate allows absorption and digestion to occur appropriately, not too much or too little absorbed

Question 5

How does surface area affect absorption?

Answer 5

Greater surface area means faster rate of absorption
Anatomical adaptations maximises surface area: length of intestine, circular folds (plicae circulares), villi and microvilli (brush boarder)

Question 6

How does transport of molecules across epithelia affect absorption?

Answer 6

The lumen of our intestine is continuous with the outside world - intestinal epithelium is a barrier
Two pathways in the barrier - to get things from the lumen into body they can be taken through the cell (transcellular) or between the cell through the tight junctions (paracellular)

Question 7

What are paracellular pathways?

Answer 7

Solutes do not cross cell membrane, only barrier is tight junctions binding cells together, relatively non-selective (if solute small enough it can get across), passive gradient (requires gradient)

Question 8

What are transcellular pathways?

Answer 8

Solute must cross two cell membranes
Cell membrane are lipid bilayers (if solute is not lipid soluble it requires a transport protein)
Requires energy source for electrochemical gradient (Na/K ATPase)

Question 9

How do you maximise absorption across available surface area?

Answer 9

To maximise absorption across available surface area we reduce nutrients into smallest possible unit (chemical digestion) and have specific transport proteins (absorb what is required, allows active transport)

Question 10

How does removal of substance from interstitial fluid increase absorption?

Answer 10

To maintain gradients you need to remove nutrients from interstitial fluid into the blood. Large blood flow to intestine, arrangement of villi and prevents build up of interstitial fluid

Question 11

How much water do absorb each day?

Answer 11

We need to drink about 1.5L per day to replace water lost to sweat, urine and feces and as we breathe
We secrete about 8L per day into the GI tract and most of this is reabsorbed into the body
If we do not replace the losses and reabsorb the secreted water = major problem, death

Question 12

How is water absorbed?

Answer 12

Water is transported via osmosis which is a passive movement from lumen into blood. Osmotic gradient set up by absorption of salts and nutrients, mostly by the paracellular pathway

Question 13

How is sodium absorbed?

Answer 13

Sodium wants to enter the cell
It enters the cell by passive movement via paracellular pathway or active transport via the cells
The active transport is transcellular, requires transporters to cross the cell membranes and has mechanisms (Na+ transport alone, Na+ transport coupled to monosaccharides, Na+ transport coupled to amino acids)

Question 14

How is sodium absorbed alone?

Answer 14

Na+ moves into cell down its gradient from high to low concentration. Transport proteins in apical membrane are Na+ channels and Na+/H+ exchangers
To get out of cell it needs to go from low to high concentration which is active transport hence requires energy, ATP, used Na+/K+ ATPase channels to provide a driving force for sodium absorption

Question 15

How are carbohydrates absorbed passively?

Answer 15

Active transcellular pathway and passive paracellular pathway
In the paracellular path all monosaccharides (glucose, galactose and fructose) diffuse down concentration gradient

Question 16

How are carbohydrates absorbed actively?

Answer 16

The active absorption of carbohydrates involves co-transport with Na+ via transcellular pathway
Na+/K+ ATPase pumps 3 Na+ out and 2+ K in against their concentration gradient which creates a driving force for sodium going into the cell. The gradient is used as a secondary energy source to pull monosaccharides in with the sodium. Any sodium absorbed is removed by Na+/K+ ATPase and the monosaccharides leave the cell via a monosaccharide carrier

Question 17

How are amino acids passively absorbed?

Answer 17

Amino acids diffuse tight junctions down chemical gradient via paracellular pathway

Question 18

How amino acids absorbed actively?

Answer 18

Na+/K+ ATPase creates a gradient for sodium to enter and amino acid co-transporter allows sodium to move down its gradient and uses it as an energy source to move its amino acids and the amino acids diffuse out of the basolateral membrane via an amino acid carrier

Question 19

How are di and tri peptides absorbed?

Answer 19

Di/tri peptides can be transported in exchange for hydrogen
Active transport process, Na+/K+ ATPase creates a negative cell membrane potential which is a driving force that pulls H+ into the cell. The H+ pulls the di/tri peptides in via H+ co-transporter. These will be broken into smaller amino acids and diffuse out the basolateral membrane via the amino acid carrier

Question 20

How are fats absorbed?

Answer 20

Products of lipid digestion can diffuse right across the membrane. Micelles deliver monoglycerides and free fatty acids to the brush boarder which diffuse across the phospholipid bilayer into the cell. Micelles are not absorbed, bile salts absorbed in ileum
Monoglycerides and fatty acids are transported into endoplasmic reticulum which resynthesises them back into triglycerides, the golgi packages the lipids into chylomicrons (stabilised by amphipathic proteins) which are exocytosed and triglycerides diffuse into lacteals of villi

Question 21

How are bile salts absorbed?

Answer 21

Bile salts in the micelles are absorbed in the ileum and the colon after fat absorption is complete. This is an active transport process and occurs in apical membranes which is Na+ dependent and have bile acid co-transporters. In the colon passive absorption of bile salts. 95% of bile salts are absorbed

Question 22

How are vitamins absorbed?

Answer 22

Fat soluble vitamins are carried in lipid droplets and end up in micelles, they are diffused across membrane as monoglycerides and fatty acids are absorbed. Water soluble vitamins are absorbed by a Na+ dependent mechanism, very similar to monosaccharide and amino acid absorption

Question 23

What happens to the things that aren’t absorbed?

Answer 23

Elimination : Expulsion of residues of digestion, feces formed in large intestine. Transferred to rectum via peristaltic waves - mass movements. Elimination from body by defecation reflex

Question 24

How is B12 absorbed?

Answer 24

B12 is absorbed in the ileum, bound to intrinsic factor which has specific B12 transporter, B12 receptor allows it to be internalised through endocytosis, once inside cell B12 and intrinsic factor unbinds, B12 binds to a protein which stabilises it in bloodstream and it is transported out