HUBS192 Lecture 27 - Absorption

Question 1

what is absorption?

Answer 1

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

Question 2

why is there minimal absorption in the mouth and the oesophagus?

Answer 2

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

Question 3

why is there minimal absorption in the stomach?

Answer 3

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)

Question 4

what is absorbed in the mouth, oesophagus and stomach?

Answer 4

lipid soluble substances

Question 5

what is the main site for absorption?

Answer 5

the small intestine

Question 6

what is absorbed in the small intestine?

Answer 6

absorbs 90% of the water and sodium required each day and all other nutrients

Question 7

what does the large intestine/colon absorb?

Answer 7

9% of the water and sodium required each day

Question 8

what are the 5 sites of absorption?

Answer 8

1) mouth
2) oesophagus
3) stomach
4) small intestine
5) large intestine

Question 9

what are the 4 factors affecting absorption?

Answer 9

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

Question 10

what does the correct motility rate of propulsion allow?

Answer 10

allows for digestion and absorption to occur simultaneously

Question 11

what are the 5 ways that motility affects absorption?

Answer 11

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

Question 12

how does the available surface area for absorption affect absorption?

Answer 12

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

Question 13

what are the 4 anatomical adaptations?

Answer 13

1) length of the intestine (6m)
2) circular folds or plicae circulars
3) villi
4) microvilli

Question 14

what are the 2 pathways molecules are transported across the epithelium?

Answer 14

1) paracellular pathway

2) cellular pathway

Question 15

what is the paracellular pathway?

Answer 15

the ‘gap’ between cells which is the tight junction, which is the only barrier that binds cells together

Question 16

what is the cellular pathway?

Answer 16

transport across the cell membranes through the cytoplasm

Question 17

what does it mean that the paracellular pathway is relatively non-selective and passive?

Answer 17

• passive = requires a gradient

- relatively non-selective = if the solute is small enough it can get across (solutes do not cross cell membranes)

Question 18

what are cell membranes?

Answer 18

lipid bilayers

Question 19

how many membranes must solutes cross in the cellular pathway?

Answer 19

solutes must cross 2 cell membranes

Question 20

what does a solute require if it is not lipid soluble?

Answer 20

it requires a transport protein

Question 21

how is absorption maximised across the available surface area?

Answer 21

• reduces nutrients into the smallest possible unit through chemical digestion
• specific transport proteins by absorbing what is required and allowing active transport

Question 22

how does the removal of substance from the interstitial fluid affect absorption?

Answer 22

-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

Question 23

what are the 7 specific substances absorbed?

Answer 23

1) water
2) sodium
3) carbohydrates (sugars)
4) proteins
5) lipids (fats)
6) bile salts
7) vitamins

Question 24

why do we need to drink 1.5 L of water per day?

Answer 24

to replace the water lost in sweat, urine and faeces as we breathe

Question 25

what is the total volume of water delivered to the small intestine per day?

Answer 25

9-10 L per day

Question 26

what is the mechanism of water absorption?

Answer 26

osmosis

-passive movement of water from the lumen into the blood

Question 27

how is the osmotic gradient established from absorption?

Answer 27

osmotic gradient set up by the absorption of salts and nutrients

Question 28

what are the 2 pathways of absorption of sodium (Na+)?

Answer 28

1) passive movement via the paracellular pathway

2) active transport via the cells

Question 29

what is the active transport via the cells of sodium absorption?

Answer 29

transcellular absorption that requires transporters to cross the cell membranes

Question 30

what are the 3 mechanisms of active transport via the cells of sodium absorption?

Answer 30

1) Na+ transport alone
2) Na+ transport coupled to monosaccharides (e.g glucose)
3) Na+ transport coupled to amino acids

Question 31

what is cellular sodium absorption through the mechanism of sodium transport alone?

Answer 31

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

Question 32

what are the mechanisms of the absorption of carbohydrates?

Answer 32

1) passive absorption

2) active absorption

Question 33

what is the passive absorption of carbohydrates?

Answer 33

passive absorption of monosaccharides (glucose, galactose, fructose) down their concentration gradient via a paracellular pathway

Question 34

what is the active absorption of carbohydrates?

Answer 34

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

Question 35

what are the 2 mechanisms of protein absorption?

Answer 35

1) passive absorption

2) active absorption

Question 36

what is the passive absorption of proteins?

Answer 36

absorption of amino acids that diffuse down their concentration gradient s via a paracellular pathway

Question 37

what is the active absorption of proteins?

Answer 37

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

Question 38

what is the absorption of the products of fat digestion?

Answer 38

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

Question 39

what are the 4 steps in the formation of chylomicrons?

Answer 39

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