Lecture 27 GI physio Absorption

Question 1

whats the process of absorption?

Answer 1

net passage of substances from lumen across lining of intestine into the interstitial fluid and then into the blood or lymph

Question 2

wheres the main site of absorption?

Answer 2

Small intestine
90% water and sodium
All nutrients

Question 3

what are the sites of absorption?

Answer 3

Mouth, esophagus and stomach
Small intestine
Large intestine

Question 4

whats the absorption of Mouth, esophagus and stomach?

Answer 4

Minimal

Lipid soluble substances

Question 5

whats the absorption of Small intestine?

Answer 5

Main site
90% water and sodium
All nutrients

Question 6

whats the absorption of Large intestine?

Answer 6

9% of water and sodium

Question 7

what factors affect absorption?

Answer 7

motility
SA
transport
removal

Question 8

what do you need for motility?

Answer 8

correct rate of propulsion for digestion and absorption

exposure to absorptive surfaces

Question 9

what do you need for available SA?

Answer 9

Rate of absorption proportional to SA

Anatomical adaptations maximise SA (villi, epithelial cells, microvilli)

Question 10

whats the problem of transport of molecule across epithelium?

Answer 10

intestinal epithelium is a barrier to absorption

Question 11

what are the 2 pathways for transport of molecules across epithelium?

Answer 11

Paracellular (between cells)

Transcellular (across cell membranes, through cytoplasm)

Question 12

whats the paracellular pathway?

Answer 12

Passive (requires gradient)
non selective (need to be small enough)

solutes dont cross cell membranes
only barrier is tight junction that binds cells together

Question 13

whats the transcellular pathway?

Answer 13

solutes must cross 2 cell membranes lipid bilayer (solutes not lipid soluble need transport proteins)

Question 14

whats transport proteins?

Answer 14

selective

allow active transport

Question 15

how do you Maximise absorption across epithelium?

Answer 15

Reduction in size
- Chemical digestion

Specific transport proteins
- Selective absorption

Question 16

how do you remove substances from interstitial space?

Answer 16

Large blood flow
arrangement of vessels in villi
position of lacteal

Question 17

what are the specific substances absorbed?

Answer 17

1. Water
2. Na
3. Bile salts
4. Vitamins
5. Carbs
6. Proteins
7. Lipids

Question 18

How much water has to be absorbed?

Answer 18

Drink ≈ 1.5 L day-1

Total water delivered to SI ≈ 9-10 L/day

Question 19

whats the mechanism of water absorption?

Answer 19

osmosis
passive

movement of water from lumen into blood

Question 20

what sets up osmotic gradient?

Answer 20

absorption of salts and nutrients

Question 21

whats the mechanism for Na absorption?

Answer 21

passive via paracellular pathway

active vis transcellular

Question 22

whats the Transport mechanisms for active absorption of Na+?

Answer 22

Na+ transport alone
Na+ absorption coupled to glucose
Na+ absorption coupled to amino acids

Question 23

Na will move INTO cells (high to low concentration) but how does Na get OUT of cells?

Answer 23

uses energy (ATP) i.e. ACTIVE

Question 24

whats the 2 mechanisms for carbohydrate absorption (products of carb digestion)?

Answer 24

passive or active absorption

Question 25

whats the passive absorption of carbs?

Answer 25

monosaccharides - glucose, galactose, fructose diffuse down gradient via paracellular pathway

Question 26

whats the active absorption of carbs?

Answer 26

cotransport with Na+ monosaccharides glucose and galactose via transcellular pathway Na+ coupled absorption of products of carbohydrate digestion

Question 27

whats the 2 mechanisms for protein absorption (products of protein digestion)?

Answer 27

passive or active absorption

Question 28

whats the passive absorption of proteins?

Answer 28

amino acids diffuse down gradient via paracellular pathway

Question 29

whats the active absorption of proteins?

Answer 29

co transport with Na+ Amino acids coupled to Na+ via transcellular pathway similar process that occurs for glucose

Question 30

Products of fat digestion are lipid soluble. whats the absorption of lipids?

Answer 30

FFA and monoglycerides diffuse across cell membrane into cell Dont need a transporter Delivered to brush border via micelles

Question 31

absorption of products of lipid digestion. is the whole micelle absorbed?

Answer 31

no fatty acids and monoglycerides diffuse into cell

Question 32

absorption of products of lipid digestion what happens to FFA and monoglycerides in the cell?

Answer 32

synthesized into triglycerides packaged into chylomicrons chylomicrons exit cell via exocytosis, enter lacteals.

Question 33

How do chylomicrons exit cell?

Answer 33

via exocytosis, enter lacteals.

Question 34

where does Bile salt absorption occur?

Answer 34

distal portions of small intestine to promote fat absorption.

Question 35

Is the Majority of secreted bile salts reabsorbed?

Answer 35

yes

Question 36

whats bile sale salt absorption?

Answer 36

Active transport in terminal ileum. - Apical Na+-dependent bile acid transporter. Passive absorption in jejunum.

Question 37

where does Passive absorption of bile salt occur?

Answer 37

jejunem

Question 38

where does Active transport of bile salt absorption occur?

Answer 38

terminal ileum. | - Apical Na+-dependent bile acid transporter.

Question 39

what are fat soluble vitamins and hows it absorbed?

Answer 39

Vitamins A, D, E & K | Absorbed with fats

Question 40

what are water soluble vitamins and hows it absorbed?

Answer 40

Vitamin C and B12 | Na+ dependent absorption

Question 41

Hows Vitamin B12 absorbed?

Answer 41

distal ileum Would be digested as passes through duodenum/jejunum so needs Intrinsic Factor (produced in stomach) bound to it to prevent digestion.

Question 42

whats elimination?

Answer 42

dont absorb it all Expulsion of residues of digestion eg faeces

Question 43

wheres faeces formed and hows it eliminated?

Answer 43

LI Transferred to rectum via peristaltic contractions Faeces eliminated from the body via defecation reflex.

Question 44

vitamin B12 would be digested as it passes through duodenum/jejunum, what does it need to prevent digestion?

Answer 44

Intrinsic Factor (produced in stomach) bound to it