L31 - absorption 2 : salt&water

Question 1

describe GI order

Answer 1

stomach -> duodenum -> jejenum -> ileum -> ileocecal valve -> ascending colon -> transverse colon -> descending colon -> sigmoid colon -> rectum

Question 2

describe colon structure

Answer 2

no villi - flat
contains crypts
mainly mucus secreting goblet cells

Question 3

where in colon does Na (and water) absorption occur

Answer 3

bottom of crypts, capillary action in lumen causes suction to bottom

Question 4

what part of the GI tract does most water absorption occur

Answer 4

jejenum

Question 5

what are the transport pathways for water

Answer 5

1. paracellular through tight junctions between cells

2. transcellular via aquaporins

Question 6

explain the standing gradient osmosis

Answer 6

water absorption into capillaries driven by Na

Question 7

modes of Na absorption into cells of GI tract contributing to standing gradient osmosis

Answer 7

1. Na/H antiport (proximal bowel)
2. Na and AA/ MS cotransporter (jejenum)
3. Na/Cl cotransporter (jejenum)
4. restricted movement through ion channels (colon)

Question 8

how is an osmotic gradient set up by solutes in interstitial fluid

Answer 8

1. Na+ into interstitial space via Na/K ATPase pump

2. HCO3- and Cl- in cell leave into interstitial space following +ve charge

Question 9

how does the osmotic gradient lead to water absorption into capillaries

Answer 9

1. high [solute] in interstitial space, water travels here to dilute due to osmotic gradient
2. increased water vol destends cellular channels and increasing pressure
3. water exits cells into interstitial then blood due to increased pressure

Question 10

name the types of diarrhoea

Answer 10

congenital
bacterial
osmotic

Question 11

what is constipation

Answer 11

slow movement of faeces allows too much water absorption leading to very hard stool

Question 12

what is irritable bowel syndrome

Answer 12

combination of constipation and diarrhoea

Question 13

what is congenital diarrhoea

Answer 13

deficiency of normal ion transport systems due to genetics

Question 14

what transporter is most commonly absent in congenital diarrhoea, and how does it cause diarrhoea

Answer 14

Cl- / HCO3- antiport in colon

affects Na transport and therefore osmotic gradient

Question 15

briefly explain cholera toxin mechanism as an example of bacterial diarrhoea

Answer 15

1. binds to enterocyte increasing cAMP within cell
2. cAMP affects the CFTR (cl-) channel and Na / H pump
3. leads to increase in [Cl] and [Na] in ileum lumen
4. water follows into lumen

Question 16

explain osmotic diarrhoea

Answer 16

anything preventing sugar/AA/fat/electrolyte reabsorption leading to water retention

Question 17

examples of things that can cause osmotic diarrhoea

Answer 17

laxatives
antibiotics
hypermotility of intestine (moving too quickly)