salt and water transport

Question 1

what does jejunum, ileum and colon reabsorb/ secrete

Answer 1

jejunum: reabsorbs: na+, k+, cl- and h20
secretes nothing

ileum: reabsorbs nothing, secretes Hco3-

colon: reabsorbs Na+, cl- and h2o
secretes: k+ and hco3-

Question 2

give 3 examples of adaptations in small intestine for absorption/ secretion

Answer 2

villi/microvilli = large SA

enterocyte actin myofilaments rhythmically contract to move microvilli for max exposure and response to intestinal contents

central lymphatic and blood vessels = large absorptive capacity for water, nutrients and electrolytes

Question 3

function of intestinal epithelium

enterocytes (3 things it does)
goblet cells (1 thing it does)
enteroendocrine cells (2 things it does)
paneth cells (1 thing it does)

Answer 3

enterocytes: barrier, secrete digestive enzymes and water/electrolyte exchange

goblet cells: mucus secretion

enteroendocrine cells: sensory and endocrine function

paneth cells: innate immune function

Question 4

what is purpose of basolateral Na+/k+ pump

how does it help na/glucose/aa transporter, how does this lead to cl- movement

how does na+/k+ pump lead to NaCl co-transport

Answer 4

to establish electrochemical gradient for absorption of electrolytes and water from gut . 3na+ out/2k+ in

this gradient allows na+ coupled to glucose/aa to move down conc grad from lumen across apical membrane (this is electrogenic and drives parallel absorption of cl-)

na+/k+ atpase establishes electrochemical gradient. na+/h+ (Cation) exchanger allows transport of H+ into gut lume. this works together with hco3-/cl- (anion) exchanger, overall leading to nacl absorption

Question 5

oral rehydration therapy: what does this contain and how does it work

Answer 5

oral rehydration solution contains na+ coupled with glucose
the cotransporter SGLT-1 is preserved in most diarrhoeal diseases, it binds 2 na+ molecules to one glucose transporting them into the cell

Question 6

what drives the cl- secretion and how does it occur

Answer 6

NKCC transporter (Na+/k+/Cl-)
located basolateral side of cell at level of crypt
as intracellular cl- conc inc, cl- is secreted into lumen via CFTR channels
CFTR channels mainly found in ileum and colon

Question 7

how is water secreted into small intestine lumen

Answer 7

cl- secretion drags na+ and water across tight junctions

Question 8

in large intestine, what is water and electrolyte flow driven by

how does na+ enter the cell

what increases number of na+ channels

when is k+ secreted

what is the point of the hco3-/cl- exchanger

what is purpose of gap junctions

what does na+ and cl- movement create

Answer 8

na+/k+ atpase

na+ channels (facilitated diffusion). and na+/h+ antiporter.

diffusion under control of aldosterone

when lumen conc low

to provide buffer for acids produced by bacteria and move cl- ions

ensure no ion backflow

osmotic gradient for transcellular water movement

Question 9

1) how do pathological changes in transepithelial transport occur (4 diff answers)
2) 4 different examples of pathological changes (4 answers)

Answer 9

1) congenital, acquired, gastrointestinal infections, bacterial enterotoxins

2) 1. stimulation of net fluid and electrolyte secretion
2. inc propulsive muscle contractions
3. mucosal destruction and inc permeability
4. nutrient malabsorption

Question 10

what does cholera toxin do and what does it lead to

how is it treated

when do effects reduce

which part of s.intestine does cholera toxin target most

Answer 10

irreversibly activates cAMP causing upregulation of cAMP regulated CFTR channels. This leads to:
massive Na+, K+ and Cl- and water efflux leads to profuse watery diarrhoea

> ORT

> following enterocyte turnover

> jejunum

Question 11

how does lactose intolerance cause osmotic diarrhoea

Answer 11

lactase deficiency > lactose not digested into glucose and galactose > remains in lumen > osmotic diarrhoea