Fluid Balance in Intestines

Question 1

chemical gradient

Answer 1

concentration of solute measured as osmolarity/osmolality
measured in milliosmoles
the area with a higher solute conc has a higher osmolarity/osmolality
water moves to this area of higher solute conc by osmosis
in the body Na+ is the main compnent that contribute to osmolarity

similar concept for charge ions
area with lots of positive ions will move down conc gradient to balance out the charge

Question 2

solvent drag

Answer 2

because water is polar it is attracted to both negative and positive ions and when these ions move the water moves with it. this is solvent drag

Question 3

water secretion and absorption

Answer 3

both are passive processes
water can move in both directions across interstitial membrane by osmosis
water is secreted at proximal portion of duodenum

Question 4

absorption of water at interstitium

Answer 4

food nutrients are abosorbed at intestitium and water is taken with the ions
the chyme in the interstitial lumen is the same water potential as the interstitium due to water efflux from intestines
Na+enters a cell due to conc gradient via SGLT1 transporter which also absorbs glucose and water via solvent drag
proteins are absorbed by Na+ linked amino acid transporters
Na+ is also transported in via sodium-hydogen antiporter which also sends out hydrogen
Na+ conc inside the cells are maintained by Na-K ATPase
this pumps Na+ ions into interstitium which increases its osmolarity and pulls Cl- ions towards it and also H20 because of solvent drag

Question 5

Oral rehydration therapy

Answer 5

this medical technique uses the glucose and Na+ being absorbed in the intestine to also absorb water due to solvent drag
potassium and sodium citrate are also involved for diarrhoea

Question 6

intestinal secretion

Answer 6

water needed in GI tract for absorption and digestion
enterocytes move to tip of villi and change as it goes
Ca2+ enters cell from food which activates calcium activated chloride channels which allows efflux of Cl- and Na+ follow sue to electrochemical gradient this increases osmolarity outside cell so water follows

chlorine can also exit via CFTR channel. this channel is regulated by cAMP which means it can control diarrhoea by increasing absorption of Na+ and water. and it is where CF affects the GI tract by mutating this protein and allowing less water into the GI tract for absorption and digestion which can lead to blockages

concentrations of Na+, Cl- and K= are maintained for apical efflux by NKCC1 pump and sodium potassium ATPase

Question 7

Enterochromaffin cells

Answer 7

also line GI tract
respond to stimuli such as PH, nutrients and toxins
have lots of vesicles with seratonin on basolateral membrane
release serotonin when stimulated onto enteric nervous system
which then acts on blood vessels to dilate them for more water leakage, stimulate acetylcholine to act on smooth muscles to contract them to increase propulsion through GI tract
they also cause enterocytes to increase Cl- secretion and decrease Na+ absorption which leads to water moving into the lumen

Question 8

Ach induced secretion

Answer 8

Ach activate M1 and M3 receptors to increase Cl- secretion this causes K+ to enter the cell via potassium channels in basolateral membrane which cause Ca2+ to move out of CFTR on apical membrane
Na+ also cant enter the cell due to presence of K+ so they move into lumen with Cl- which leads to water moving with them
this causes mast cells to secrete histamine, prostagladin and serotonin

Question 9

Electrolytes in Diarrhoea

Answer 9

diarrhoea increases secretion of Na+, K+, Na+ and water
Loss of Na+ activates RAAS increasing aldesterone increases absorption of Na+ from kidneys and GI tract while decreasing H+ and K+ which can lead to hypokalaemia

diarrhoea also deplete bicarbonate levels which leads to acidiosis
patients may be hypokalaemic but their body doesnt recognise it as the K+ conc in the ECF remains constant but is being depleted inside the cells only when the internal K+ conc is depleted will the body realise

Question 10

constipation

Answer 10

less than 3 bowel movements a week
caused by: lack of fibre/fluid, bowel obstruction, latrogenic, Idiopathic, can be a symptim of other disease

Question 11

constipation treatments

Answer 11

water is absorbed from stool with Na+
when constipated the transit time is reduced so more water and Na+ is removed making it harder to pass
so bulk forming laxitives are given to increase osmolarity of faeceal matter so water moves down its potential gradient to the faeces making it easier to pass
stimulating laxitives act on enterocytes to increase cAMP levels to increase CFTR expression and chloride secretion which also increase osmolarity of faeceal matter so water moves to it