Physiology of Fluid Balance

Question 1

how is water absorbed in GI tract

Answer 1

passive process driven by transport of solutes (sodium) from lumen of intestines to the bloodstream

Question 2

contents of faeces

Answer 2

water
cellulose
bilirubin
bacteria

Question 3

definition of diarrhoea

Answer 3

loss of fluid and solutes from GI tract in excess of 500ml per day

Question 4

is water ingested and secreted balanced with water absorbed

Answer 4

yes

Question 5

how is absorption of water in the GI driven

Answer 5

mainly by reabsorption of sodium - provides osmotic force fo reabsorption of water

Question 6

how may water move during absorption

Answer 6

via transcellular or paracellular routes

Question 7

how is sodium/H+ exchange stimulated

Answer 7

luminal bicarbonate

Question 8

where does parallel sodium/H+ and Chloride/bicarbonate exchange occur

Answer 8

occurs in ileum and proximal colon

Question 9

where do epithelial sodium channels occur

Answer 9

occurs in the colon (distally)

Question 10

how is parallel absorption of chloride driven by odium/glucose co-transport and sodium/amino acid co-transport

Answer 10

collectively the overall transport of sodium generates a transepithelial potential in which the lumen in negative - driving parallel absorption of chloride

Question 11

describe sodium/glucose co-transport and sodium/amino acid co-transport

Answer 11

most important in postprandial period in jejunum

secondary active transport and electrogenic

Question 12

describe sodium/H+ exchange

Answer 12

occurs in duodenum and jejunum

stimulated by luminal bicarbonate

Question 13

describe parallel sodium/H+ and Chloride/bicarbonate exchange

Answer 13

occurs in ileum and colon

most important in interdigestive period

Question 14

describe epithelial sodium channels

Answer 14

occurs in the colon (distally)

regulated by aldosterone

Question 15

where does sodium/glucose co-transport and sodium/amino acid co-transport take place

Answer 15

occurs throughout small intestine

Question 16

how is parallel absorption of chloride driven by odium/glucose co-transport and sodium/amino acid co-transport

Answer 16

collectively the overall transport of sodium generates a transepithelial potential in which the lumen in negative - driving parallel absorption of chloride

Question 17

action of aldosterone in epithelial sodium channels

Answer 17

opens channel
inserts channels into membrane from intracellular vesicle pool
increases synthesis of channels and NA+/K+-ATPase

Question 18

describe NHE1

Answer 18

cellular pH housekeeper

Question 19

how is Na+/H+ exchange stimulated

Answer 19

by the alkaline environment of the lumen due t presence of bicarbonate from the pancreas

Question 20

describe parallel sodium/H+ and Chloride/bicarbonate

Answer 20

primary mechanism of sodium absorption in the interdigestive period - absorption is electroneutral
does not contribute greatly to postprandial absorption

Question 21

how is parallel sodium/H+ and Chloride/bicarbonate regulated

Answer 21

via intracellular cAMP, cGMP and calcium - all reducing NaCl absorption (a cause of diarrhoea due to infection of E.Coli)

Question 22

describe epithelial sodium channels

Answer 22

mediates electrogenic sodium absorption in the distal colon;

highly efficient and important in sodium conservation

Question 23

how is epithelial sodium channels increased

Answer 23

via aldosterone;

not regulated by cAMP or cGMP

Question 24

action of aldosterone in epithelial sodium channels

Answer 24

opens channel
inserts channels into membrane from intracellular vesicle pool
increases synthesis of channels and NA+/K+-ATPase

Question 25

how does chloride absorption occur

Answer 25

passively via transcellular or paracellular routes other; Cl--HCO3- exchange (ileum, proximal and distal colon) parallel Na+-H+ and Cl--HCO3- exchange (ileum and proximal colon)

Question 26

secondary messengers that indirectly activate CFTR

Answer 26

cAMP cGMP calcium

Question 27

chloride absorption in large intestine

Answer 27

the driving force is provided by lumen negative potential due to electrogenic movement of sodium through epithelium sodium channels

Question 28

how does chloride secretion occur

Answer 28

at a basal rate | usually overshadowed by a higher rate of absorption

Question 29

where does chloride secretion occur

Answer 29

from crypt cells

Question 30

processes involved on the basolateral membrane during chloride secretion

Answer 30

Na+/K+ATPase Na+/K+/2Cl- co-transporter (NKCC1) K+ channels (IK1 and BK)

Question 31

process of chloride secretion

Answer 31

Low intracellular Na+ drives inward movement of Na+, K+ and Cl- via NKCC1 K+ recycles via K+ channels, but intracellular concentration of Cl- increases providing electrochemical gradient for Cl- to exit cell via CFTR on the apical membrane Lumen negative potential develops providing voltage-dependent secretion of Na+ through paracellular pathway

Question 32

how is CFTR indirectly activated

Answer 32

bacterial enterotoxins hormones and neurotransmitters immune cells products some laxatives

Question 33

secondary messengers that indirectly activate CFTR

Answer 33

cAMP cGMP calcium

Question 34

where does the chloride conductance mediated by CFTR result from

Answer 34

opening of channels at apical membrane | insertion of channels from intracellular vesicles into membrane

Question 35

causes of diarrhoea

Answer 35

``` infectious agents - viruses and bacteria impaired absorption of NaCl chronic disease toxins drugs psychological factors hypermotility non-absorbable or poorly absorbable solutes in intestinal lumen (lactase deficiency) excessive secretion ```

Question 36

outcomes of diarrhoea

Answer 36

dehydration - loss of sodium and water metabolic acidosis - loss of bicarbonate hypokalaemia - loss of potassium

Question 37

treatment of diarrhoea

Answer 37

maintenance of fluid and electrolyte balance (first priority) use of anti-infective agents (if appropriate) use of non-antimicrobial antidiarrhoeal agents (symptomatic)

Question 38

how may the absorption of NaCl be impaired

Answer 38

congenital defects (e.g. congenital chloridorrhoea – absence of Cl--HCO3- exchanger) inflammation infection (e.g. enterotoxins from some strains of E.coli and campylobacter sp.) excess bile acid in colon

Question 39

how may excessive secretion be a cause of diarrhoea

Answer 39

cholera; cholera toxin (CTX) enters enterocyte enzymatically inhibits GTPase activity of the Gs subunit increased activity of adenylate cyclase increased concentration of cAMP cAMP stimulates CFTR hypersecretion of Cl-, with Na+ and water following

Question 40

describe rehydration therapy for diarrhoea

Answer 40

exploits SGLT1; 2 Na+ bind Affinity for glucose increases, glucose binds Na+ and glucose translocate from extracellular to intracellular 2 Na+ dissociate, affinity for glucose falls Glucose dissociates Cycle is repeated