Fluid/Electrolyte Balance

Question 1

how can fluid/electrolyte balance loss occur?

Answer 1

breakdown in selectively permeable barrier in the GI tract
may require oral/IV treatment

disease/surgical removal of colon

Question 2

list the main facts of water homeostasis

Answer 2

intake - 2L
excretion - urine (1.2L), GI tract (0.1L)
insensible loss - 0.7L
GI tract turnover - 9L

Question 3

why might water move into the GI tract?

Answer 3

osmotic imbalance:
disaccharide digestion of sugars draws water into intestine
high plasma osmollality draws water into a compartment

Question 4

explain the cause and classification of diarrhoea

Answer 4

fluid accumulation in the GI tract
- increases gut motility and decreases absorption
- more fluid build up

normal stool weight 200g/day (more than this = diarrhoea)

Question 5

explain osmotic diarrhoea

Answer 5

excess water in gut:
- non-absorbable solutes
- failure to digest/absorb nutrients

caused by laxatives/antacids, digestive enzyme deficiencies, inflammation, short bowel syndrome, orlistat (lipase inhibitor)

Question 6

list the main electrolytes to possibly get lose in GI tract

Answer 6

Na
Cl
K
Mg
HCO3
(H2O)

Question 7

explain the mechanism of potassium balance

Answer 7

total body potassium - 3600mmol (95% intracellularly)
most excreted through kidney (30-100mmol/day)

(hypo/hyperkalaemia = arrythmia risk)

Question 8

name some of the mechanisms of ion transport

Answer 8

ATPase
co-transporter: symporter
exchange carrier: antiporter
ion channel

Question 9

explain the main membrane transporter in the intestine

Answer 9

sodium drives absorption of water
gradient provides energy for active transport of minerals, vitamins and metabolites

Question 10

describe the Na/glucose co-transporter in the SI

Answer 10

SGLT1 transporter - moves glucose/Na into cell
GLUT2 transporter - moves glucose out of cell (same side as Na/K ATPase)

Question 11

explain secretory diarrhoea

Answer 11

endotoxins (cholera, rotavirus, salmonella) stimulate colonic electrolyte secretion

enterotoxins -> beta-2 adrenergic receptor -> cAMP -> PKA (phosphorylation) -> CFTR (Cl-) ion channel increased -> severe diarrhoea

loss of Na, K, Cl, HCO3

Question 12

explain inflammatory diarrhoea

Answer 12

caused by intestinal transporter damage due to inflammation
- (crohns, ulcerative colitis)
- irritable colon
- infectious disease (shigella, salmonella)