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REGUB Path - Disorders of Potassium Balance (i) (Intro (42 x 1/3 x 4 = 56…
REGUB Path - Disorders of Potassium Balance (i)
Intro
humans = 60% H2O
for someone 70kg: total body water = 70 x 60% = 42L
2/3 of total body H2O = intracellular (1/3 extracellular)
intracellular K conc = 140 mmol/L
42L x 2/3 x 140 = 3920 mmol of K intracellularly
K is mostly intracellular unlike Na
extracellular K conc = 4mmol/L
42 x 1/3 x 4 = 56 mmol of K extracellularly
our diet contains much more than this (recommended daily intake 100-120 mmol/day)
paleolithic diet was extremely high in K
plasma K = 14 mmol (approx 1/4 of extracellular component)
dietary intake is so large compared to plasma levels that unless it is moved intracellularly it would be lethal
normal plasma K = 3.5 to 5.2 mmol/L
K regulated by 3 steps
Intake
Cellular distribution
insulin = most potent driver, moves glucose, K+ and phosphate into cells
catechols
pH
when acidotic H+ ions move in, K+ out (risk of hyperkalaemia) (e.g. DKA in hyperglycaemia)
when alkalotic: H+ moves out, K+ into cells (risk of hypokalaemia)
cell turnover
cell destruction releases K into blood (tumour lysis syndrome i.e. large nos of cancer cells die together, hypothermia, haemolysis)
cell construction (growth) decreases K serum levels (e.g. txing megaloblastic anaemia, cancer)
osmolality
if extracellular compartment is hypertonic water will flow out of cells, carrying K+ with it
Na-K-ATPase pump
lowers serum K by driving it into cells (2 Na out, 2 K in)
activated @ B2 Rs by catechols + insulin
Renal excretion
a properly functioning kidney can compensate for a wide range of K intake
persistent K disorders generally indicate a failure of renal K handling
all filtered K reabsorbed by proximal tubule + loop of Henle, secreted back into urine @ cortical collecting duct (made up of principal cells for K excretion + intercalated cells for H excretion)
principal cell has 3 channels
all are unregulated by aldosterone
ENaC (epithelial Na channel): resorbs Na, creates -ve charge in tubule, K secreted
Na-K-ATPase: Na moves out of principal cell into blood, K moves into cell
outward K rectifying channel: K secreted into urine
tubule's -ve charge = principle determinant of K secretion
disrupted by Cl- resorption
maintained by unresorbable anions (e.g. bicarb/HCO3-), reduce chloride (leaves duct paracellularly)
regulated by 2 processes: distal tubular flow + aldosterone (must be balanced)
hypovolaemia: aldosterone increased due to low BP + tubular flow decreased - still balanced
hypervolaemia: aldosterone decreased + tubular flow increased - still balanced
hypokal: aldosterone + tubular flow increased
hyperkal: aldosterone + tubular flow decreased