Potassium Homeostasis
Major Roles of Potassium
intracellular volume
excitability
Potassium Homeostasis
distribution and concentration of K
intake and excretion of K; problems associated with flux thru blood while keeping low conc.
renal mechanisms of excretion and retention
Regulation of renal excretion
K concentrations and amounts
concentration (mM): intracellular = 150; extracellular = 4
amount (mEq) : " = 3435; " = 65
amount (%) : " = 98; " = 2
example of distribution with values expressed in mEq of K/day
Þ intestinal absorption (90) Þ ECF (65)
- to and from tissue stores (3435) – controlled by insulin, epinephrine, aldosterone
- to urine (90-95) – controlled by plasma K, ADH, and aldosterone
Renal Excretion of K
amount = GFR x conc. = 180L/day x 4 mEq/L = 720 mEq/day
daily intestinal intake » 100 mEq/day
for homeostasis: a) large renal tubular reabsorption. b) minor, but regulated tubular secretion
Cellular Mechanisms of K absorption
passive in proximal tubule
Na, K, 2Cl cotransporter in Loop of Henle
H,K ATPase in distal tubule and collecting duct
Cellular Mechanisms of K secretion
apical K channel in Loop of Henle (recycles a major portion of K)
apical K channel releases K in exchange for Na (EnaC) in DT and CD – aldo regulates
K reabsorption during K depletion during normal (increased) K intake (in %)
PT= 67 (67); TAL= 20 (20); DT= 3 (10-50 secreted); CCD= 9 (5-30 secreted); urine = 1 (15-80)
K distribution between intra- and extracellular fluid compartments
keeps plasma K constant: Epi, insulin, aldo can cause plasma K displacement: acid-base balance, Posm, cell lysis, exercise
Renal K excretion
maintains K balance: plasma [K], aldo, ADH
as plasma [K] Ý , renal excretion Ý
as aldo Ý , renal secretion Ý ; as ADH Ý , renal secretion Ý
displaces K balance: flow rate of tubular fluid, acid-base balance
as tubular flow rate Ý , distal K secretion Ý
as pH Ý , distal K secretion Ý
