Monday, January 1, 2012 CC-BY-NC
Renal: Potassium and hydrogen regulation

Maintainer: admin

1Potassium Regulation


  • most abundant intracellular ion
  • important for function of excitable tissues
    • major determinant of resting membrane potentials
  • hyperkalemia: high potassium concnetration in extracellular fuid
  • hypokalemia: low ^
  • hypo/hyperkalemia both cause abnormal rhythms of the heart
    • ex: hyperkalemia and electorcardiogram: as potassium concentration increaes, P wave gets taller, and QRS gets wider, and eventually person goes into cardiac arrest

1.2Kidney and potassium balance

  • balance is maintained by kidny because 90% of potassium intake is excreted into urine.
  • in kidney:
    • freely filtered at glomerulus
    • but then tubule reabsorb most of the filtered K so very little K actually apppear in the urine
    • however, K is secreted at the cortical collecting duct
      • coupled with Na+ reabsorption by K+/Na+ APTase at basolateral membrane and K+ chanel at apical luminal membrane
    • change in K excretion is mainly due to K secretion in the cortical collecting duct and some in distal convoluted tubule.

1.2.1Regulation of Secretion

  • by aldosterone
    • hypercolemia --> aldosterone secretion in adrenal cortex --> cortical collecting duct's secretion of potassium increase --> more excretion
      • hypercolemia can also increase secretion of potassium at CCD directly.
  • K secretion can occur when renin aldosterone system is activated by other causes:
    • (remember the pathways that decrease sodium excretion, since aldosterone also inhibits sodium excretion)
    • decrease in plasma volume -- >angiotensin --> aldosterone secretion incresases --> sodium reabsorption increase and potassium secretion increase
  • hyperaldosteronism:
    • common cause: adenoma of the adrenal gland that produces aldosterone
    • effects: hypertension, hypokalemia,
    • renin is suppressed (not sure why)
    • metabolic alkalosis (loss of hydrogen ions in urine)

2Hydrogen Regulation


  • tightly regulated because metabolic reactions are senstivie to pH.
  • gain of hydrogen ion from CO2 (volatile acid)
    • production of nonvolatile acid (phosphoric acid, sulfuric acid, lactic acd) from metaoblism of organic molecules and proteins
    • gain of hydrogen due to loss of bicarbonate
      • due to diarrhea (loss of gastric fluid)
      • into urine
  • loss of hydrogen ion:
    • hyperventilation (loss of CO2)
    • utilization of hydrogen ion in metabolism
    • loss of hydrogen in vomitus (loss of gastric fluid) and loss into the urine.
    • (think of it as shifting the balance of the equation H2CO3 <--> HCO3- + H+
  • carbonic anhydrase converts CO2+H2O to H2CO3, which is broken down into HCO3- and H+
    • therefore, loss of one bicarbonate from the body is equivalent to gain of one H+
    • gain of one bicarbonate is loss of one H+
  • buffer: any substances that reversibly bind to hydrogen ions
    • extracellular buffer: Co2/HCO3- system --> keep pH around 7.4
    • intracellular buffers: phosphates, proteins


  • respiratory system (controls CO2)
  • kidneys (by controlling HCO3-)
    • when alkalosis (high pH >7.4) : kdiney excrete HCO3 to gain H+
    • when acidosis (pH < 7.4) : kidney produce new HCO3- and add it to the plasma
      • pH is determined by ratio of concentrations of HCO3- to CO2
  • usually kdineys reabsorb all the filtered HCO3- (80% reabsorption occurs in glomerulus)
    • except in the case of alkalosis

2.3Mechanism of HCO3 reabsorption

  • H2O and CO2 get converted to H2CO3 by carobnic anhydrase in tubular epithelial cells
  • H2CO3 get broken down itno H+ and HCO-
  • HCO- diffuse into the intersittial fluid (gain of one HCO3-)
  • H+ diffuse inot tubular lumen and combines with filtered HCO3- , producing H2O and CO2
  • H2O and CO2 diffuse into the epithelai cells, process repeat.
  • netgain: one HCO3-

2.4Mechanims of HCO3 generation

  1. same process as above, except that this time, the hydrogen ion diffused into the tubular lumen combines with nonbicarbonate buffer (such as HPO42-.
    • so body gains HCO3- from H2CO3 and didn't use up a HCO3- to accept the hydrogen ion.
  2. filtered glutamine diffuses into epithelial cells along with sodium and get converted into NH4+ and HCO3-
    • HCO3- diffuse into the interstitual fluid
    • NH4+ diffuse into the tubular lumen (antitransporter for NH4+ and NA+)
      • (so also loss of hydrogens)