Session 5 Flashcards
Describe the normal distribution of potassium across the fluid compartments.
98% in ICF, mainly within skeletal muscle but also in liver RBCs and bone; 2% in ECF.
Describe the internal balance of potassium regulation.
Potassium moves between the ECF and ICF via Na-K-ATPase and K channel action. Provides short term control.
How is potassium uptake into cells increased?
Via the action of insulin, aldosterone and catechoamines; due to increased potassium levels in the ECF or due to alkalosis.
What promotes potassium removal from cells?
Exercise, cell lysis, increased ECF osmolality, Low ECF potassium levels, acidosis.
Describe the external balance of potassium control.
Adjustment of renal potassium secretion to provide long term control of potassium levels.
How does insulin affect potassium levels?
Potassium in splanchnic blood stimulates potassium secretion by the pancreas; insulin causes increased Na-K-ATPase activity; more potassium is taken into muscle cells and the liver.
How does aldosterone affect potassium?
Potassium in the blood stimulates aldosterone release; aldosterone stimulates Na-K-ATPase so more potassium is taken up by cells.
How do catechoamines affect potassium?
Act on beta-2 adrenoceptors to stimulate Na-K-ATPase; causes increased potassium uptake by cells.
How does exercise affect potassium levels?
Potassium is released during the recovery phase of an action potential; skeletal muscle damage during exercise releases potassium; exercise causes catechoamine release. All factors cause increased potassium levels which are buffered by uptake of potassium by non-contracting cells.
What effect does acidosis have on potassium levels?
Causes hydrogen to move into cells; potassium moves out of cells in a reciprocal movement; results in hyperkalaemia.
How does alkalosis affect potassium levels?
Hydrogen moves out of cells; potassium moves into cells in a reciprocal movement; causes hypokalaemia.
What affect does hyperkalaemia have on blood pH?
More potassium moves into cells; hydrogen moves out of cells in reciprocal action; acidosis results as blood pH drops.
What effect does hypokalaemia have on blood pH?
More potassium leaves cells; hydrogen moves into cells in reciprocal action; alkalosis results as blood pH rises.
Where is most potassium normally absorbed in the kidneys?
Proximal tubule.
How is potassium resorbed in the proximal tubule?
Passively by paracellular diffusion.
How is potassium resorbed in the thick ascending limb of the kidney?
Actively using Na-K-ATPase in the basolateral membrane and Na-K-2Cl transporters in the apical membrane.
Where in the kidney can potassium resorption not be controlled (i.e. it is fixed)?
PT, TAL, intercalated cells of DCT, CCD and MCD.
Where in the kidney is potassium resorption variable (i.e. it can be regulated)?
Principal cells of the DCT and CCD.
How is potassium secreted in the principal cells of the DT and CCD?
Na-K-ATPase moves K from ECF into tubular cells; high intracellular K levels create gradient for secretion; apical ENaC moves sodium to create an electrical gradient for K secretion; K is secreted via apical K channels.
How is potassium secretion increased in the DT and CCD?
- Increased ECF K levels stimulate Na-K-ATPase and increase apical K channel permeability so more potassium can be excreted.
- Stimulates aldosterone release which increases transcription of Na-K-ATPase, K channels and ENaC so potassium secretion increases.
- Alkalosis increases K secretion.
- Increased tubular flow rate increases secretion as greater gradient.
- Increased Na delivery in DT increases excretion as more Na is absorbed so greater gradient.
How is potassium resorbed in the DT and CCD?
By intercalated cells: active process mediated by apical H-K-ATPase.
What is hyperkalaemia?
Potassium raised above 5.0 mmol/L.
