Potassium Balance Flashcards
What is the typical daily intake of potassium? Name some foods with high potassium.
About 50-125mmol. Potassium is found in particularly leafy vegetables and most fruit and fruit juice and in potatoes, especially if they are fried (high salt) or baked.
Where is most potassium in the body found?
About 95% is within cells (around 150mmol/L), the rest is extracellular (around 4.5mmol/L). Note that different cell types also have different amounts of potassium.
What is the main regulator between the intracellular and extracellular compartments?
3Na+/2K+ ATPase pump. This is known as internal balance.
Name some hormones which are responsible for regulating internal balance.
insulin, adrenaline, aldosterone and it can be affected by pH.
What is external balance?
The homeostasis that occurs between what is taken into the body in the diet and what is excreted out and it is the kidneys that play a major role in this. External balance regulates urinary K+ excretion/retention to affect the overall K+ balance in the body.
What are the two types of potassium regulation?
- Acute regulation – Which is distribution of K+ through the ICF and ECF compartments
- Chronic regulation – Achieved by the kidney adjusting K+ excreting and reabsorption
List the functions of potassium.
- Its levels are so high intracellularly, it has an important role in determining intracellular fluid osmolality and hence cell volume.
- It determines the resting membrane potential (RMP).
- It affects vascular resistance.
Clinically, define the terms Hyperkalemia and Hypokalemia.
• Plasma [K+] > 5.5mM = Hyperkalemia
• Plasma [K+] <3.5mM = Hypokalaemia
Plasma [K+] MUST NOT rise above 6.5mmol/L
What is an ionic gradient?
Two gradients combined, the combination of chemical and electrical gradients. Mainly potassium and sodium which determine these gradients.
What is the Nernst Equation?
E = (RT/zF) x (ln[X]0)/[X]i E = Nerst equilibrium potential R = Ideal gas constant T = Temperature (in kelvin) z = Charge of ion F = Faradays number X = the element (either Na or K)
What does the Nernst equation tell us?
The Nerst equation tells us the equilibrium potential (i.e. when net movement stops) and we can use this to calculate the membrane potential. Because you have conc. gradient pushing K+ out but the electrical positivity on outside pushing it in.
What happens to the Ek (Nernst equilibrium potential for K) values when patient is hyperkalaemic and hypokalaemic?
Normal: [K+]o = 3.5mM and [K+]i = 140Mm - EK = -98.5
Hyperkalemia: [K+]o = 7mM and [K+]i = 140mM - EK = -80
Hypokalemia: [K+]o = 1.5mM and [K+]i = 140mM EK = -121.5
In hyperkalemia, the equilibrium potential for K+ is more positive, i.e. so RMP is closer to a position of depolarisation. When you drop the K+ equilibrium potential in hypokalemia you move the RMP closer to hyperpolarisation.
What part of the body do the changes in RMP have an effect on and how?
Changes in RMP can severely affect the heart, this is by causing cardiac cell membrane potential depolarisations/hyperpolarisations, this produces characteristic changes in ECG.
Causes of Hypokalaemia
Renal or extra-renal loss (stool, sweat) of K+ or by restricted intake. E.g.
• Long standing use of diuretics without KCl compensation
• Hyperaldosteronism/Conn’s syndrome
• Prolonged vomiting -> fluid loss/(Na+ loss? )= Increased aldosterone secretion = K+ excretion in kidneys
• Profuse diarrhoea (diarrhoea fluid contains 50mM K+)
What does hypokalaemia result in?
In decreased release of adrenaline, aldosterone and insulin, to prevent shifting into the cell.