Potassium Balance
The concept of potassium balance including typical daily dietary
intakes along with output of these potassium
The difference between Na+ and K+ filtered loads and in the urinary
excretion of the two cations
The main role of sodium and potassium in the ECF of stimulating the
electrogenic Na+/K+ exchange pump on cell membrane
Hypokalaemia (low plasma [K+]) and hyperkalaemia (high plasma [K+])
- definitions and effect on body
The main roles of potassium in the body viz determining intracellular
osmolality and cell volume; in determining resting membrane potential
hence the role of inappropriate K+ concentrations eg. in cardiac
arrhythmias, heart block and asystole muscular weakness; in
determining vascular resistance and hence blood pressure
The role of the proximal convoluted tubules and loop of Henle in
obligatory reabsorption of K+
The role of the distal convoluted tubule in the facultative adjustment of
K+losses
The control of K+ secretion by various factors, the most important
being aldosterone, high plasma [K+], along with increases tubular flow
and acidosis/alkalosis
The role of rennin-angiotensis-aldosterone system in Na/K balance
Conn's syndrome (primary hyperaldosteronism) and its affect on Na/K
balance, demonstrating the integration of both cations
Summary diagram of the transport of Na & K throughout the tubule
Na+ and K+ are the most prevalent cations in body fluids, so what do they do and why
is it important to regulate their plasma concentrations?
Potassium input—
Potassium typical daily intake in the UK is 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.
Potassium intake, unlike sodium, should not be restricted routinely, only in cases of
renal impairment with a low GFR. This is because K+-containing foods may include
healthy foods.
Potassium Homeostasis
The majority of potassium in the body (about 95%) is within cells, so concentration
is high intracellularly (around 150mmol/L), the remaining is extracellular which has
low concentration of potassium (around 4.5mmol/L).
Note that different cell types also have different amounts of potassium e.g. muscle
cells very high around 2700mmol/L, liver less at 250mmol/L.
This regulation between the intracellular and extracellular compartments of
potassium is mainly done by the 3Na+/2K+ ATPase pump. This is known as internal
,balance and there are several hormones that regulate the internal balance (high
intracellular, low extracellular) and it is critical this is maintained.
From moment to moment this concentration balance is mainly regulated by internal
balance, which shifts K+ between those two compartments.
The hormones that effect this balance include insulin, adrenaline, aldosterone and it
can be affected by pH.
External balance is 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.
So external balance regulates urinary K+ excretion/retention to affect the overall K+
balance in the body.
However there can also be a loss of K+ in stools and sweat, but this is unregulated.
Importance of Potassium Homeostasis:
So regulation of K+ homeostasis implies two things
Acute regulation – Which is distribution of K+ through the ICF and ECF compartments
Chronic regulation – Achieved by the kidney adjusting K+ excreting and reabsorption
So what are potassium’s functions that make it so important that we have to regulate
it in this way?
1 – Its levels are so high intracellularly, it has an important role in determining
intracellular fluid osmolality and hence cell volume
2 – Crucially, it determines the resting membrane potential (RMP), and is very
important for normal functioning of excitable cells i.e. in the repolarisation of certain
cells like myocardial, skeletal muscle and nerve cells
3 – It affects vascular resistance
The Na+/K+ ATPase pump
The Na+/K+ ATPase pump maintains high intracellular [K+] and low [Na+]. Then there
is the reverse of this outside the cell. It utilises ATP within the cell to do this.
, Internal Balance/Acute Regulation
So when we eat a meal and our [K+] levels in our plasma rise, which happens
relatively quickly, these have to be shifted quite fast into our ICF compartment
(because EC pool will change more dramatically with changes in K +). This shift into
the ICF is mainly due to hormonal control, of that of
-Insulin
-Adrenaline
-Aldosterone
-pH changes
These all act to push potassium into our cells.
It is VERY important that plasma [K+] doesn’t rise above 6.5mmol/L,
Clinically if:
Plasma [K+] > 5.5mM = Hyperkalemia
Plasma [K+] <3.5mM = Hypokalaemia
Resting Membrane Potential
Our cells rely on the creation of an ionic gradient for the membrane potential, an
ionic gradient is two gradients combined, the combination of chemical and electrical
gradients.
It is mainly potassium and sodium which determine these gradients, it is the balance
between the two gradients. Note though there are other ions that contribute to this
gradient such as Cl-.
So it is a dynamic balance between membrane conductance and the Na + and K+ that
determines RMP normally.
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. This is
normally around -95mV, but due to permeability to Na+ it brings it down to about -
70mV.
So what happens when plasma [K+] is altered above or below normal?
This equation tells us at what RMP the equilibrium point for K+ is (i.e. at what potential
is there the balance)
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
The concept of potassium balance including typical daily dietary
intakes along with output of these potassium
The difference between Na+ and K+ filtered loads and in the urinary
excretion of the two cations
The main role of sodium and potassium in the ECF of stimulating the
electrogenic Na+/K+ exchange pump on cell membrane
Hypokalaemia (low plasma [K+]) and hyperkalaemia (high plasma [K+])
- definitions and effect on body
The main roles of potassium in the body viz determining intracellular
osmolality and cell volume; in determining resting membrane potential
hence the role of inappropriate K+ concentrations eg. in cardiac
arrhythmias, heart block and asystole muscular weakness; in
determining vascular resistance and hence blood pressure
The role of the proximal convoluted tubules and loop of Henle in
obligatory reabsorption of K+
The role of the distal convoluted tubule in the facultative adjustment of
K+losses
The control of K+ secretion by various factors, the most important
being aldosterone, high plasma [K+], along with increases tubular flow
and acidosis/alkalosis
The role of rennin-angiotensis-aldosterone system in Na/K balance
Conn's syndrome (primary hyperaldosteronism) and its affect on Na/K
balance, demonstrating the integration of both cations
Summary diagram of the transport of Na & K throughout the tubule
Na+ and K+ are the most prevalent cations in body fluids, so what do they do and why
is it important to regulate their plasma concentrations?
Potassium input—
Potassium typical daily intake in the UK is 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.
Potassium intake, unlike sodium, should not be restricted routinely, only in cases of
renal impairment with a low GFR. This is because K+-containing foods may include
healthy foods.
Potassium Homeostasis
The majority of potassium in the body (about 95%) is within cells, so concentration
is high intracellularly (around 150mmol/L), the remaining is extracellular which has
low concentration of potassium (around 4.5mmol/L).
Note that different cell types also have different amounts of potassium e.g. muscle
cells very high around 2700mmol/L, liver less at 250mmol/L.
This regulation between the intracellular and extracellular compartments of
potassium is mainly done by the 3Na+/2K+ ATPase pump. This is known as internal
,balance and there are several hormones that regulate the internal balance (high
intracellular, low extracellular) and it is critical this is maintained.
From moment to moment this concentration balance is mainly regulated by internal
balance, which shifts K+ between those two compartments.
The hormones that effect this balance include insulin, adrenaline, aldosterone and it
can be affected by pH.
External balance is 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.
So external balance regulates urinary K+ excretion/retention to affect the overall K+
balance in the body.
However there can also be a loss of K+ in stools and sweat, but this is unregulated.
Importance of Potassium Homeostasis:
So regulation of K+ homeostasis implies two things
Acute regulation – Which is distribution of K+ through the ICF and ECF compartments
Chronic regulation – Achieved by the kidney adjusting K+ excreting and reabsorption
So what are potassium’s functions that make it so important that we have to regulate
it in this way?
1 – Its levels are so high intracellularly, it has an important role in determining
intracellular fluid osmolality and hence cell volume
2 – Crucially, it determines the resting membrane potential (RMP), and is very
important for normal functioning of excitable cells i.e. in the repolarisation of certain
cells like myocardial, skeletal muscle and nerve cells
3 – It affects vascular resistance
The Na+/K+ ATPase pump
The Na+/K+ ATPase pump maintains high intracellular [K+] and low [Na+]. Then there
is the reverse of this outside the cell. It utilises ATP within the cell to do this.
, Internal Balance/Acute Regulation
So when we eat a meal and our [K+] levels in our plasma rise, which happens
relatively quickly, these have to be shifted quite fast into our ICF compartment
(because EC pool will change more dramatically with changes in K +). This shift into
the ICF is mainly due to hormonal control, of that of
-Insulin
-Adrenaline
-Aldosterone
-pH changes
These all act to push potassium into our cells.
It is VERY important that plasma [K+] doesn’t rise above 6.5mmol/L,
Clinically if:
Plasma [K+] > 5.5mM = Hyperkalemia
Plasma [K+] <3.5mM = Hypokalaemia
Resting Membrane Potential
Our cells rely on the creation of an ionic gradient for the membrane potential, an
ionic gradient is two gradients combined, the combination of chemical and electrical
gradients.
It is mainly potassium and sodium which determine these gradients, it is the balance
between the two gradients. Note though there are other ions that contribute to this
gradient such as Cl-.
So it is a dynamic balance between membrane conductance and the Na + and K+ that
determines RMP normally.
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. This is
normally around -95mV, but due to permeability to Na+ it brings it down to about -
70mV.
So what happens when plasma [K+] is altered above or below normal?
This equation tells us at what RMP the equilibrium point for K+ is (i.e. at what potential
is there the balance)
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
