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Potassium HomeostasisBy ScholarRx
Updated June 7, 2021
access_time15 min
Learning Objectives (6)
After completing this brick, you will be able to:
● Briefly explain how abnormal acid-base balance alters plasma
potassium concentration.
● 1
● Identify the tubular sites of K reabsorption and secretion and
+
describe the factors controlling K+ secretion by the cortical
collecting duct, including aldosterone.
● 2
● Calculate the normal filtered load of K+.
● 3
● Describe the mechanism regulating potassium movement between
intracellular and extracellular fluid and what factors regulate it.
● 4
● List the normal range of dietary K+ intake and loss from the body.
● 5
● Describe the importance of potassium (ie, the role of extracellular K+
in maintaining normal nerve and muscle function).
● 6
cableCASE CONNECTION
DG is a 58-year-old woman who comes to your office to discuss her
leg cramps and muscle aches. “It’s been pretty constant for the past 4
days,” she reports. “I haven’t increased my exercise or worked any
,harder. And I can’t seem to find any relief.” When you review DG’s
medications with her, she says, “I went to an urgent care clinic 2
weeks ago because I had some swelling in my ankles, and they gave
me a diuretic.” She pulls the bottle from her backpack to show you a
prescription for furosemide, 20 mg daily. “That’s the only medication I
take.” DG’s vitals and physical exam are unremarkable. You order an
electrolyte panel, and the results return later that day. Her sodium is
normal, but her potassium is low.
How will you explain to DG the cause of her muscle cramping?
Consider your answer as you read, and we’ll revisit at the end of the
brick.
GO TO CONCLUSION arrow_downward
What Is Potassium Homeostasis?
Potassium (K+) is a cation that plays an important role in regulating
the function of excitable tissues such as nerves, cardiac muscles, and
skeletal muscles; in buffering body fluids (acid-base balance); and in
regulating intracellular volume. Disturbances in K+ balance can
induce arrhythmias and sudden death. So potassium homeostasis—
maintaining the optimal balance of potassium in the body—is finely
regulated by mechanisms we will discuss in some detail.
Most of the K+ in our bodies (98%) is located in the intracellular fluid
(ICF), and the other 2% is in the extracellular fluid. K+ is responsible
for setting the cell’s resting membrane potential. Changes in the
membrane potential affect the excitability of tissues by opening or
closing the Na+ channels that are in charge of action potentials. Small
,shifts of K+ into or out of cells can greatly change the extracellular K+
concentration, alter the extracellular/intracellular K+ ratio, and change
membrane potentials with potentially lethal consequences. The normal
range of serum K+ values is 3.5-5.0 mEq/L.
The regulation of K+ depends on the balance between dietary K+
intake and K+ excretion. The average intake of K+ is 80-100 mEq/day.
Around 90% of the K+ is absorbed in the gastrointestinal (GI) tract.
Absorption of K+ can be impaired by diarrhea or conditions causing
malabsorption, like Crohn disease. Most of the excretion of K+
(~90%) is done through the kidneys. The remaining 10% is excreted
through the GI tract.
What Happens When Potassium
Levels are Abnormal?
Hypokalemia: Low Levels of K+ in the
Extracellular Fluid
Low levels of K+ in the extracellular fluid (ECF) is called
hypokalemia. When there is an imbalance of K+ inside and outside of
the cell, the body tries to correct itself. Since there is a lower amount
of K+ on the outside of the cell, K+ from the inside of the cell moves
outward. Normally, hyperpolarization occurs when more K+ leaves
the cell.
Causes. Low K+ intake, gastrointestinal K+ losses (diarrhea), and
renal K+ losses (diuretics) are common causes of hypokalemia.
Consequences. In muscle, hypokalemia leads to an ascending
weakness. In the heart, it causes ST-segment depression, low-
amplitude T-waves, prolonged QT interval, and classic “U” wave.
Other manifestations of hypokalemia include thready pulse, alkalosis,
, shallow respiration, irritability, lethargy, drowsiness, fatigue, and
confusion.
Hyperkalemia: High Levels of K+ in the
Extracellular Fluid
On the other hand, hyperkalemia, or high levels of K+ in the ECF,
would result in a shift of K+ into the cell. This results in a more
positive membrane potential (depolarization). However, this causes
Na+ channels to remain in an inactive state since the membrane
potential does not become negative enough for the Na+ channels to
return to the closed state. This decreases the conductivity of the
nerves and muscle cells and causes a slowing of impulses in the
myocardium (eg, a prolonged QRS interval on electrocardiogram
[ECG]).
Causes. Some causes of hyperkalemia include acute and chronic
kidney injury (reduced glomerular filtration rate) and cell lysis
(rhabdomyolysis).
Consequences. Decreases in the conductivity of the nerves and
muscle cells result in weakness, paralysis, and conduction heart
blocks. Hyperkalemia can also result in ascending muscle weakness
with flaccid paralysis and ECG changes. Potentially fatal arrhythmias
can occur.
Regulation of K+ Movement Between
Intracellular and Extracellular Fluid
As mentioned earlier, most of the K+ is located in the ICF. Movement
of K+ is achieved via various channels. The Na+-K+-ATPase induces
the movement of two K+ into the cell in exchange for three Na+ out of
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