USMLE® Step 1 style questions USMLE
Content Reviewers:Viviana Popa, MD
Potassium or Kalium is a positive ion, or a cation, noted with a K. About 98% of total body potassium is found in the intracellular fluid, or the ICF for short, which makes for an intracellular potassium concentration of about 150 milliequivalents per liter.
However, since we can only measure the plasma level of potassium, which is about 4.5 milliequivalents per liter, that level is often used to define the normal extracellular concentration of potassium.
Maintaining the normal potassium concentration in the ECF and ICF is essential for the normal functioning of excitable cells like nerve cells and muscle cells, including cardiomyocytes.
Now, across all cell membranes, when there’s no stimulus, there are negative electrical charges on the inside and positive electrical charges on the outside.
This creates a potential difference called the resting membrane potential.
Once there’s a stimulus- like when a muscle contracts-, an electrochemical impulse is generated and transmitted along the cell membrane and that generates an action potential.
Okay, now, we get potassium from our diet.
The daily recommended potassium intake is about 40 to 50 milliequivalents per liter which is about 1.6 to 2 grams of potassium - which is the equivalent of 5 bananas per day.
Once ingested, potassium is reabsorbed in the blood by the GI tract and travels unbound to plasma proteins.
Most of potassium gets inside the cells, a little amount can be lost through sweat and the GI tract and the rest is filtered by the kidneys and excreted.
Knowing this, potassium needs to be carefully regulated in order for its concentration to remain constant.
Potassium balance depends on the total amount of potassium in the body which in turn is determined by potassium intake and excretion and it’s called the external potassium balance.
Potassium balance also depends on the distribution of potassium between the ECF and ICF and is also called the internal potassium balance.
Okay, let’s start with external potassium balance.
On a daily basis, the urinary excretion of potassium must be equal to the dietary potassium, minus small amounts of potassium that can be lost through sweat or through the gi tract.
Now, if potassium excretion is less than potassium intake, then this is a positive potassium balance and hyperkalemia, or increased potassium levels in the blood, can occur.
If potassium excretion is greater than intake, then this is a negative potassium balance and hypokalemia, or low levels of potassium in the blood, can occur.
So the kidneys have a pretty important job of keeping the potassium balance at a cozy neutral.
The renal corpuscle, in turn, is made up of the glomerulus, which is a tiny clump of capillaries, and Bowman’s capsule surrounding it.
So, blood gets to the glomerulus through the afferent arteriole, which is a branch of the renal artery, and leaves the glomerulus through the efferent arterioles.
These vessels act like a coffee filter, allowing everything but red blood cells and proteins to pass from the bloodstream into Bowman’s capsule - which is connected to the renal tubule.
And the resulting fluid is called filtrate.
Now, upon exiting the glomerulus, the efferent arterioles divide into capillaries a second time, forming the peritubular vessels, which wrap around the segments of the renal tubule: the proximal convoluted tubule, the U- shaped loop of Henle, which has a descending and ascending limb, the distal convoluted tubule, and the collecting duct.
As filtrate passes through the renal tubule, ions like potassium are filtered from the capillaries into the lumen of the tubule, and reabsorbed from the lumen into the capillaries, depending on the amount of potassium in the bloodstream.
First, potassium is freely filtered across glomerular capillaries and moves on in the proximal convoluted tubule or in the PCT where 67 percent of the potassium is reabsorbed.
Interestingly, 67 percent of the water is also reabsorbed in the PCT, leaving the lumen full of all kinds of solutes, like potassium.
As a consequence, some potassium gets dragged passively from the lumen, into the PCT cells and then in the bloodstream.
Some of the potassium can also simply get dragged along with water and this is called solvent drag.
Further on, in the thick ascending limb of Henle, an additional 20 percent of the filtered potassium is reabsorbed using the sodium-potassium-chloride cotransporter or NKCC2.
Some medications, like loop diuretics, can block the NKCC2 cotransporter, and this results in increased sodium, chloride, and potassium excretion.
Finally, the distal convoluted tubule and collecting duct are responsible for adjustments in potassium excretion, specifically adjustments when dietary potassium intake varies.
So if a person has a low potassium diet, then more potassium will be reabsorbed in these segments.
Conversely, when there’s a normal or a high intake of potassium, more potassium will be secreted by the principal cells of the distal and convoluted tubule into the lumen.
This is mostly regulated by aldosterone, a hormone produced in the adrenal glands in response to angiotensin II - which in turn is a part of the renin-angiotensin-aldosterone system that typically responds to low blood pressure.
Aldosterone increases sodium and water reabsorption, to bring blood pressure back up, and this also results in increased potassium secretion.
So first, Aldosterone induces the synthesis of more sodium channels on the apical surface of the distal tubular cells, as well as the collecting duct cells.
This allows for more sodium to enter the cells, so more sodium that can be used for the sodium-potassium ATPase, found on the basolateral surface of the tubular cells.
The sodium-potassium ATPase is a pump that gets two potassium ions inside the cells, and pumps three sodium ions outside the cells.
As a result, sodium is pumped out of the cells and eventually into the bloodstream, while potassium enters the cells.
Aldosterone also increases the number of sodium-potassium ATPases and as a result, even more potassium will be pumped into the cell.
Potassium homeostasis is the body's ability to maintain a constant balance of potassium in the body. Potassium is a cation mostly located inside the cell and is essential for maintaining the function of excitable tissues. The kidneys play a fundamental role, being responsible for the external potassium balance, particularly the cells of the distal convoluted tubule and collecting duct, which are considered the fine-tuning components of potassium reabsorption and secretion. Kidneys remove excess potassium from the bloodstream and excrete it in the urine.
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