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Renal system anatomy and physiology
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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.
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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