SERUM ELECTROLYTES: FUNCTIONS, TEST, NORMAL VALUES - MEDICINE - 2025

The serum electrolytes are ions, electrically charged minerals which are dissolved in the circulatory stream that is part of the extracellular water. They fulfill important bodily functions and their imbalances have serious consequences for health.

The most important electrolytes that are tested in routine tests include sodium (Na +), potassium (K +), calcium (Ca ++), phosphate (HPO42-), chlorine (Cl–), and magnesium (Mg ++). Bicarbonate (HCO3–) or carbon dioxide (CO2), hydrogen ions (H +), and / or blood pH can also be ordered for diagnosing acid / base imbalances and, in some cases, iron.

Sodium-potassium pump (Source: BruceBlaus. When using this image in external sources it can be cited as: Blausen.com staff (2014). «Medical gallery of Blausen Medical 2014». WikiJournal of Medicine 1 (2). DOI: 10.15347 /wjm/2014.010. ISSN 2002-4436.Derivative by Mikael Häggström via Wikimedia Commons)

60% of human body weight is water. The water is distributed in several compartments that have different compositions. The total volume of water found within the cells of the body is called total intracellular water.

The volume of fluid that surrounds each cell of the body and from which the cells feed and eliminate their waste is called interstitial water. The volume of water that is part of circulating blood is called intravascular water volume or plasma volume.

The interstitial water and the intravascular or plasma water, added together, form the extracellular water volume. Electrolytes are distributed differently in the various compartments. For example, sodium is an ion that is more concentrated in the extracellular fluid than in the intracellular fluid, whereas potassium is the other way around.

What are they?

Electrolytes are ions that are distributed in body fluids and are distributed in different ways in the different water compartments of the body and fulfill different functions.

- Sodium and potassium

Sodium is a highly concentrated ion in the extracellular fluid, whereas potassium is highly concentrated in the intracellular fluid. These concentration differences are maintained by the active function of the Na + / K + pumps, which remove 3 Na + and 2 K + into the cell, consuming ATP (adenosine triphosphate).

This large difference in sodium concentration between the intracellular and extracellular fluid provides the energy for the coupled transport of many other substances across the membrane. For example, in some cells glucose enters together with sodium or calcium enters coupled with the passive diffusion of sodium.

The activity of the Na + / K + pumps is hormonally adjusted (by the thyroid) to regulate caloric expenditure at rest.

The gradients (concentration differences) of sodium and potassium across the membranes of muscle and nerve cells are used to generate electrochemical impulses that are used for the function of neurons and different types of muscles.

Active sodium transport out of the cell is very important to maintain intracellular water volume, protecting cells from injury. If the pumps are turned off sodium accumulates inside the cell and water enters through osmosis and the cell swells and can rupture.

Many pathologies are accompanied by alterations in serum sodium and / or potassium values, for example, kidney dysfunctions can cause increased excretion of ions, so their serum values ​​tend to drop, or vice versa, they can decrease elimination reason why they accumulate and their serum values ​​increase.

- Calcium and phosphorus

Calcium accumulates in intracellular compartments within some cytoplasmic organelles. The amount of free calcium in both the extracellular fluid and the intracellular fluid is small and highly regulated.

There are large deposits of calcium and phosphorus in the bone matrix. Inside cells, calcium is coupled with many functions.

It participates in muscle contraction and exocytosis processes related to the secretory function of many cells, such as glandular cells, and the release of neurotransmitters for neuronal communication.

Phosphorus has very important functions to maintain bone structure, but it is also part of some so-called “high-energy” compounds such as ATP (adenosine triphosphate), ADP (adenosine diphosphate), cAMP (cyclic adenosine monophosphate) and GTP, among others. It is also part of DNA and RNA, which are nucleic acids.

These high-energy molecules serve as the direct fuel suppliers for most of the chemical reactions that occur in the body. Among these, some also participate in intracellular signaling chains as second messengers.

- Chlorine

Chlorine, like sodium, is considered an extracellular ion because the intracellular concentration of these ions is very low. Chlorine has various functions: in the digestive system it is used by the cells of the stomach to form hydrochloric acid and thus participates in the digestion of fats and proteins.

Another very important function of chlorine in the blood system is its participation in the exchange of bicarbonate in red blood cells. Bicarbonate is a form of blood transport of CO2 (carbon dioxide).

The CO2 produced by the cells enters the bloodstream and inside the red blood cell it binds to water and through an enzyme called carbonic anhydrase, which accelerates this reaction, forms carbonic acid, which dissociates into H + and bicarbonate (reversible reaction).

The bicarbonate exits the red blood cell through a Cl– / HCO3– exchanger that removes the bicarbonate and puts chlorine into the red blood cell.

It has to do with the osmotic balance of the liquid compartments of the body. It is found in the cerebrospinal fluid and its serum concentration can be altered in various pathologies that involve the renal excretion system and in some acid-base alterations.

- Magnesium

Magnesium is found in bones and teeth, but it is an essential mineral for most tissues. It performs functions as a cofactor in many enzymatic reactions. It is an intracellular ion and has to do with muscle and neuronal function.

Magnesium ion (Source: Pumbaa (original work by Greg Robson) via Wikimedia Commons)

Test

After a fasting period of 6 to 8 hours, a venous blood sample is taken to perform the test. Potassium, sodium, calcium, chlorine, phosphate, magnesium, and bicarbonate are commonly measured. Other ions may be included at the request of the treating physician. Some tests do not include phosphate and magnesium, unless specifically requested.

Sometimes these tests are included in what is called the Basic Metabolic Panel (BMP) which includes, in addition to the electrolytes mentioned above, creatinine, glucose and urea.

Normal values

References

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4. Iatridis, PG (1991). Best and Taylor's Physiological Basis of Medical Practice. JAMA, 266 (1), 130-130.
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