Understanding Fluid and
Electrolyte Balance

By Dr. Vik, MBBS, MRCP(GB), PhD
Understanding fluid and electrolyte balance

The human vascular system contains within it a complex combination of electrolytes and fluids.

These are maintained in perfect balance and harmony in different spaces within the body.

In this article, I will briefly explain normal fluid and electrolyte balance and talk about the role of electrolytes in maintaining physiological homeostasis.

Body Fluid Compartments

For an understanding of fluid and electrolyte balance, it is important to appreciate the distribution of total body fluid.

In essence, the fluid that is present in the body is divided into two different compartments - the extracellular compartment and the intracellular compartment (Krogh, 1938). 

Fluid outside the cells

The extracellular compartment consists of the fluid that lies outside the cells in the body.

Extracellular fluid accounts for around 20% of total body weight, which is around 13 liters of fluid in an individual who weighs 60 kg (Guyton, 2006).

The extracellular fluid contains 2 components:

  • Interstitial fluid

  • Plasma (Halperin, 2002).

There is a complex exchange of various ions and substances between the interstitial fluid and the plasma. This typically occurs through small ports that are present within blood capillaries.

Fluid inside the cells

The intracellular fluid is the fluid that is present within the cells.

This intracellular fluid accounts for 40% of an individual’s total body weight (Guyton, 2006). Unlike the extracellular fluid, the intracellular fluid is highly variable, depending on the type of cell.

When put together, the intracellular and extracellular fluid form the basic components of blood. Of course, blood also contains other components, including red blood cells and hemoglobin.

Electrolytes and fluid balance

Electrolytes in extracellular fluid

Within the extracellular fluid are present differing concentrations of electrolytes, including sodium and potassium.  These electrolytes are responsible for maintaining an electrical charge within the fluid.

Sodium and potassium are positively charged anions that are present in high concentrations in the plasma while chloride and similarly negatively charged electrolytes are present in higher concentrations in the interstitial fluid.

Electrolytes in intracellular fluid

On the other hand, the intracellular fluid contains very small amounts of sodium and chloride.

However, it does have a high concentration of phosphate and potassium along with small amounts of magnesium and sulfate ions. There are no calcium ions in the intracellular fluid.

Electrolyte imbalance due to fluid loss

When patients are ill, fluid and electrolytes may be lost from the body in the form of sweat, excessive urine output or diarrhea.

This can result in an imbalance between the electrolyte composition and fluid content in the intracellular and extracellular fluid. A loss of electrolytes may also be caused by the use of certain medications.

This phenomenon can also occur in healthy individuals. For instance, endurance athletes tend to lose a lot more fluid than they can consume and absorb through the stomach (Rehrer, 2001).

The impact of the loss of electrolytes

The loss of electrolytes in the body can have a significant impact on normal function.

Replacing these electrolytes with a suitable electrolyte rich solution is essential to maintain normal homoeostasis and blood volume.

It is important to replace the essential major elements, including sodium, potassium and magnesium, since losing these electrolytes can have numerous detrimental effects on various vital systems.

Electrolytes lost during normal living

While illness and certain activities can result in electrolyte and fluid loss, electrolyte loss also occurs as a part of the natural physiological processes.

For example, ‘insensible loss’ of fluid can occur through simple breathing or normal bowel activity. Sweating from the skin can result in the loss of around half a liter of fluid per day.

Whichever way that electrolytes are lost, when these losses exceed what is normal, an imbalance may occur within the fluids and electrolytes in the body.

Conclusion - understanding fluid and electrolyte balance

There is a delicate balance of fluid content and electrolytes in the human body that can be affected by natural processes and by diseases.

It is important to replace electrolytes and fluid losses. Not doing so can cause a person not to feel well, to be unhealthy.

Works cited

Guyton, A. C. (2006). TThe body fluid compartments: Extracellular and intracellular fluids; interstitial fluid and oedema. In J. C. Hall, Medical Physiology (pp. 291 - 298). Philadelphia: Elsevier Inc.

Halperin, M. L. (2002). Clinical approach to disorders of salt and water balance: emphasis on integrative physiology. Critical care clinics, 249 - 272.

Krogh, A. (1938). Extracellular and intracellular fluid. Scandinavica, 9 - 18.

Rehrer, N. J. (2001). Fluid and electrolyte balance in ultra-endurance sport. Sports Medicine , 701 - 715.

(Return from Understanding Fluid and Electrolyte Balance to What is an Electrolyte)

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