Fluids and electrolytes in our body


A normal adult`s body composes of approximately 60% of the fluid. In many organisms, fluids weigh more than their solid mass becasue fluids are vital for all our bodily processes. They are the primary components in almost every cell of our body. Fluids and electrolytes go hand in hand, an imbalance in any one of them could affect both of them. Fluid and electrolytes keep our body hydrated, cool, active and lubricated when necessary.  Loss of fluids or electrolytes can bring a drastic change in our body resulting in an extreme state of thirsty.


Table Of Contents

1. Introduction

2. Factors affecting fluid and electrolyte levels

3. Fluid compartments

4. Mechanism of fluid movement in our body

5. Fluid loss

6. Electrolytes

7. Mechanism of fluid regulation in our body

    7.1 Osmosis and Osmolality

    7.2 Diffusion

    7.3 Filtration

8. Fluid loss and gain from the body


Factors affecting fluid and electrolyte levels

Our body`s fluid and electrolytes status is linked to the age, gender, body fat, activity levels and the underlying pathological conditions. Proportionately, the younger children have more body fluid than older people on every kilogram of body weight. Similarly, men have a higher percentage of fluid composition than women. On the other hand, people with obesity (more body fat) have lesser fluid content than others. More active the individual is more chances that he might quickly dehydrated. Some of the illnesses such as cholera, diarrhoea, gastroenteritis, excessive bleeding, accidents and severe vomiting can cause an imbalance in the fluids and electrolytes in our body.

Fluid compartments

Fluid compartments(Source-OpenStax College)


Our body fluid is not uniformly distributed across various cells. Depending upon the location of fluid, the 2 fluid compartments are the intracellular fluid compartment and the extracellular compartment. About 35% of body fluid is found inside the intracellular fluid (ICF) compartment. ICF is located primarily in the skeletal muscle mass. On the contrary,   extracellular fluid (ECF) shares the maximum percentage of the body fluid. ECF is again subdivided into intravascular, interstitial, and transcellular fluid spaces. In addition to the ECF and ICF, the fluid present within the blood vessels is known as intravascular fluid ( in simple terms it is the blood plasma). In an adult body, plasma weighs about 3-6 litres. Plasma is vital for blood flow and maintenance of blood cells.  Fluid can also be distributed between the cells. The space between the cells is called interstitial space. Interstitial space can accommodate about 11 to 12 Liters of fluid. Another form of fluid is the lymph - an example of interstitial fluid. In addition to the interstitial fluid compartment, there is also transcellular space.  Some of the transcellular fluids are (CSF) cerebrospinal fluid, pericardial fluid surrounding heart, synovial fluid in the joints, and pleural fluids surrounding the lungs, sweat and secretions from bodily systems such as digestive system.


Mechanism of fluid movement in our body

Until death, our body fluids are constantly under the influence of mechanical, hydrostatic, osmotic and diffusion gradients.  Fluids move from ICF to ECF and vice-versa but they always move as a result of pressure gradients and the pumping action of the heart. Fluid movement is essential to maintain the homeostasis (balance) throughout the body. Loss of fluid and electrolytes can cause an imbalance in the equilibrium expressed as thirsty, tiredeness, dryness of skin and mucus membranes, sometimes serious reactions if the fluid loss is chronic.


Fluid loss

Fluid and electrolytes are necessary fr many biological processes, however, organisms are vulnerable to abnormal fluid loss. Extreme hot weather and excess of body workup can cause dehydration. Diarrheal and metabolic disorders are closely linked with the fluid loss. Some of the signs of fluid are diminished urine output, increased heart rate, decreased BP, dryness of skin and mucus membranes, constipation, weight loss, stiffness of joints, infections of the GI system, lack of tears and nasal secretions.



Electrolytes are the charged ionic masses found in the body. They move along with the fluids . Some of the most common electrolytes in our body fluids are sodium, potassium, calcium, magnesium, and hydrogen ions. Depending upon their electric potential, electrolytes can be cations( positively charged such as potassium and sodium ) and the anions( negatively charged such as chlorides).  The major anions in our body are chloride, bicarbonate, phosphate, sulfate, and proteinase ions whereas the cations are sodium and potassium.  Electrolytes carry fluid along when they move from one compartment to another. The electrolyte concentration is expressed in terms of mill equivalents (mEq) per litre. An imbalance in the electrolytes and fluids beyond a certain point is a danger sign. Some of the electrolyte levels in our body are as shown in the table below.

Table Showing The Approximate Electrolyte Content In Our Body


Name of the electrolyte

Levels in mEq











Extracellular Fluid (Plasma)


Sodium (Na+)



Potassium (K+)



Calcium (Ca++)



Magnesium (Mg++)



Chloride (Cl−)



Bicarbonate (HCO3 −)



Phosphate (HPO4 −−)



Sulfate (SO4 −−)



Organic acids








Name of the electrolyte

Levels in mEq







Intracellular Fluid chamber

Sodium (Na+)



Potassium (K+)



Calcium (Ca++)



Phosphate  and sulphates (--)








Mechanism of fluid regulation in our body

Osmosis and Osmolality

When 2 different solutions are isolated by an impermeable membrane, the fluid tends to get attracted to the lower concentration area. The movement of fluids through the membrane is known as osmosis. The total number of dissolved particles present in a unit of fluid is known as the osmolality of a solution. Osmolality has a direct effect on the movement of fluid between the 2 fluid compartments. This is becasue osmolality creates a state of concentration gradient.  


This similar to osmosis but they do not necessarily require a membrane or a medium to produce fluid movements. It happens as a result of the natural tendency of a substance to move from a high concentration area to the lower concentration area. One of the best examples of diffusion is the gaseous movement in the lungs and the mechanism of inspiration and expiration.


Filtration is the result of hydrostatic pressure in the capillaries. Due to the high pressure, the fluid moves from the areas of higher hydrostatic pressure are towards the areas of low hydrostatic pressure. The concentration gradient is created by the strength of the filtrate. For example, the urine formation process, in which the high concentration of urine ( filtrate)  assists the kidneys to filter about 180 litres of plasma per day.


Fluid loss and gain from the body

Fluid can gain through the intake of liquid foods and fluids such as water, juice, coffee, tea and many other substances. Thirsty is the normal part of fluid maintenance mechanism and it hardly affects our body.  However, chronic disease states can trigger the loss or gain of the fluid levels disproportionately. For example, cardiac illnesses, accidents, ADH imbalance, severe fever,  and some of the kidney disorders can have a serious impact on our body. In order to combat the inadvertent effects of fluid loss, many of our body systems work actively. The paragraphs below explain how different body organs and systems participate in fluid movement through in and out of the body.

  1. Kidneys excrete about 1 to 2 litres (in a normal adult). At the same time, our body needs 3-4 litres of fluid for all bodily processes including the urinary excretion. The standard rule is that the urine output must be approximately 1 mL of urine per KG body weight per hour (1 mL/kg/h) in any age group.

  2. Skin loses a considerable amount of fluid. Normal skin evaporates about  600 mL/day through perspiration. However insensible perspirations and disease conditions such as fever can cause excessive loss of fluids and electrolytes.

  3. The lungs can cause insensible water loss resulting in the loss of 400 mL of fluids every day. Loss can further go high in the case of increased rate or depth of respiration, exhaustion, fatigue and active exercise resulting in forceful breathing.

  4. GI System, on the other hand, loses about 100 to 200 mL per day. But the person suffering from infectious diseases of the gastrointestinal system is more prone for the loss of as much as 8 litres which is fatal.  





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