A normal adult’s body contains approximately 60% of fluid with the rest being solid mass which shows how important is the fluid component. Not just human beings but in many organisms, fluids weigh more than their solid mass. When we say fluids, it is understood that electrolytes are also under consideration and they both are inseparable and are the chief components of our body. Therefore, a little imbalance in any one of them could affect the balance of both. Fluid and electrolytes keep our body hydrated, cool, lubricated, stable, active and healthy. Loss of fluids or electrolytes can bring in a drastic change in our body resulting in an extreme state of thirsty and hypovolemic( shock due to a drop in fluid levels) shock. A hypovolemic shock can make our body completely dysfunctional for a period of time.
Factors affecting fluid and electrolyte levels
Our body fluid and electrolytes status depends upon the age, body fat, activity levels and the morbidities(the underlying disease conditions). The body of the younger children contain more fluid content than the older people when we take into the account of per kilogram basis. Similarly, men have a higher percentage of fluid composition than women. On the other hand, people with obesity (more body fat) have the excess of fluid content when compared to the normal adults. Some chronic illnesses such as cholera, diarrhoea, gastroenteritis and excessive bleeding can substantially result in fluid loss and dehydration. Severe vomiting and diarrheal disorders associated with fluid balance and metabolism increases the risk of imbalance in the fluids and electrolytes in our body.
Fluid compartments(Source-OpenStax College)
Fluids are not uniformly distributed throughout the body. Depending upon the location of distribution , we find 2 compartments in our body ; intracellular fluid compartment and the extracellular compartment. About 35% of body fluid is observed within the intracellular fluid (ICF) compartment which is located primarily inside our body cells. On the contrary, extracellular fluid (ECF) shares the maximum percentage(60-65%) of the total body fluid. The ECF is further categorized into intravascular, interstitial, and transcellular fluid spaces. In addition to the ECF and ICF, the fluid found inside the blood vessels is known as intravascular fluid which is nothing but the blood plasma. In a healthy adult, the total plasma weighs about 3-6 litres. It is an essential component of blood flow and maintenance of body`s cellular metabolism by supplying the necessary fluid for the metabolism. Furthermore, a proportion of the fluids present in between the the cells is called interstitial space which weighs about 11 to 12 Litres of the total body fluid. Some fluid is also found in the lymph - an example of interstitial fluid. In addition to the interstitial fluid compartment, some of the transcellular fluids are such as (CSF) cerebrospinal fluid found surrounding the brain and spinal cord, pericardial fluid surrounding the heart, synovial fluid in the joints, and pleural fluids surrounds the lungs.
Mechanism of fluid movement in our body
Our body fluids are constantly under the influence of mechanical, hydrostatic, osmotic and diffusion pressure gradients. This results in the constant motion of fluids from ICF to ECF and vice-versa , but what makes them move? One of the major force driving the fluid movement is the 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 imbalance in the equilibrium manifested by thirsty, tiredness, dryness of skin and mucus membranes. Our body might react seriously if the fluid loss is extremely high . Excessive draining of body fluid manifest in the form of shock and unconsciousness.
Fluids and electrolytes are necessary for many biological processes therefore ,organisms suffer if there is unusual fluid loss. Extreme hot weather and excess of body workup can cause dehydration. Additionally, diarrhea and metabolic disorders can increases the risk of fluid loss. Some of the signs of fluid loss 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 elements found in the body. They travel within the body through 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 fluids 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)
Bicarbonate (HCO3 −)
Phosphate (HPO4 −−)
Sulfate (SO4 −−)
Name of the electrolyte
Levels in mEq
Intracellular Fluid chamber
Phosphate and sulphates (--)
Mechanism of fluid regulation in our body
Osmosis and Osmolality
When 2 different solutions are isolated by a semipermeable(partly transparent for fluids) membrane, the fluid tends to move from the higher concentration area to the lower. If the fluids moves in the presence of a membrane, it is known as osmosis. The total number of particles present in a unit of dissolved fluid is known as the osmolality of a solution. Higher the osmolality, greater the fluid concentration therefore, the fluid moves from the higher concentration towards the area of lower concentration.
This similar to osmosis however diffusion does not necessarily require a membrane or a medium to travel through. It occurs 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 of gases in the lungs during the process of inhalation and the exhalation.
Filtration is the consequence of hydrostatic pressure exerted by the capillaries. Due to the high pressure, the fluid moves from the areas of higher hydrostatic pressure towards the areas of lower hydrostatic pressure. The concentration gradient is created as a result of the formation of urine which is of high in concentration and such high concentration of urine (filtrate) helps the kidneys to filter about 180 litres of plasma per day.
Fluid loss and gain from the body
Fluids can be gained through the more intake of semi-liquid foods and fluids such as water, juice, coffee, tea and many other substances. Thirsty is the normal part of fluid maintenance mechanism , it hardly affects our body. However, chronic disease states can trigger the sudden 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.
Kidneys excrete about 1 to 2 litres (in a normal adult) of urine a day. At the same time, our body needs 3-4 litres of fluid per day for all its bodily processes. 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. Therefore, an adult weighing an average of 70 kilograms may pass 1680ML of urine per day however it varies between individuals.
Normal skin evaporates about 600 mL of fluid per day through perspiration(sweat). However, insensible perspiration and disease conditions such as fever which can result in the excessive loss of fluids and electrolytes.
The lungs can lose around 400 ML of fluid a day through insensible water loss. Loss can further go high if the individual experiences exhaustion, fatigue and active exercise which results in forceful breathing.
GI System loses about100 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.