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The fluids of the body are primarily composed of water, which in turn contains a multitude of substances.<1> One such group of substances includes electrolytes such as sodium, potassium, magnesium, phosphate, chloride, etc. Another group includes metabolites, such as oxygen, carbon dioxide, glucose, urea, etc. A third important group of substances contained within the water of our body, which includes proteins, most of which are vital for our existence. Examples of proteins include coagulation factors, immunoglobulins, albumin, and various hormones.<1> As the distribution of the fluid in the body and the substances found within is critical for the maintenance of intracellular and extracellular functions pivotal to survival, the body has developed mechanisms to control compartment composition tightly. However, various clinical pathologies can alter the fluid composition and its constituents in the multiple compartments of the human body, which can have deleterious effects on our health and often require intensive interventions to monitor and maintain normal physiological conditions.<2> This article will primarily cover the physiologic composition of water in the human body, differentiate the various compartments in the body and their associated volumes and compositions, depict how to measure the different volumes, and delve into the clinical relevance associated with disturbances of the normal physiological conditions.


Cellularly, the distribution of fluid compartments within the body is vital to its health, function, and survival.A 70 kg man is made up of 60% water, which is equal to 42 l.The body's fluids are divided into two main compartments: intracellular fluid volume (ICFV) and extracellular fluid volume (ECFV).

Each space works in unison with each other and has different functions paramount for normal physiological function.


In various compartments, water is distributed according to several principles.In order to understand the different principles, it is imperative to realize that ingestion and excretion of water and electrolytes are closely regulated so as to maintain consistent total body water (TBW) and total body osmolarity (TBO).

Several different factors mediate the redistribution of water between the two ECF compartments: hydrostatic pressure, oncotic pressure, and the osmotic force of the fluid. Combining these two components yields the Starling equation: Jv = Kfc <(Pc - Pi> - n (Op-Oi)>.<7> This equation determines the rate of fluid across the capillary membrane (Jv) and takes the difference between the hydrostatic pressures of the capillary fluid (Pc) and the interstitial fluid (Pi), as well as the oncotic pressure of the capillary fluid (Op) and the interstitial fluid (Oi). It also takes into account the osmotic force between the two compartments (n).

There is also a connection between intracellular fluid and interstitial fluid.Having a cell membrane between these two environments makes them very closely related.The interstitial space is generally where waste products diffuse into and out of cells. .A gradient of osmotic pressure between the two spaces allows water to freely flow through the membrane. .The osmotic gradient will be balanced if ECF osmolarity increases; however, the entire body osmolarity will remain higher than what is typical, and the cell will shrink.When there is a decrease in ECF osmolarity, water will move from the ECF to the ICF to achieve an osmolar equilibrium; however, the total body osmolarity remains lower than normal, and the cell will swell.In addition, if isosmotic fluid entered the extracellular space, then the ICF would not change and the ECFV would rise.

Related Testing

There are certain parts of this information that can seem abstract, especially when we talk about compartments that are more theoretical.Therefore, it is of vital importance to be able to measure the volumes of each compartment.Using the indicator-dilution method* 9> is a method of measuring space. This method involves the introduction of measurable substances into a specific compartment that are distributed uniformly.It is possible to measure individual volumes directly, and others by subtracting the volumes of related compartments.

Clinical Significance

Aside from the significance of the study of water balance has on our physiologic understanding of the human body, the idea behind it is commonly seen in pathology and is presented clinically on a daily basis. Various conditions lead to an imbalance of water in the different compartments of the body; the specific imbalance can show in different ways and can be treated differently as well. The following presents five clinical scenarios where alterations in water balance can present. Each will have an accompanying analysis of ECF volume, ECF osmolarity, ICF volume, and ICF osmolarity.

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