Extracellular Fluid: Composition And Functions

The extracellular fluid is all the fluid present in an organism and located outside the cells. It includes interstitial fluid, plasma, and the small amounts present in some special compartments.

The interstitial fluid represents the fluid in which all the cells of the body are immersed and corresponds to what is called the “internal environment”. Its composition and characteristics are essential for the maintenance of cellular integrity and functions, and are regulated by a series of processes that together are called “homeostasis”.

Eukaryotic human cell representation

Plasma is the volume of fluid contained in the vascular compartments. The vascular compartments contain blood formed 40% by cells and 60% by plasma, which would represent the interstitial fluid of blood cells.

Special compartments are sites in which small volumes of fluid are confined and that include aqueous humor and fluids: cerebrospinal, pleural, pericardial, synovial joints, serous secretions such as the peritoneum and the content of some glands such as the digestive.

Article index

  • one

    Composition of extracellular fluid

    • 1.1

      Volumetric composition of extracellular fluid

    • 1.2

      Chemical composition of extracellular fluid

    • 1.3

      Composition of plasma

    • 1.4

      Composition of interstitial fluid

  • two

    Functions of the extracellular fluid

    • 2.1

      Plasma functions

  • 3

    References

Composition of extracellular fluid

An animal cell all surrounded by extracellular fluid (Source: OpenStax College [CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)] via Wikimedia Commons)

Volumetric composition of extracellular fluid

Body fluids are aqueous solutions, which is why all these fluids are also known as total body water, and their volume in liters, as a liter of water weighs a kilo, is estimated as 60% of body weight. In a man weighing 70 kg, that would represent a total volume of water of 42 liters.

Of this 60%, 40% (28 liters) is contained within the cells (intracellular fluid, ICL) and 20% (14 liters) in the extracellular spaces. Due to the small volume of the so-called special compartments, it is customary to consider the extracellular fluid as consisting only of interstitial fluid and plasma.

It is said then that three quarters of the extracellular fluid is interstitial fluid (about 11 liters) and a quarter is plasma fluid (3 liters).

Chemical composition of extracellular fluid

When considering the chemical composition of the extracellular fluid, the relationships that its two compartments maintain with each other and those that the interstitial fluid maintains with the intracellular fluid must be taken into consideration, since the exchange relationships of substances between them determine their composition.

With respect to the intracellular fluid, the interstitial fluid is kept separated from it by the cell membrane, which is practically impermeable to ions, but permeable to water. This fact, together with intracellular metabolism, means that the chemical composition of both liquids differs considerably, but that they are in osmotic balance.

Regarding plasma and interstitial fluid, both sub-extracellular compartments are separated by the capillary endothelium, which is porous and allows the free passage of water and all small dissolved particles, except for most of the proteins, which due to their large size can not pass.

Thus, the composition of plasma and interstitial fluid is very similar. The main difference is given by the higher concentration of plasma proteins, which in osmolar terms is about 2 mosm / l, while the interstitial is 0.2 mosm / l. Important fact that conditions the presence of an osmotic force in the plasma that opposes the exit of liquid into the interstitium.

As proteins generally have an excess of negative charge, this fact determines what is called the Gibbs-Donnan equilibrium, a phenomenon that allows to maintain electroneutrality in each compartment, and makes the positive ions are slightly more concentrated where there is more protein. (plasma) and negatives behave in the opposite way (more in the interstitium).

Composition of plasma

The plasma concentrations of the different components, expressed in mosm / l, are as follows:

– Na +: 142

– K +: 4.2

– Ca ++: 1.3

– Mg ++: 0.8

– Cl-: 108

– HCO3- (bicarbonate): 24

– HPO42- + H2PO4- (phosphates): 2

– SO4- (sulfate): 0.5

– amino acids: 2

– creatine: 0.2

– lactate: 1.2

– glucose: 5.6

– proteins: 1.2

– urea: 4

– others: 4.8

Based on these data, the total osmolar concentration of the plasma is 301.8 mosm / l.

Composition of interstitial fluid

The concentrations of the same components, in the interstitial fluid, also in mosm / l, are:

– Na +: 139

– K +: 4

– Ca ++: 1.2

– Mg ++: 0.7

– Cl-: 108

– HCO3- (bicarbonate): 28.3

– HPO42- + H2PO4- (phosphates): 2

– SO4- (sulfate): 0.5

– amino acids: 2

– creatine: 0.2

– lactate: 1.2

– glucose: 5.6

– proteins: 0.2

– urea: 4

– others: 3.9

The total osmolar concentration of the plasma is 300.8 mosm / l.

Features 

extracellular fluid

The main function of the extracellular fluid is fulfilled immediately at the interface between the interstitial fluid and the intracellular fluid, and consists of providing the cells with the elements necessary for their function and survival, and serving them at the same time as an “emultory “By receiving the waste products of your metabolism. In the following image you can see circulating red blood cells and extracellular fluid:

The exchange between the plasma and the interstitial fluid allows the replacement in the latter of the substances that it has delivered to the cells, as well as the delivery to the plasma of the waste products that it receives from them. Plasma, for its part, replaces what is delivered to the interstitium with material from other sectors and delivers waste products to other systems, for their elimination from the body.

Thus, the functions of provider and collector of the extracellular fluid, related to cellular function, have to do with the dynamic exchanges that occur between cells and interstitial fluid, between the latter and plasma and finally between plasma and its substances. suppliers or their recipients of waste material.

An indispensable condition for the internal environment (interstitial fluid) to be able to carry out its functions of supporting cellular activity, is the need to preserve a relative constancy in the value of certain relevant variables related to its composition.

These variables include volume, temperature, electrolyte composition including H + (pH), concentrations of glucose, gases (O2 and CO2), amino acids and many other substances whose low or high levels can be harmful.

Each of these different variables has regulatory mechanisms that manage to maintain their values ​​within adequate limits, achieving a global equilibrium as a result that is known as homeostasis. The term homeostasis therefore refers to the set of processes responsible for the multifactorial constancy of the internal environment.

Plasma functions

Plasma is the circulating component of the extracellular fluid, and it is the liquid medium that provides the necessary mobility to the cellular elements of the blood, facilitating their transport, and therefore their functions, which are not located in a specific sector, but rather they have to do with the transport link that through this mobility they carry out between various sectors.

Red blood cells suspended in plasma (Source: Arek Socha at www.pixabay.com)

Plasma osmolarity, somewhat higher than interstitial due to proteins, is a determining factor in the amount of fluid that can move between both compartments. It generates an osmotic pressure of about 20 mm Hg that opposes the hydrostatic pressure within the capillaries and allows a balance to be reached in the liquid exchange and the conservation of the volume of both sectors.

The plasma volume, together with the compliance of the walls of the vascular tree, is a determining factor of the filling pressure of the circulatory system, and therefore of the arterial pressure. Modifications in more or less than that volume produce changes in the same direction in said pressure.

Plasma also contains in solution a number of substances, especially proteins, which are involved in the body’s defense processes against the invasion of potentially pathogenic noxes. These substances include antibodies, early response proteins, and those of the complement cascade.

Another important detail related to plasma function refers to the presence in it of the factors involved in the blood clotting process. Process aimed at healing wounds and preventing blood loss that could lead to severe hypotension that endangers the life of the body.

References

  1. Ganong WF: Celular & Molíquido extracellular Basis of Medical Physiology, in: Review of Medical Physiology , 25th ed. New York, McGraw-Hill Education, 2016.
  2. Guyton AC, Hall JE: The Body Fluid Compartments, in: Textbook of Medical Physiology , 13th ed, AC Guyton, JE Hall (eds). Philadelphia, Elsevier Inc., 2016.
  3. Kurtz A, Deetjen P: Wasser- und Salzhaushalt, In: Physiologie, 4th ed; P Deetjen et al (eds). München, Elsevier GmbH, Urban & Fischer, 2005.
  4. Oberleithner H: Salz- und Wasserhaushalt, in: Physiologie , 6th ed; R Klinke et al (eds). Stuttgart, Georg Thieme Verlag, 2010.
  5. Persson PB: Wasser- und Eliquido extracellulartrolythaushalt, in: Physiologie des Menschen mit Pathophysiologie , 31th ed; RF Schmidt et al (eds). Heidelberg, Springer Medizin Verlag, 2010.

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