INTERSTITIAL FLUID: COMPOSITION AND FUNCTIONS - BIOLOGY - 2024

The interstitial fluid is the substance that occupies the so-called "interstitial space", which is nothing more than the space that contains and surrounds the cells of an organism and that represents the interstitium that remains between them.

The interstitial fluid is part of a larger volume that is total body water (ACT): this represents around 60% of the body weight of a young adult of normal consistency and 70 kg of weight, which would be 42 liters, which are distributed in 2 compartments, one intracellular (LIC) and the other extracellular (LEC).

Interstitial fluid and intracellular fluid (Source: Posible2006 via Wikimedia Commons)

Intracellular fluid occupies 2 thirds (28 liters) of total body water, that is, 40% of body weight; while the extracellular fluid is a part (14 liters) of the total body water or, what is the same, 20% of the body weight.

The extracellular fluid is considered, in turn, divided into two compartments, one of which is precisely the interstitial space, which contains 75% of the extracellular fluid or 15% of body weight, that is, about 10.5 liters; meanwhile the remainder (25%) is blood plasma (3.5 liters) confined in the intravascular space.

Composition of interstitial fluid

When talking about the composition of the interstitial fluid, it is obvious that the main component is water, which occupies almost all of the volume of this space and in which particles of a different nature are dissolved, but predominantly ions, as will be described later.

Interstitial fluid volume

Total body water is distributed in the intra- and extracellular compartments, and the latter, in turn, is subdivided into interstitial fluid and plasma volume. The values ​​given for each compartment were obtained experimentally by making measurements and estimating these volumes.

The measurement of a compartment can be done using a dilution method, for which a certain quantity or mass (m) of a substance "X" is administered that mixes uniformly and exclusively with the liquid to be measured; a sample is then taken and the concentration of "X" is measured.

From the point of view of water, the different liquid compartments, despite being separated by membranes, are freely communicated with each other. That is why the administration of substances is done intravenously, and the samples to be analyzed can be taken from plasma.

The volume of distribution is calculated by dividing the administered amount of "X" by the concentration of "X" in the sample (V = mX / CX). Substances that are distributed in total body water, in extracellular fluid (inulin, mannitol, sucrose) or in plasma (Evans blue or radioactive albumin) can be used.

Approximate distribution of body fluid (Source: OpenStax College via Wikimedia Commons)

There are no exclusively distributed substances in intracellular or interstitial fluid, so the volume of these compartments must be calculated based on the others. The volume of the intracellular fluid would be the total body water minus the volume of the extracellular fluid; while the volume of the interstitial fluid would be the extracellular fluid subtracted from the plasma volume.

If in a 70 kg man the volume of the extracellular fluid is 14 liters and the plasma fluid 3.5 liters, the interstitial volume would be about 10.5 liters. This coincides with what has already been stated that the volume of the interstitial space is 15% of the total body weight or 75% of the volume of the extracellular fluid.

Particulate composition of interstitial fluid

The interstitial fluid is a compartment that can be considered as a continuous liquid phase, located between the other two compartments that are the plasma, from which it is separated by the endothelium of the capillaries, and the intracellular fluid from which the outer cell membranes separate it..

Interstitial fluid, like other body fluids, has in its composition a great variety of solutes, among which electrolytes acquire both quantitative and functional importance, because they are the most abundant and determine the distribution of the fluid between these compartments.

From the electrolytic point of view, the composition of the interstitial fluid is very similar to that of plasma, which is even also a continuous phase; but it presents significant differences with that of intracellular fluid, which may even be different for different tissues made up of different cells.

The cations present in the interstitial fluid and their concentrations, in meq / liter of water, are:

- Sodium (Na +): 145

- Potassium (K +): 4.1

- Calcium (Ca ++): 2.4

- Magnesium (Mg ++): 1

That together add up to a total of 152.5 meq / liter. As for the anions, these are:

- Chlorine (Cl-): 117

- Bicarbonate (HCO3-): 27.1

- Proteins: <0.1

- Others: 8.4

For a total of 152.5 meq / liter, a concentration that is equal to that of the cations, so the interstitial liquid is electroneutral. Plasma, for its part, is also an electro-neutral liquid, but it has somewhat different ionic concentrations, namely:

Cations (which together add up to 161.1 meq / liter):

- Sodium (Na +): 153

- Potassium (K +): 4.3

- Clacio (Ca ++): 2.7

- Magnesium (Mg ++): 1.1

Anions (which together add up to 161.1 meq / liter)

- Chlorine (Cl-): 112

- Bicarbonate (HCO3-): 25.8

- Proteins: 15.1

- Others: 8.2

Differences between interstitial fluid and plasma

The great difference between plasma and interstitial fluid is given by plasma proteins, which cannot cross the endothelial membrane and are, therefore, non-diffusible, thus creating a condition, together with endothelial permeability to small ions, for Gibbs equilibrium -Donnan.

In this equilibrium, the non-diffusible protein anions alter diffusion a little, causing the small cations to be retained in the plasma and have higher concentrations there, while the anions are repelled towards the interstitium, where their concentration is slightly higher.

Another result of this interaction consists in the fact that the total concentration of electrolytes, both anions and cations, is higher on the side where the non-diffusible anions are found, in this case plasma, and lower in the interstitial fluid.

It is important to highlight here, for comparative purposes, the ionic composition of the intracellular fluid (ICF), which includes potassium as the most important cation (159 meq / l of water), followed by magnesium (40 meq / l), sodium (10 meq / l) and calcium (<1 meq / l), for a total of 209 meq / l

Among the anions, proteins represent about 45 meq / l and other organic or inorganic anions about 154 meq / l; together with chlorine (3 meq / l) and bicarbonate (7 meq / l), they add up to a total of 209 meq / l.

Interstitial fluid functions

Cell habitat

The interstitial fluid represents what is also known as the internal environment, that is, it is like the "habitat" of the cells to which it provides the necessary elements for their survival, also serving as a receptacle for those final waste products of metabolism cellular.

Exchange of materials

These functions can be fulfilled due to the communication and exchange systems that exist between plasma and interstitial fluid and between interstitial fluid and intracellular fluid. The interstitial fluid thus functions, in this sense, as a kind of exchange interface between plasma and cells.

Everything that reaches the cells does so directly from the interstitial fluid, which in turn receives it from the blood plasma. Everything that leaves the cell is poured into this liquid, which then transfers it to the blood plasma so that it can be taken to where it must be processed, used and / or eliminated from the body.

Maintain tissue osmolality and excitability

Maintaining the constancy of the volume and osmolar composition of the interstitium is decisive for the conservation of cell volume and osmolality. That is why, in man, for example, there are several physiological regulatory mechanisms intended to fulfill this purpose.

The concentrations of some electrolytes in the interstitial fluid, apart from contributing to the osmolar balance, also have, along with other factors, very important roles in some functions related to the excitability of some tissues such as nerves, muscles and glands.

The values ​​of interstitial potassium concentration, for example, together with the degree of permeability of the cells to it, determine the value of the so-called “cellular resting potential”, which is a certain degree of polarity that exists across the membrane and which makes the cell about -90 mV more negative inside.

The high concentration of sodium in the interstitium, together with the internal negativity of the cells, determines that when the permeability of the membrane to this ion increases, during the state of excitation, the cell depolarizes and produces an action potential that triggers phenomena such as muscle contractions, neurotransmitter release, or hormone secretion.

References

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