The Body Fluid Compartments
Extracellular and Intracellular Fluids:
The fluids of the body
may be classified into two main divisions: the fluid within
cells ( intracellular fluid) and the fluid outside the
cell (extracellular fluid). The extracellular fluid can be
further divided into interstitialfluid, plasma, lymph,
cerebrospinal fluid, and milk (in mammals).
The human body and even its individual body fluids may be conceptually divided into various fluid
compartments, which, although not literally anatomic compartments, do represent a
real division in terms of how portions of the body's water ,solutes, and suspended elements
are segregated. The two main fluid compartments are the intracellular and
extracellular compartments. The intracellular compartment is the space within
the organism's cells; it is separated from the extracellular
compartment by cell membranes.
About two thirds of the total body water of humans is held in the cells,
mostly in the cytosol, and the
remainder is found in the extracellular compartment. The extracellular fluids
may be divided into three types: interstitial fluid in the "interstitial
compartment" (surrounding tissue cells and bathing them in a solution of
nutrients and other chemicals), blood plasma and lymph in the "intravascular compartment"
(inside the blood vessels and lymphatic vessels), and small amounts of trans cellular fluid such
as ocular and cerebrospinal fluids in
the "trans-cellular compartment". The interstitial and intravascular
compartments readily exchange water and solutes but the third extracellular
compartment, the trans cellular, is thought of as separate from the other two
and not in dynamic equilibrium with them.
Intracellular
compartment
The intracellular fluid is all fluid
contained inside the cells. Most of this is cytosol, the matrix in which cellular organelles are suspended. The cytosol and
organelles together compose the cytoplasm. The cell membranes are the outer barrier. In humans,
the intracellular compartment contains on average about 28 litres of fluid, and
under ordinary circumstances remains in osmotic equilibrium. It contains moderate
quantities of magnesium and sulphate ions.
Extracellular
compartment
The interstitial, intravascular and
trans cellular compartments comprise the extracellular compartment. Its extracellular fluid(ECF) contains
about one-third of total body water.
Interstitial compartment
The interstitial compartment (also
called "tissue space") surrounds tissue cells. It is filled
with interstitial fluid. Interstitial
fluid provides the immediate microenvironment that
allows for movement of ions, proteins and nutrients across the cell barrier. This
fluid is not static, but is continually being refreshed by the blood capillaries and recollected by lymphatic capillaries. In the average
male (70 kg) human body, the interstitial space has approximately 10.5
litres of fluid.
Intravascular compartment
The main intravascular fluid in mammals
is blood, a
complex mixture with
elements of a suspension (blood cells), colloid(globulins), and solutes (glucose and ions). The blood represents both the intracellular
compartment (the fluid inside the blood cells) and the extracellular
compartment (the blood plasma). The other intravascular fluid
is lymph. It too
represents both the intracellular compartment (the fluid inside its lymphocytes) and the extracellular compartment (the
noncellular matrix of the lymph, which is roughly equivalent to serum). The average volume of plasma in the average
(70 kg) male is approximately 3.5 liters. The volume of the intravascular
compartment is regulated in part by hydrostatic pressure gradients, and by
reabsorption by the kidneys.
Transcellular compartment
The third extracellular compartment, the
transcellular, consists of those spaces in the body where fluid does not
normally collect in larger amounts,[3][4] or where
any significant fluid collection is physiologically nonfunctional.[5] Examples
of transcellular spaces include the eye, the central nervous system, the peritoneal and pleural cavities, and the joint capsules. A small amount of fluid, called transcellular fluid, does exist
normally in such spaces. For example, the aqueous humor, the vitreous humor, the cerebrospinal fluid, the serous fluid produced by the serous membranes, and the synovial fluid produced by the synovial membranes are all transcellular
fluids. They are all very important, yet there is not much of each. For
example, there is only about 150 mL of cerebrospinal fluid in the entire
central nervous system at any moment. All of the aforementioned fluids are
produced by active cellular processes working with blood plasma as the raw
material, and they are all more or less similar to blood plasma except for
certain modifications tailored to their function. For example, the
cerebrospinal fluid is made by various cells of the CNS, mostly the ependymal cells, from blood plasma.
the next passively across a semi
permeable membrane until the hydrostatic and osmotic pressure gradients balance
each other. Many medical conditions can cause fluid shifts. When fluid moves
out of the intravascular compartment (the blood vessels), blood pressure can
drop to dangerously low levels, endangering critical organs such as the brain, heart andkidneys; when it shifts out of the cells (the
intracellular compartment), cellular processes slow down or cease from
intracellular dehydration; when excessive fluid accumulates in the interstitial
space, edema develops;
and fluid shifts into the brain cells can cause increased cranial pressure.
Fluid shifts may be compensated by fluid replacement or diuretics.
Third spacing
Third spacing is the unusual
accumulation of fluid in a transcellular space. In medicine, the term is most
commonly used with regard to burns, but also can refer to ascites and pleural effusions. With regard to severe burns,
fluids may pool on the burn site (i.e. fluid lying outside of the interstitial
tissue, exposed to evaporation) and cause depletion of the fluids. Withpancreatitis or ileus, fluids may "leak out" into the peritoneal cavity, also causing depletion of the
intracellular, interstitial or vascular compartments.
Patients who undergo long, difficult
operations in large surgical fields can collect third-space fluids and become
intravascularly depleted despite large volumes of intravenous fluid and blood
replacement.
The precise volume of fluid in a
patient's third spaces at any moment is dynamic and is virtually impossible to
accurately quantify clinically.
Third spacing conditions may
include peritonitis, pyometritis, and pleural effusions. Hydrocephalus and glaucoma are theoretically forms of third
spacing, but they do not substantially affect corporeal blood volume and are not usually called third
spacing. Hydrocephalus can substantially disrupt intracranial blood
volume because of the pressure buildup.
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