Fluid compartments
Encyclopedia
Fluid compartments in the mammal
ian body broadly comprise two compartments, each with several subdivisions:
intracellular fluid, which makes up approximately 60-65% of body water, and
extracellular fluid
, which makes up the other 35-40% of body water (for all practical purposes, the only solvent
in the body is water
).
The intracellular compartment contains on average about 28 liters of fluid, and under ordinary circumstances remains in osmotic equilibrium with the ECF.
s of a given tissue. It is filled with interstitial fluid
. Interstitial refers to a "small opening or space between objects". Together with the vascular space, the interstitial space comprises the extracellular space. When excessive fluid accumulates in the interstitial space, edema
develops. 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 and recollected by lymph
atic channels. In the average male (70 kg)human body, the interstitial space has approximately 10.5 liters of fluid.
In the lung
s there is an interstitial space between capillaries
(tiny blood vessel
s) and the alveoli (the microscopic air-filled sacs in the lungs responsible for absorbing oxygen
from the atmosphere
). For gas exchange
to occur, carbon dioxide
must diffuse
across the endothelium
of the capillaries across the interstitial space, and across the alveolar epithelium; oxygen must diffuse the other direction.
which is a complex fluid with elements of a suspension
(blood cells), colloid
(globulins) and solvent
(glucose and ions).The average volume of plasma in the average (70 kg) male is approximately 3.5 liters.The volume of the intravascular lumen is regulated in part by hydrostatic pressure gradients,and by reabsorption by the kidneys.
and pleural cavity
. Still, small amount of fluid does exist normally in such spaces, and function e.g. as lubricant in the case of pleural fluid. Also, the lumen of the gastrointestinal tract
is often classified as belonging to the third space as well, although it has substantial fluid content physiologically. In this classification system, the 1st and 2nd space generally refer to the intravascular space (i.e. within the vessels) and the extravascular space (i.e. the interstitial and intracellular spaces), respectively.
Fluid which collects in the second space (ie. the combined interstitial and intracellular space) is more readily available for the body to utilize (such as for the correction of ionic imbalances in other compartments) than fluid in the third space, because fluid in the second space is physiologically more active with the intravascular ("first") space than third space fluid is.
gradients. Water will move from one chamber into 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 space (the blood vessels), blood pressure can drop to dangerously low levels, endangering critical organs such as the brain
, heart
and kidney
s. When fluid shifts out of the cells (the intracellular space), cellular processes slow down or cease from intracellular dehydration. Fluid shifts into the brain cells can cause increased cranial pressure. Fluid shifts may be compensated by fluid replacement
or diuretic
s.
, fluids may pool on the burn site (ie. fluid lying outside of the interstitial tissue, exposed to evaporation) and cause depletion of the fluids in the first and second compartments. With pancreatitis
or ileus
, fluids may "leak out" into the peritoneal cavity
, also causing depletion of the first and second compartments.
Patients who undergo long, difficult operations in large surgical fields collect third-space fluids and become intravascularly depleted despite large volumes of intravenous fluid and blood replacement. Extensive tissue swelling occurs when the third space fills with excess fluid, known as edema.
Clinically, it is common that the actual volume of fluid in a particular patient's third space is difficult to accurately quantify.
Third spacing conditions may include peritonitis
, pyometritis, and pleural effusion
s.
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ian body broadly comprise two compartments, each with several subdivisions:
intracellular fluid, which makes up approximately 60-65% of body water, and
extracellular fluid
Extracellular fluid
Extracellular fluid usually denotes all body fluid outside of cells. The remainder is called intracellular fluid.In some animals, including mammals, the extracellular fluid can be divided into two major subcompartments, interstitial fluid and blood plasma...
, which makes up the other 35-40% of body water (for all practical purposes, the only solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
in the body is water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
).
Intracellular fluid
Intracellular fluid is found inside the bilayered plasma membrane, and is the matrix in which cellular organelles are suspended, and chemical reactions take place.The intracellular compartment contains on average about 28 liters of fluid, and under ordinary circumstances remains in osmotic equilibrium with the ECF.
Interstitial compartment
The interstitial compartment (also called extravascular compartment or tissue space) is the space that surrounds the cellCell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s of a given tissue. It is filled with interstitial fluid
Interstitial fluid
Interstitial fluid is a solution that bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid, which also includes plasma and transcellular fluid...
. Interstitial refers to a "small opening or space between objects". Together with the vascular space, the interstitial space comprises the extracellular space. When excessive fluid accumulates in the interstitial space, edema
Edema
Edema or oedema ; both words from the Greek , oídēma "swelling"), formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin or in one or more cavities of the body that produces swelling...
develops. Interstitial fluid provides the immediate microenvironment
Microenvironment
Microenvironment carries different meanings depending on the context.* Medical: a small or relatively small usually distinctly specialized and effectively isolated habitat or environment...
that allows for movement of ions, proteins and nutrients across the cell barrier. This fluid is not static, but is continually being refreshed and recollected by lymph
Lymph
Lymph is considered a part of the interstitial fluid, the fluid which lies in the interstices of all body tissues. Interstitial fluid becomes lymph when it enters a lymph capillary...
atic channels. In the average male (70 kg)human body, the interstitial space has approximately 10.5 liters of fluid.
In the lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
s there is an interstitial space between capillaries
Capillary
Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...
(tiny blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
s) and the alveoli (the microscopic air-filled sacs in the lungs responsible for absorbing oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
from the atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
). For gas exchange
Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...
to occur, carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
must diffuse
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
across the endothelium
Endothelium
The endothelium is the thin layer of cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. These cells are called endothelial cells. Endothelial cells line the entire circulatory system, from the heart...
of the capillaries across the interstitial space, and across the alveolar epithelium; oxygen must diffuse the other direction.
Intravascular compartment
The main intravascular fluid in mammals is bloodBlood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
which is a complex fluid with elements of a suspension
Suspension (chemistry)
In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
(blood cells), colloid
Colloid
A colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...
(globulins) and solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
(glucose and ions).The average volume of plasma in the average (70 kg) male is approximately 3.5 liters.The volume of the intravascular lumen is regulated in part by hydrostatic pressure gradients,and by reabsorption by the kidneys.
Third space
The third space is space in the body where fluid does not normally collect in larger amounts, or where any significant fluid collection is physiologically nonfunctional. Major examples of third spaces include the peritoneal cavityPeritoneal cavity
The peritoneal cavity is a potential space between the parietal peritoneum and visceral peritoneum, that is, the two membranes that separate the organs in the abdominal cavity from the abdominal wall...
and pleural cavity
Pleural cavity
In human anatomy, the pleural cavity is the potential space between the two pleura of the lungs. The pleura is a serous membrane which folds back onto itself to form a two-layered, membrane structure. The thin space between the two pleural layers is known as the pleural cavity; it normally...
. Still, small amount of fluid does exist normally in such spaces, and function e.g. as lubricant in the case of pleural fluid. Also, the lumen of the gastrointestinal tract
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
is often classified as belonging to the third space as well, although it has substantial fluid content physiologically. In this classification system, the 1st and 2nd space generally refer to the intravascular space (i.e. within the vessels) and the extravascular space (i.e. the interstitial and intracellular spaces), respectively.
Fluid which collects in the second space (ie. the combined interstitial and intracellular space) is more readily available for the body to utilize (such as for the correction of ionic imbalances in other compartments) than fluid in the third space, because fluid in the second space is physiologically more active with the intravascular ("first") space than third space fluid is.
Fluid shift
Fluid shifts occur when the body's fluids move between the fluid compartments. Physiologically, this occurs by a combination of hydrostatic pressure gradients and osmotic pressureOsmotic pressure
Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....
gradients. Water will move from one chamber into 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 space (the blood vessels), blood pressure can drop to dangerously low levels, endangering critical organs such as the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
, heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
and kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
s. When fluid shifts out of the cells (the intracellular space), cellular processes slow down or cease from intracellular dehydration. Fluid shifts into the brain cells can cause increased cranial pressure. Fluid shifts may be compensated by fluid replacement
Fluid replacement
Fluid replacement or fluid resuscitation is the medical practice of replenishing bodily fluid lost through sweating, bleeding, fluid shifts or other pathologic processes. Fluids can be replaced via oral administration , intravenous administration, rectally, or hypodermoclysis, the direct injection...
or diuretic
Diuretic
A diuretic provides a means of forced diuresis which elevates the rate of urination. There are several categories of diuretics. All diuretics increase the excretion of water from bodies, although each class does so in a distinct way.- Medical uses :...
s.
Third spacing
Third spacing is the physiological concept where body fluids accumulate in the third space (see above). In the practice of medicine, the term is most commonly used with regard to burns, but also can refer to ascites and pleural effusions. With regard to severe burnsBurn (injury)
A burn is a type of injury to flesh caused by heat, electricity, chemicals, light, radiation or friction. Most burns affect only the skin . Rarely, deeper tissues, such as muscle, bone, and blood vessels can also be injured...
, fluids may pool on the burn site (ie. fluid lying outside of the interstitial tissue, exposed to evaporation) and cause depletion of the fluids in the first and second compartments. With pancreatitis
Pancreatitis
Pancreatitis is inflammation of the pancreas. It occurs when pancreatic enzymes that digest food are activated in the pancreas instead of the small intestine. It may be acute – beginning suddenly and lasting a few days, or chronic – occurring over many years...
or ileus
Ileus
Ileus is a disruption of the normal propulsive ability of the gastrointestinal tract.Ileus is commonly defined simply as bowel obstruction. However, authoritative sources define it as decreased motor activity of the GI tract due to non-mechanical causes...
, fluids may "leak out" into the peritoneal cavity
Peritoneal cavity
The peritoneal cavity is a potential space between the parietal peritoneum and visceral peritoneum, that is, the two membranes that separate the organs in the abdominal cavity from the abdominal wall...
, also causing depletion of the first and second compartments.
Patients who undergo long, difficult operations in large surgical fields collect third-space fluids and become intravascularly depleted despite large volumes of intravenous fluid and blood replacement. Extensive tissue swelling occurs when the third space fills with excess fluid, known as edema.
Clinically, it is common that the actual volume of fluid in a particular patient's third space is difficult to accurately quantify.
Third spacing conditions may include peritonitis
Peritonitis
Peritonitis is an inflammation of the peritoneum, the serous membrane that lines part of the abdominal cavity and viscera. Peritonitis may be localised or generalised, and may result from infection or from a non-infectious process.-Abdominal pain and tenderness:The main manifestations of...
, pyometritis, and pleural effusion
Pleural effusion
Pleural effusion is excess fluid that accumulates between the two pleural layers, the fluid-filled space that surrounds the lungs. Excessive amounts of such fluid can impair breathing by limiting the expansion of the lungs during ventilation.-Pathophysiology:...
s.