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1. A nonsolid substance (that is, liquid or gas) that tends to flow or conform to the shape of the container in which it is kept.
2. Consisting of particles or distinct entities that can readily change their relative positions, that is, tending to move or capable of flowing.
[L. fluidus, fr. fluo, to flow]


/flu·id/ (floo´id)
1. a liquid or gas; any liquid of the body.
2. composed of molecules which freely change their relative positions without separation of the mass.

amniotic fluid  the liquid within the amnion that bathes the developing fetus and protects it from mechanical injury.
cerebrospinal fluid  (CSF) the fluid contained within the ventricles of the brain, the subarachnoid space, and the central canal of the spinal cord.
follicular fluid  the fluid in a developing ovarian follicle.
interstitial fluid  the extracellular fluid bathing most tissues, excluding the fluid within the lymph and blood vessels.
intracellular fluid  the portion of the total body water with its dissolved solutes which are within the cell membranes.
prostatic fluid  the secretion of the prostate gland, which contributes to formation of the semen.
Scarpa's fluid  endolymph.
seminal fluid  semen.
synovial fluid  synovia; the transparent, viscid fluid secreted by the synovial membrane and found in joint cavities, bursae, and tendon sheaths.


Etymology: L, fluere, to flow
1 a substance, such as a liquid or gas, that is able to flow and to adjust its shape to that of a container because it is composed of molecules that are able to change positions with respect to each other without separating from the total mass.
2 a body fluid, either intracellular or extracellular, involved in the transport of electrolytes and other vital chemicals to, through, and from tissue cells. See also blood, cerebrospinal fluid, lymph.

fluid/electrolyte management

a nursing intervention from the Nursing Interventions Classification (NIC) defined as regulation and prevention of complications from altered fluid and/or electrolyte levels. See also Nursing Interventions Classification.


A liquid or gas which conforms to the shape of its container. See Body fluid, Cerebrospinal fluid, Pericardial fluid.


1. A nonsolid substance, such as a liquid or gas, which tends to flow or conform to the shape of the container.
2. Consisting of particles or distinct entities that can readily change their relative positions; tending to move or capable of flowing.
[L. fluidus, fr. fluo, to flow]


(floo'id) [L. fluidus, flowing]
A nonsolid, liquid, or gaseous substance. See: secretion

allantoic fluid

Fluid found in the fetal membrane that develops from the yolk sac.

amniotic fluid

A clear fluid that surrounds the fetus in the amniotic sac. Its primary functions are to suspend and protect the growing fetus, allow freedom of movement, maintain even constant temperature, and aid normal development of the fetal lungs. Volume increases from about 50 ml at 12 gestational weeks to around 800 ml at 38 weeks. The fluid is constantly being circulated by the fetus swallowing fluid, urinating, and inhaling/exhaling fluid during fetal respiration. Samples of amniotic fluid may be collected by amniocentesis to identify fetal chromosomal abnormalities, state of health, and maturity. Synonym: liquor amnii.
See: amniocentesis; oligohydramnios; polyhydramnios

ascitic fluid

Clear, pale, straw-colored fluid occurring in ascites. The fluid is normally sterile; its specific gravity is normally 1.005 to 1.015; the cellular content is less than 250 white blood cells per cubic millimeter, and its protein content is low. Cancer, heart failure, liver failure, peritonitis, and tuberculosis may alter the amount or character of ascites.

body fluid

A fluid found in one of the fluid compartments of the body. The principal fluid compartments are intracellular and extracellular. A much smaller segment, the transcellular, includes fluid in the tracheobronchial tree, the gastrointestinal tract, and the bladder; cerebrospinal fluid; and the aqueous humor of the eye. The chemical composition of fluids in the various compartments is carefully regulated. In a normal 154 lb (70 kg) adult human male, 60% of total body weight (i.e., 42 L) is water; in a normal adult female is 55% of total body weight is water (39 L).
See: acid-base balance; fluid replacement; fluid balance

Bouin fluid

See: Bouin fluid
Enlarge picture

cerebrospinal fluid

Abbreviation: CSF
The sodium-rich, potassium-poor tissue fluid of the brain and spinal cord. The fluid supplies nutrients and removes waste products; it is also a watery cushion that absorbs mechanical shock to the central nervous system. Synonym: spinal fluid See: lumbar puncture


The fluid is formed by the choroid plexuses of the lateral and third ventricles. That of the lateral ventricles passes through the foramen of Monro to the third ventricle, and through the aqueduct of Sylvius to the fourth ventricle. There it may escape through the central foramen of Magendie or the lateral foramina of Luschke into the cisterna magna and to the cranial and spinal subarachnoid spaces. It is reabsorbed through the arachnoid villi into the blood in the cranial venous sinuses, and through the perineural lymph spaces of both the brain and the cord. See: illustration


The fluid is normally watery, clear, colorless, and almost entirely free of cells. The initial pressure of spinal fluid in a side-lying adult is about 100 to 180 mm of water. On average, the total protein is about 15 to 50 mg/dL, and the concentration of glucose is about twothirds the concentration of glucose in the patient's serum. Its pH, which is rarely measured clinically, is slightly more acidic than the pH of blood. Its concentration and alkaline reserve are similar to those of blood. It does not clot on standing. Turbidity suggests an excessively high number of cells in the fluid, typically white blood cells in infections such as meningitis or red blood cells in intracerebral hemorrhage.

CSF may appear red following a recent subarachnoid hemorrhage or when the lumbar puncture that obtained the CSF caused traumatic injury to the dura that surround the fluid. Centrifugation of the fluid can distinguish between these two sources of blood in the spinal fluid: the supernatant is usually stained yellow (xanthochromic) only when there has been a recent subarachnoid hemorrhage.

Many conditions may cause increases in CSF total protein: infections, such as acute or chronic meningitis; multiple sclerosis (when oligoclonal protein bands are present); Guillain-Barré syndrome; and chronic medical conditions like cirrhosis and hypothyroidism (when diffuse hypergammaglobulinemia is present). The concentration of glucose in the CSF rises in uncontrolled diabetes mellitus and drops precipitously in meningitis, sarcoidosis, and some other illnesses. Malignant cells in the CSF, demonstrated after centrifugation or filtering, are hallmarks of carcinomatous meningitis.


The CSF is normally sterile. Meningococci, streptococci, Haemophilus influenzae, Listeria monocytogenes, and gram-negative bacilli are recovered from the CSF only in cases of meningitis. Syphilitic meningitis is usually diagnosed with serological tests for the disease, such as the venereal disease research laboratory (VDRL) test, the rapid plasma reagin (RPR) test, or the fluorescent treponemal antibody test. Cryptococcal infection of the CSF may be demonstrated by India ink preparations, or by latex agglutination tests. Tuberculous meningitis may sometimes be diagnosed with Ziehl-Neelsen stains, but more often this is done with cultures. These last three infections (syphilis, cryptococcosis, and tuberculosis) are much more common in patients who have acquired immunodeficiency syndrome (AIDS) than in the general population.


crevicular fluid

Gingival sulcular fluid.

extracellular fluid

The body fluid outside of cells. It includes the interstitial, intravascular, and cerebrospinal fluids. Water is the common solvent of all these fluids. Approximately 36% of an adult's body fluids and 47% of and infant's body fluids are extracellular.

extravascular fluid

Interstitial fluid.

follicular fluid

The liquid rich in heparin sulfate, hyaluronic acid, anti-müullerian hormone, defensins, and other chemicals that surrounds developing oocytes as they mature in the ovary.

gingival fluid

Gingival sulcular fluid.

gingival sulcular fluid

Abbreviation: GSF
In dentistry, the fluid that seeps through the gingival epithelium. It increases with gingival inflammation. Cellular elements within GSF include bacteria, desquamated epithelial cells, and leukocytes. Electrolytes and some organic compounds are also present. Synonym: crevicular fluid; gingival fluid

interstitial fluid

Water and dissolved substances inside tissues but outside of cells and vessels. Interstitial fluid is largely the ultrafiltrate of arterial blood, having been pushed through capillary walls by hydrostatic force; therefore it has a salt concentration similar to blood serum. Normally, approximately 29% of an adult's body fluids and 40% of an infant's body fluids are interstitial fluids. Excess interstitial fluid is returned to the circulation by the lymphatics. An accumulation of excess interstitial fluid is called edema.
Synonym: extravascular fluid

intracellular fluid

Abbreviation: ICF
The potassium-rich, sodium-poor watery solution inside cells. Approx. 55 to75% of total body water is intracellular.

intraocular fluid

Fluid within the anterior and posterior chambers of the eye. Synonym: aqueous humor

intravascular fluid

That portion of the total body fluid contained within blood and lymphatic vessels.

peritoneal fluid

The clear straw-colored serous fluid secreted by the cells of the peritoneum. The few milliliters present in the peritoneal cavity moisten the surfaces of the two peritoneal layers and allow them to glide over each other as the intestinal tract changes shape during the process of digestion and absorption. In certain disease states (such as right-sided heart failure, cirrhosis, or ovarian malignancy) the amount of peritoneal fluid is increased.
See: ascites

pleural fluid

Fluid secreted by serous membranes in the pleurae that reduces friction during respiratory movements of the lungs. When excessive pleural fluid is secreted and not absorbed, a pleural effusion accumulates.

Scarpa fluid

See: Scarpa, Antonio

seminal fluid


serous fluid

Fluid secreted by serous membranes that reduces friction in the serous cavities (pleural, pericardial, and peritoneal).

spinal fluid

Cerebrospinal fluid

synovial fluid

Clear viscid lubricating fluid of the joint, bursae, and tendon sheaths, secreted by the synovial membrane of a joint. It contains mucin, albumin, fat, and electrolytes.
Synonym: synovia See: synovial joint

transcellular fluid

The extracellular fluid that lubricates the potential spaces of the body, such as the pleura and pericardium.

Zenker fluid

See: Zenker, Friedrich Albert von


A nonsolid substance (i.e., liquid or gas) that tends to flow or conform to the shape of the container in which it is kept.
[L. fluidus, fr. fluo, to flow]

fluid (floo´id),

n a liquid or gaseous substance.
fluid, crevicular,
n a clear, usually unnoticeable fluid that can serve as a defense mechanism against infection by carrying antibodies and other therapeutic substances between the connective tissue and sulcus or pocket. Also called
gingival sulcus fluid or
sulcular fluid.
fluid delivery,
n the continual fluid stream of an ultrasonic instrument, either over or through the vibrating tip, which is necessary to maintain a stable instrument temperature throughout a procedure.
fluid, dentinal,
n the fluid content within the dentinal tubules of the dentin of the tooth.
fluid, lacrimal,
n a watery secretion of the lacrimal gland, commonly called tears. The fluid is secreted into the lacrimal lake, an area located between the eyeball and the upper eyelid. It helps bathe the sensitive cornea. Tearing can result from eye irritation, or during periods of emotional distress.
fluid, synovial,
n the small amount of fluid occurring in normal joints. Its principal function is to lubricate the joint surfaces and nourish the articular cartilage, such as in the temporomandibular joint.
fluid, total body,
n all the fluids contained in the body. There are two main types: the intracellular fluid, which is contained totally within the cells, and the extracellular fluid, which is contained entirely outside the cells.
fluid wax,
n See wax, fluid.


1. a liquid or gas; any liquid of the body.
2. composed of molecules which freely change their relative positions without separation of the mass.

allantoic fluid
the fluid contained within the allantois.
amniotic fluid
the fluid within the amnion that bathes the developing fetus and protects it from mechanical injury.
ascitic fluid
see ascites.
fluid balance
a state in which the volume of body water and its solutes (electrolytes and nonelectrolytes) are within normal limits and there is normal distribution of fluids within the intracellular and extracellular compartments. The total volume of body fluids should be about 60% of the body weight, and it should be distributed so that one-third is extracellular fluid and two-thirds intracellular fluid. Although this distribution remains constant in a healthy animal, there is continuous movement of fluid into and out of the various compartments. See also dehydration, water intoxication.
body f's
the fluids within the body, composed of water, electrolytes and nonelectrolytes. The volume and distribution of body fluids vary with age, sex and amount of adipose tissue. Throughout life there is a slow decline in the volume of body fluids; obesity decreases the relative amount of water in the body.
Although the body fluids are continuously in motion, moving in and out of the cells, tissue spaces and vascular system, physiologists consider them to be 'compartmentalized'. Fluid within the cell membranes is called intracellular fluid and comprises about two-thirds of the total body fluids. The remaining one-third is outside the cell and is called extracellular fluid. The extracellular fluid can be further divided into tissue fluid (interstitial fluid), which is found in the spaces between the blood vessels and surrounding cells, and intravascular fluid, which is the fluid component of blood.
The maintenance of a proper balance between the intracellular and extracellular fluid volumes is essential to health. In patients with heart failure and renal failure the balance becomes upset, producing either localized or generalized edema. Excessive fluid loss produces fluid volume deficit causing cellular dehydration and impaired cellular function.
Bouin's fluid
a histological fixative.
cerebrospinal fluid
the fluid contained within the ventricles of the brain, the subarachnoid space, and the central canal of the spinal cord. See also cerebrospinal fluid.
fluid dram
see fluid dram.
fluid extract
a liquid preparation of a vegetable drug, containing alcohol as a solvent or preservative, or both, of such strength that each milliliter contains the therapeutic constituents of 1 gram of the standard drug it represents.
fetal fluid
allantoic plus amniotic fluids.
interstitial fluid
the extracellular fluid bathing the cells in most tissues, excluding the fluid within the lymph and blood vessels.
isotonic fluid
having the same tonicity or osmotic pressure as blood.
lacrimal fluid
aqueous fluid secreted by the lacrimal glands; called also tears.
fluid line
in radiographs, the interface between fluid and gas, as in the gastrointestinal tract, will show as a straight line.
fluid loss
by vomiting, diarrhea, polyuria, water deprivation. See dehydration.
fluid mosaic model
the modern concept of the structure of a biological membrane developed by S.J. Singer and G.L. Nicolson. In it the membrane consists of protein molecules partly embedded in a discontinuous bilayer of phospholipids that form the matrix of a mosaic of functional cell units.
fluid ounce
see fluid ounce.
pericardial, pleural, peritoneal fluid
normally present in amounts sufficient only to lubricate the movement of viscera within the respective cavities. Composition similar to blood serum.
fluid replacement
see fluid therapy (below).
fluid restriction
the limitation of oral fluid intake to a prescribed amount for each 24-hour period.
fluid retention
see edema.
spinal fluid
the fluid within the spinal canal.
fluid splashing sounds
audible when gas and fluid are free in a cavity, e.g. abomasum in cases of abomasal displacement; can be elicited by shaking a small animal or part of a large animal (i.e. succussion) or by simultaneous percussion and auscultation.
synovial fluid
fluid therapy
aims to replace fluids lost by disease process or by restriction of intake, or to maintain a high rate of fluid excretion to ensure removal of toxins, or to administer therapeutic or anesthetic agents slowly over a long period. The amounts and route of administration vary with the need of the patient. Normal solutions include 5% dextrose and Ringer's solution; alkalinizing fluids include lactated Ringer's and 1.3% sodium bicarbonate; acidifying solutions include isotonic saline and 1.9% ammonium chloride.
fluid thrill
see thrill.
fluid volume deficit
an imbalance in fluid volume in which there is loss of fluid from the body not compensated for by an adequate intake of water. The major causes are: (1) insufficient fluid intake, and (2) excessive fluid loss from vomiting, diarrhea, suctioning of gastric contents, or drainage through operative wounds, burns or fistulae. Decreased volume in the intravascular compartment is called hypovolemia. Because water moves freely between the compartments, extracellular fluid deficit causes intracellular fluid deficit (cellular dehydration), which leaves the cells without adequate water to carry on normal function.
fluid volume excess
an overabundance of water in the interstitial fluid spaces or body cavities (edema) or an excess of fluid within the blood vessels (hypervolemia) and water intoxication.
Factors that contribute to the accumulation of edematous fluid are: (1) dilatation of the arteries, as occurs in the inflammatory process; (2) reduced effective osmotic pressure, as in hypoproteinemia, lymphatic obstruction and increased capillary permeability; (3) increased venous pressure, as in congestive heart failure, thrombophlebitis and cirrhosis of the liver; and (4) retention of sodium due to increased reabsorption of sodium by the renal tubules.
fluid wave
see thrill.

Patient discussion about fluid

Q. What causes fluid to leak from the legs?

A. It may be caused by chronic venous insufficiency - when the veins of your leg are not competent enough to drain the blood from the legs. It causes accumulation of blood in your legs, and the elevated pressure of the blood causes the water in the blood to leak out of the blood vessels.

You may read more here: www.nlm.nih.gov/medlineplus/ency/article/000203.htm

More discussions about fluid
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