# diffusion coefficient

Also found in: Dictionary, Thesaurus, Financial, Acronyms, Encyclopedia, Wikipedia.

## coefficient

[ko″ĕ-fish´ent]
1. an expression of the change or effect produced by the variation in certain variables, or of the ratio between two different quantities.
2. in chemistry, a number or figure put before a chemical formula to indicate how many times the formula is to be multiplied.
absorption coefficient absorptivity.
Bunsen coefficient the number of milliliters of gas dissolved in a milliliter of liquid at atmospheric pressure (760 mm Hg) and a specified temperature. Symbol, α.
confidence coefficient the probability that a confidence interval will contain the true value of the population parameter. For example, if the confidence coefficient is 0.95, 95 per cent of the confidence intervals so calculated for a large number of random samples would contain the parameter.
correlation coefficient a numerical value that indicates the degree and direction of relationship between two variables; the coefficients range in value from +1.00 (perfect positive relationship) to 0.00 (no relationship) to −1.00 (perfect negative or inverse relationship).
diffusion coefficient see diffusion coefficient.
coefficient of digestibility the proportion of a food that is digested compared to what is absorbed, expressed as a percentage.
dilution coefficient a number that expresses the effectiveness of a disinfectant for a given organism. It is calculated by the equation tcn = k, where t is the time required for killing all organisms, c is the concentration of disinfectant, n is the dilution coefficient, and k is a constant. A low coefficient indicates the disinfectant is effective at a low concentration.
linear absorption coefficient the fraction of a beam of radiation absorbed per unit thickness of absorber.
mass absorption coefficient the linear absorption coefficient divided by the density of the absorber.
phenol coefficient see phenol coefficient.
sedimentation coefficient the velocity at which a particle sediments in a centrifuge divided by the applied centrifugal field, the result having units of time (velocity divided by acceleration), usually expressed in Svedberg units (S), which equal 10−13 second. Sedimentation coefficients are used to characterize the size of macromolecules; they increase with increasing mass and density and are higher for globular than for fibrous particles.

## diffusion

[dĭ-fu´zhun]
1. the state or process of being widely spread.
2. the spontaneous mixing of the molecules or ions of two or more substances resulting from random thermal motion; its rate is proportional to the concentrations of the substances and it increases with the temperature.

In the body fluids the molecules of water, gases, and the ions of substances in solution are in constant motion. As each molecule moves about, it bounces off other molecules and loses some of its energy to each molecule it hits, but at the same time it gains energy from the molecules that collide with it.

The rate of diffusion is influenced by the size of the molecules; larger molecules move less rapidly, because they require more energy to move about. Molecules of a solution of higher concentration move more rapidly toward those of a solution of lesser concentration; in other words, the rate of movement from higher to lower concentration is greater than the movement in the opposite direction.

Other factors influencing the rate of diffusion from one substance to another are the size of the chamber in which the diffusion is taking place and the temperature within the chamber. The rate of diffusion increases as the size of the chamber increases. Molecular motion never ceases except at absolute zero; as the temperature increases so does the rate of motion of molecules. Thus, the higher the temperature, the greater the molecular activity and, consequently, the greater the rate of diffusion.

Many of the substances passing through the cell membrane are transported actively or passively by the process of diffusion. For certain hormones and other substances, there are transport proteins in the plasma membrane that bind to substances and transport them across the membrane; this type of transport is called facilitated diffusion. Without this constant motion of molecules there would be no exchange of nutrients and end products of cellular metabolism between the intracellular and extracellular fluid and the cell could not survive. The diffusion of water across cell membranes is called osmosis.

The diffusion of gases through the respiratory membrane is essential to normal respiration. The rapidity and ease with which oxygen and carbon dioxide are diffused through the membrane are affected by the thickness of the membrane and its surface area, the diffusion coefficient of the gas in the water within the membrane, and the difference between the partial pressures of the gases in the alveoli and the blood.

The respiratory membrane is normally less than 1 micron in thickness, yet it is composed of three layers within the alveolus (surfactant and fluid layers and alveolar epithelium), an interstitial space between the alveolar epithelium and capillary membrane, and two layers in the capillary membrane. The thickness of the respiratory membrane can be affected by the presence of edematous fluid and by fibrotic changes in the membrane resulting from certain pulmonary diseases. An increase of fluid within the respiratory membrane and alveoli reduces the rate of diffusion because the gases must pass through the additional fluid as well as the other layers of the membrane. Thickening of the epithelial layers of the membrane, as in fibrosis, imposes additional restriction on the passage of gases.

The difference in the partial pressure of a gas in the alveoli and that same gas in the blood is a measure of the net tendency of that gas to pass through the respiratory membrane. The term partial pressure refers to the amount of pressure being exerted by a particular gas in a mixture of gases, the word partial referring to the part that is a particular gas in relation to the whole mixture. The partial pressure of oxygen, for example, reflects the number of oxygen molecules striking the surface of the membrane at any given point. The difference in the partial pressure refers to the difference in the amount of pressure being exerted by the oxygen molecules on the alveolar side of the membrane and the amount of pressure being exerted by the oxygen striking the same point from the opposite side. When the partial pressure of oxygen in the alveoli is greater than that of the oxygen in the blood, the oxygen molecules move across the membrane in the direction of the blood. The same is true in regard to carbon dioxide, which moves in the opposite direction when its partial pressure in the blood is greater than that in the alveoli. Partial pressures of oxygen and carbon dioxide are discussed in more detail under blood gas analysis. See illustration. Simple diffusion. From Applegate, 2000.
diffusion coefficient the number of milliliters of a gas that will diffuse at a distance of 0.001 mm over a square centimeter surface per minute, at 1 atm of pressure. The diffusion coefficient for any given gas is proportional to the solubility and molecular weight of the gas. The diffusion coefficient for oxygen is 1.0, for carbon dioxide it is 20.3, and for nitrogen it is 0.53. The diffusion capacity of a gas varies directly with the diffusion coefficient.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

## dif·fu·sion co·ef·fi·cient

the mass of material diffusing across a unit area in unit time under a concentration gradient of unity.
Synonym(s): diffusion constant
Farlex Partner Medical Dictionary © Farlex 2012

## dif·fu·sion co·ef·fi·cient

(di-fyū'zhŭn kō'ĕ-fish'ĕnt)
The mass of material diffusing across a unit area in unit time under a concentration gradient of unity.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012
References in periodicals archive ?
where C(x,t) is chloride concentration at depth x and immersion time t, [C.sub.s] is chloride concentration at the surface, x is depth, t is immersion time, and D is chloride diffusion coefficient.
Wang et al., "Correlation of apparent diffusion coefficients measured by 3T diffusion-weighted MRI and SUV from FDG PET/CT in primary cervical cancer," European Journal of Nuclear Medicine and Molecular Imaging, vol.
Although empirical (and certainly not valid over the entire [sigma]/[R.sub.pore] range), expression (6) also takes into account that sufficiently large molecules ([sigma] [right arrow] [infinity]) can no longer diffuse through the clay anymore (G [right arrow] [infinity] corresponding to a zero effective/apparent/pore diffusion coefficient).
Still on the table 2, observed that an inverse behavior of immersion time over the sucrose and water diffusion coefficients, i.e., increased immersion time decreases water and solute diffusion coefficients.
These results are in accordance with Luu and Wu , where the authors compared the diffusion coefficient of different microstructures and concluded that regular ferrite shows the highest values.
The difference in this simulation of the surface-coated concrete is the diffusion coefficient matrix, which depends on the coating and concrete area.
Overall liquid diffusion coefficient of yellow-poplar heartwood was 3.15 x [10.sup.-8] [m.sup.2]/S (Table 5).
Kinetic parameters like diffusion coefficients and heterogeneous rate constants were also derived by using the data given in Tables 1 and 2.
Since the diffusion coefficient is dependent on the activation energy, the analysis for both initial stage and long term stage of absorption showed that there is a very clear difference between the resin and the composites.
Role of apparent diffusion coefficient values in differentiation between malignant and benign solitary thyroid nodules.
Keywords: MRI brain, Apparent Diffusion Coefficient, Typical meningioma, Atypical, Anaplastic.

Site: Follow: Share:
Open / Close