# diffusion

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## 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

(di-fyū'zhŭn),
1. The random movement of molecules or ions or small particles in solution or suspension under the influence of brownian (thermal) motion toward a uniform distribution throughout the available volume; the rate is relatively rapid among liquids and gases, but takes place very slowly among solids.
2. Light scattering.
Farlex Partner Medical Dictionary © Farlex 2012

## dif·fu·sion

(di-fyū'zhŭn)
1. The random movement of molecules or ions or small particles in solution or suspension toward a uniform distribution throughout the available volume.
2. Synonym(s): dialysis (1) .
Medical Dictionary for the Health Professions and Nursing © Farlex 2012

## diffusion

(dif-ū′zhŏn) [L. diffusio, pouring out, spreading out]
DIFFUSION OF RESPIRATORY GASES
The tendency of the molecules of a substance (gas, liquid, or solid) to move from a region of high concentration to one of lower concentration. In the body, oxygen and carbon dioxide move by diffusion. The diffusion of water to an area of greater solute concentration is called osmosis. See: illustration

### facilitated diffusion

The movement of a substance (such as glucose) through a cell membrane along a concentration gradient with the help of membrane proteins acting as carrier molecules.

### water diffusion

The movement of water into and out of cells, e.g., cells of the central nervous system after a stroke. Decreases in water diffusion are found when brain cells have been deprived of blood and oxygen.
Medical Dictionary, © 2009 Farlex and Partners

or

## passive transport

the movement of molecules of a particular substance from regions of high concentration to regions of low concentration of that substance, i.e. down a CONCENTRATION GRADIENT. The rate of diffusion in gases is much greater than in liquids, due to differences in their molecular structure. For example, carbon dioxide will diffuse 10,000 times more rapidly in air than in water. diffusion pressure deficit (DPD) see WATER POTENTIAL.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005

## diffusion

1. Scattering of light passing through a heterogeneous medium, or being reflected irregularly by a surface, such as a sandblasted opal glass surface. Diffusion by a perfectly diffusing surface occurs in accordance with Lambert's cosine law. In this case, the luminance will be the same, regardless of the viewing direction. 2. The passive movement of ions or molecules through a medium or across a semi-permeable membrane (e.g. the ciliary epithelium) in response to a concentration gradient until equilibrium is reached. It is one of the three mechanisms that create aqueous humour. See diffuse light; diffuse reflection; ultrafiltration.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann

## dif·fu·sion

(di-fyū'zhŭn)
1. The random movement of molecules or ions or small particles in solution or suspension toward a uniform distribution throughout the available volume.
2. Synonym(s): dialysis (1) .
Medical Dictionary for the Dental Professions © Farlex 2012
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