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equilibrium (e-kwi-lib´re-um)

1. The condition of being evenly balanced; a state of repose between two or more antagonistic forces that exactly counteract each other. 2. In chemistry, a state of apparent repose created by two reactions proceeding in opposite directions at equal speed; in chemical equations, sometimes indicated by two opposing arrows (<->) instead of the equal sign.dynamic e; See also equilibrium constant. [L. aequilibrium, a horizontal position, fr. aequus, equal, + libra, a balance]
acid-base e. acid-base balance
Donnan e. when a semipermeable membrane or its equivalent (e.g., a solid ion-exchanger) separates a nondiffusible substance, such as protein, from diffusible substances, the diffusible anions and cations are distributed on the two sides of the membrane so that 1) the products of their concentrations are equal, and 2) the sum of the diffusible and nondiffusible anions on either side of the membrane is equal to the sum of the concentrations of diffusible and nondiffusible cations; the unequal distribution of diffusible ions thus produced creates a potential difference across the membrane (membrane potential).Gibbs-Donnan e;
dynamic e. equilibrium (2)
genetic e. the condition of a dynamic genetic system in which the several rates of change between all possible pairs of parts are such that the composition is invariant.
Gibbs-Donnan e. Donnan e
Hardy-Weinberg e. that state in which the genetic structure of the population conforms to the prediction of the Hardy-Weinberg law; it is not a stable e., although for a large mating population it may be approximated.random mating e;
homeostatic e. See homeostasis.
nitrogenous e. a condition in which the amount of nitrogen excreted from the body equals that taken in with the food; nutritive e. so far as protein is concerned.
nutritive e. condition in which there is a perfect balance between intake and excretion of nutritive material, so that there is no increase or loss in weight.physiologic e;
physiologic e. nutritive e
radioactive e. a situation (not a true e.) in which a particular atom is being produced by the radioactive breakdown of a precursor while it is itself breaking down, the two breakdowns matching so that after a period of time the ratio of radioactivity of product and precursor is constant with time.
random mating e. Hardy-Weinberg e
secular e. a type of radioactive e. in which the half-life of the precursor (parent) radioisotope is so much longer than that of the product (daughter) that the radioactivity of the daughter becomes equal to that of the parent with time.
stable e. e. in which, after every small perturbation, the original state will tend to be restored.
transient e. a type of radioactive e. in which the half-life of the parent radioisotope is longer than that of the daughter so that the ratio of activities of parent and daughter become constant as they decrease with time.
unstable e. e. in which the response to a small perturbation will tend to make the perturbation greater (e.g., a logged feedback process of zero order).


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