From Big Medical Encyclopedia

IMPEDANCE (English impedance, from armor. to interfere with impedire) — physical. the size characterizing environmental resistance for fluctuations of various origin.

Depending on a type of fluctuation distinguish And. electric, And. mechanical and And. acoustic.

Electric impedance represents the full (complex) resistance of an electric chain to alternating current. In a habit view And. electric (Z) represents the geometrical sum of active resistance of an electric chain (R) and reactive resistance (X):

Z = √ (R 2 + X 2 ).

Active resistance In is size, a reciprocal of conductivity of fabric, and depends on the frequency of alternating current a little. The reactive component of the X complex resistance of Z for various electroconductive biol, in vivo and in vitro substrates depends on the frequency of electric current. For unit And. electric in the International System of Units the Ohm is accepted (Ω, ohm).

Measurement of size electric And. it is applied to the characteristic of electric properties of fabrics, bodies, separate cells (see. Conductivity of biological systems ). Electric And. biol, fabrics decreases at increase in frequency of the enclosed electric field that is connected with existence of a capacity component And., caused generally by the phenomenon polarization (see).

And. body tissues depends, in particular, on a condition of blood circulation (a krovenapolneniya of vessels). Therefore measurement And. separate body parts, is more often than extremities, it is the basis for studying of peripheric circulation — a so-called method reografiya (see). At a reografiya alternating current with a frequency of 20 — 30 kHz is used.

Electric And. biol, fabrics changes depending on them funkts, states. The small alternating current passing through an object at measurement does not cause damage of fabric therefore observed changes in it under these or those conditions can be connected with structural and ionic changes in fabric. Studying of components electric And. suspensions of cells allows to determine electric parameters of both cells, and their superficial membranes, to judge them change permeability (see).

Measurement of an impedance at high frequencies (higher than 1 MHz) allows to estimate total concentration of free electrolytes in cells and fabrics (see. Conductometry ). Measurement And. electric allows to register also changes physical. - chemical structure of living tissues it is normal also of pathology. Therefore this method can be used for studying of dynamics of the changes happening at various diseases and injuries and also for assessment of efficiency of their treatment.

Acoustic impedance — the size of complex resistance entered for the characteristic of resistance of any speaker systems to distribution of sound vibrations.

Account acoustic And. it is important during the studying of distribution of a sound, acoustic properties as physical. (pipes of variable section, loud-hailers, radiators and receivers of a sound, their diffusers, membranes etc.), and biol, systems (acoustic organs, speeches etc.), and also during creation of devices, corrective acoustic organs and speeches.

Acoustic And., as well as electric, includes active and reactive components. The active component is connected with power losss on the radiation of a sound the speaker system and with losses on friction. The reactive component (reactive resistance) characterizes forces of inertia and elasticity operating in system. According to it reactive resistance is called also inertial or elastic resistance.

In the SI system acoustic And. measure in newton-seconds on meter in the fifth degree (N with/m 5 ), and in the SGS system — in dynes-seconds on centimeter in the fifth degree (dynes sec./cm 5 ). The last unit is called sometimes acoustic ohm.

The mechanical impedance characterizes environmental resistance to distribution of various fluctuations (sound, ultrasonic etc.). Measurement of size mechanical And. it is used in medical - biol, researches during the studying of vibration and its action on an organism under production conditions.

In unit mechanical And. in the SI system the newton-second on meter is (N with/m). And. acoustic (Za) and mechanical (Zm) are connected by a ratio:

Zm = Za * S 2 ,

where S — the area of the speaker system.

Bibliography: Biophysics, under the editorship of B. N. Tarusov and O. R. Necklace, M., 1968, bibliogr.; From a heap to E. Bases of acoustics, the lane with English, t. 1 — 2, M., 1976.

G.E. Fedorov.