From Big Medical Encyclopedia

RHEOLOGY (Greek rheos a current, a flow + logos the doctrine) — science about deformations and flowability of substance; studies the processes connected with nonreversible, residual deformations and currents of various viscous and plastic bodies. Also the biorheology combining researches of processes of a current of biological liquids (e.g., blood, synovial, pleural and other liquids), deformations of cells of fabrics and bodies (muscles, bones, blood vessels, etc.) the person and animals enters R.

Biological objects, as a rule, show at the same time elastic and viscous or plastic and viscous properties. Studying of rheological properties of different types of fabrics and bodies and their changes in process of pathology formed the basis of creation of many methods used in diagnostic and a wedge, the purposes (see. Reogepatografiya , Reografiya , Reokardiografiya , Reooftalmografiya , Reopulmonografiya , Rheoencephalography ).

Elastic properties bodies (materials) capable to reversible deformation, i.e. recovery of the sizes after cancellation of the external force which caused deformation have. If after the termination of influence from the outside recovery is incomplete or does not happen at all, then residual deformation is called inelastic, or plastic, and by materials — plastic. A measure of deformation is relative deformation ΔX/X, equal to the relation of absolute deformation (ΔХ) to initial value of the size (X) characterizing the sizes of a body. According to Hooke's law tension about at elastic deformation of a body is proportional to relative deformation:

σ = k(ΔX/X),

where σ — tension (i.e. the physical quantity in number equal to the elastic force of F upr , S operating per acre sections of a body: σ = F upr / S, a k — the module of elasticity in number equal to tension arising in a body when relative deformation is equal to unit. As the main rheological characteristic of liquids serves their size viscosity (see).

At a rheological research of biological fabrics with elastic properties define curve stretchings (i.e. dependences of mechanical tension on relative deformation), on the Crimea calculate the size of the module of elasticity with a different length of an object. About viscoelastic character of fabric judge by existence of a so-called loop of a hysteresis, i.e. discrepancy of the curves of stretching received at increase in length and the return shortening. During the studying of stress relaxation (i.e. its time histories after step change of length), creep (time history of length after step change of the deforming force), a phase ratio between tension and deformation in the conditions of periodic change of length temporary parameters of deformation of visco-elastic bioobjects become clear. Laboratory rheological test is used for clarification of pathological changes of bodies and fabrics. Curve stretchings and the module of elasticity can be indicators of suitability of transplants of skin, blood vessels, etc. These rheological characteristics are considered during the development of prostheses of blood vessels.

Measurement of dependence of viscosity of blood (see. Viscosity ), from the speed of its current of in vitro finds application in diagnosis of diseases of the blood circulatory system. In bioobjects widely apply the mechanical models representing various combinations of elastic and viscous elements to the analysis of the mechanism of the rheological phenomena.

Bibliography: Bartenev G. M. Structure and relaxation properties of elastomers, M., 1979; L and y t f at t E. The phenomena of transfer in live organisms, the lane with English, M., 1977; McDonald D. And. Blood flow in arteries, L. — Baltimore, 1974.

D. I. Roshchupkin.