Page 4, Blood Pressure, Dr. D. Penney


Compliance:

Hooke's law, frequently used in physics and engineering, describes the way materials deform (Figure 03). For example, when a copper wire is stretched, it increases in length directly with the applied stress (force/cross-sectional area) (Figure 04). Eventually, however, a point is reached where the material deforms more rapidly, the yield point. If this is continued the breaking stress is reached. If instead, the stress is decreased before reaching the breaking point, the material springs back toward its original length. Stretch to less than the yield point permits a return to the original length, whereas beyond the yield point a hysteresis loop is generated by an incomplete return. In the latter case, permanent deformation has taken place.

The same kind of maneuver can be done with blood vessel wall by cutting off a small loop, then cutting the loop so a tissue strip is generated. The strip is then mounted in an appropriate bath and stretching device. It is found that the blood vessel wall doesn't behave in the same way as a homogeneous material. Its response to stretch is non-linear, deforming readily at first, and later greatly resisting stretch. The reason for this phenomenon has to do with the several components of the blood vessel wall and their quite different physical characteristics.




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