Structure and Stress-Strain Relationship of Soft Tissues

The mechanical properties of a soft tissue are related to itsstructure. Weshall illustrate this by the properties of thearteries and the lung. Viscoelasticity, strain rate effects,pseudo-elasticity, and constitutive equations ar discussed.The mecahnical properties of an organ is, however, not onlybased onthe tissues of the organ, but also on its geometry andrelationship to the neighboring organs. A typical example isthe blood vessel. The capillary blood vessels of the mesenteryare "rigid"; those in the bat's wing are "distensible"; whereasthe capillaries of the lung are "sheet" like: rigid in one plane,and compliant in another. The stress-strain relationship ofthe systemic arteries is highly nonlinear, stiffening exponentiallywith increasing strains; yet that of the pulmonary arteriesin the lung is linear. The systemic veins are easily collapsible;yet the pulmonary veins in the lung are not: they remain patentwhen the blood pressure falls below the alveolargas pressure.The explanation of these differences lies in the varied interactionsbetween the blood vessels and the surrounding tissues in differentorgans. The implications of these differences on blood circulationare pointed out. Therole of ultrastructure is discussed.