Therefore, different biological tissues display different degrees of stress relaxation. The reason for these differences among tissues is that tissues differ in their structural components (e.g., smooth muscle, collagen, elastin) and the arrangement of those components within the tissue. For example, the bladder, which is mostly smooth muscle, shows a high degree of stress relaxation while a tendon that is primarily composed of collagen shows virtually no stress relaxation.
To understand the functional significance of stress relaxation, one only has to relate the ability of different tissues to display stress relaxation to their biological function. For example, as urine flows into the bladder, the bladder volume increases. It is important that as the volume increases there is only a small increase in pressure, otherwise high pressures within the bladder would impede urine outflow from the kidneys and damage the kidneys. The high compliance and high degree of stress relaxation in the bladder ensures that the pressure does not increase very much. In contrast, if the aorta and other arterial vessels showed the same high degree of stress relaxation as the bladder, then blood pressure could not be sufficiently maintained.
Understanding stress relaxation is important when the compliance of a tissue is being determined. Compliance is the ratio of the change in volume divided by the change in pressure. However, because tissues such as blood vessels show some stress relaxation, it is important to measure the steady-state change in pressure after the volume is increased. If the pressure is measured before stress relaxation is complete, then the measured compliance would be lower than that measured in the steady-state. If the volume changes rapidly within a blood vessel (e.g., the aorta during cardiac systole and diastole), the measured compliance (change in volume divided by the change in pressure) will vary depending upon the rate of volume change. With more rapid changes in volume (e.g., as occurs with increased ventricular ejection velocity), the calculated compliance will be less than at more slowly changing volumes. Therefore, when a true steady-state compliance cannot be determined, the compliance is sometimes referred to as the "dynamic compliance" of the tissue or organ.