Cardiovascular Physiology Concepts
Richard E. Klabunde, Ph.D.
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Stress Relaxation
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. RK Revised 04/06/2007 |
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DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice. © 1999-2008 Richard E. Klabunde, all rights reserved. |
Stress
relaxation is a property of biological tissues that is related to their
viscoelastic properties. To understand this concept, the stress-strain
relationships for a spring and biological tissues such as a blood vessel and
bladder can be compared (see figure). When a spring is suddenly stretched
(increased strain applied), the tension (stress) that is generated is
proportional to the change in length (i.e., Hookean behavior). Furthermore, the
developed tension remains constant over time. In contrast, if a blood vessel
(e.g., aortic segment) is isolated, tied off at one end and cannulated at the
other end so that it can be filled with a known volume of fluid and have its
pressure measured at the same time, then a sudden increase in volume (increased
strain) increases pressure (increased stress); however, the
pressure does not remain steady, but declines a small amount over time. This
decline in pressure (stress) over time at a constant volume (strain) is termed
"stress relaxation." The reason that pressure falls while the
volume remains constant can be explained by the Law of LaPlace, where wall
tension (T) is proportional to pressure (P) times radius (r), T