the Vascular System
subscribed to a time constant. These properties allow arteries to respond
to rapid transient changes in transmural blood pressures.
the main pulmonary artery showing hysteresis
Top tracing: major axis. Bottom tracing: minor axis.
Hysteresis develops when the vessel is subjected to sinusoidal or
cyclic changes. If the artery is purely elastic, there will be no phase shift
between the applied pressure and the resulting change in diameter. The
viscoelastic behavior of the artery leads to phase shifts in its pressure-
observed, reflecting viscous
losses. In other words, energy is dissipated in stretching the artery and
allowing it to return to its control value. If the artery were purely elastic,
there would be no energy loss and the artery would return to its control
value along the exact path during stretching.
Examples of experimentally measured pressure-diameter relations are
shown in Fig.
for the pulmonary aorta. Since the pulmonary aorta
is normally oval, there are
different diameters, namely, the major
axis diameter and the minor axis diameter. When the major and minor
axes diameters are plotted against pressure, the hysteresis loops are