  The laboratory models of the human heart left ventricle developed in the last
decades gave a valuable contribution to the comprehension of the role of the
fluid dynamics in the cardiac function and to support the interpretation of the
data obtained in vivo. Nevertheless, some questions are still open and new ones
stem from the continuous improvements in the diagnostic imaging techniques.
Many of these unresolved issues are related to the three-dimensional structure
of the left-ventricular flow during the cardiac cycle. In this paper we
investigated in detail this aspect using a laboratory model. The ventricle was
simulated by a flexible sack varying its volume in time according to a
physiologically shaped law. Velocities measured during several cycles on series
of parallel planes, taken from two orthogonal points of view, were combined
together in order to reconstruct the phase averaged, three-dimensional velocity
field. During the diastole, three main steps are recognized in the evolution of
the vortical structures: i) straight propagation in the direction of the long
axis of a vortex-ring originated from the mitral orifice; ii) asymmetric
development of the vortex-ring on an inclined plane; iii) single vortex
formation. The analysis of three-dimensional data gives the experimental
evidence of the reorganization of the flow in a single vortex persisting until
the end of the diastole. This flow pattern seems to optimize the cardiac
function since it directs velocity towards the aortic valve just before the
systole and minimizes the fraction of blood residing within the ventricle for
more cycles.
