Figure 1.

A fluid moving inside a straight vessel presents higher velocities in its centre and slower in the external fluid layers due to friction (A). The difference in flow velocity generates a tendency of the fluid to spin away from the central jet that causes swirling flow motion when the limiting boundaries expand (B). This situation appears in cardiac chambers when the filling inflow passes through the atrio-ventricular valve into the ventricle, where this swirling tendency organizes in vortices (C).