Fig. 6.

Injection near the head demonstrates preferential return flow that is dependent upon the cardiac cycle. After injection, the heart enters a ∼16 second period of asystole. During asystole, there is little transport of dye away from the injection site. This is evident from time-varying plots of local pixel intensity. The small oscillations around baseline at the blue-dot region of interest are probably secondary to head motion (see supplementary material Movie 8). Resumption of the cardiac cycle initiates anterior-to-posterior return flow to the heart. The flow is preferentially along the trachea and dorsal to the aorta.