Fig. 3. Hemodynamic properties of the cardiac cycle (bar) in one heart segment to show the timing of filling and emptying (example: heart segment 10). The optical signal (black line) reflects the rhythmic filling (downward deflection) and emptying (upward deflection) of the heart segment and yields the information for the start of diastole (=10% filled), maximal diastole (trough), the attainment of systole (`systole'; see Materials and methods for definition) and the end of systole (=90% empty). Phase relations were based on systole set at 0% phase. The period of one cardiac cycle was normalized to 100% phase (4.7±0.7 s in juvenile leeches). Systole of the afferent vessels and the closure of the efferent latero-abdominal sphincter occurred at the same time, just before maximal heart diastole (horizontal dotted bars).