Arterial Haemodynamics in the Cephalopod Mollusc, Octopus Dofleini

Simultaneous measurements of blood pressure and blood flow were made in the dorsal aorta of the cephalopod Octopus dofleini Wülker. In resting animals the heart rate varied from 8.5 to 14 beats min−1, while aortic pressure typically ranged from about 2.5 kPa at end diastole to 5.5 kPa at peak systole. Blood flow rate varied with pressure, averaging 0.9 ml s−1, with peak flow rates of 1.4–2.8 ml s−1. A slow asymptotic decline in both pressure and flow during the 2- to 4-s diastolic period indicated that the aorta functions as an elastic reservoir, as previously predicted. Aortic impedance spectra were derived from digitized pressure and flow data. The impedance amplitude decreased continuously with increasing frequency, while the impedance phase was always negative. These results are consistent with a two-element Windkessel model of the arterial system. The apparent pressure wave velocity, determined from transit times of pressure pulses in vivo, was > 10 m s−1. This was much higher than the intrinsic wave velocity we predicted from in vitro measurements of aortic elasticity. This anomaly occurred because the length of the aorta represented less than 1 % of the wavelength of the fundamental pulse frequency, due to the low heart rate in this species. Consequently, arterial haemodynamics in O. dofleini are dominated by strong wave reflections, but do not exhibit other wave propagation effects which are typical of mammalian circulatory systems. Thus, the simple Windkessel model adequately describes the arterial circulation of this species of cephalopod.