Cardiovascular dynamics of tuna have been investigated by recording blood pressures and flows in the central circulation of both anaesthetised and swimming individuals. In anaesthetised fish (N=5), heart rate averaged 112+/−21 beats min-1 (mean +/− s.e.) and stroke volume was 0.67+/−0.24 ml kg-1 when normoxic water flowed over the gills. Ventricular diastolic pressure was zero until atrial contraction filled the ventricle. Ventral aortic pressures were high (mean 12.08+/−1.15 kPa), and blood flow was continuous in the ventral aorta throughout diastole. Dorsal aortic pressure (mean 6.3+/−1.28 kPa; N=4) and flow were both pulsatile. Pressure pulsatility (pulse pressure as a proportion of mean pressure) was about one-quarter of flow pulsatility, indicating considerable compliance in the dorsal aortic circulation. Total peripheral resistance averaged 0.17+/−0.4 kPa ml-1 kg-1 min-1 of which gill resistance averaged 48+/−15 % (N=4). For the ventral aorta, impedance modulus fell markedly from the mean value and then declined more gradually towards zero with increasing harmonic frequencies. Impedance phase was negative (−0.8 to −1.1 rad) meaning that flow leads pressure at all harmonics. In swimming yellowfin tuna (N=5), heart rate averaged 108.8+/−12.1 beats min-1 and mean ventral and dorsal aortic pressures were 11.6+/−0.5 and 6.8+/−1.2 kPa, respectively, so gill resistance was 42 % of total peripheral resistance. Average stroke volume in three swimming kawakawa was 0.54+/−0.2 ml kg- 1 at a mean heart rate of 128+/−48 beats min-1. Data from swimming fish were within the range obtained from anaesthetised tuna. A simple model of the fish circulation consisting of two sets of compliant and resistive elements coupled in series (a second-order RC network) gave reasonable predictions of arterial pressure-flow relationships. Hence, we conclude that a ‘Windkessel’ dominates central cardiovascular dynamics of tuna despite heart rates and blood pressures that fall in the mammalian range.

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