Ventricular and Arterial Dynamics of Anaesthetised and Swimming Tuna

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Abstract
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 beatsmin−1 (mean ± S.E.) and stroke volume was 0.67±0.24 mlkg−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±1.15kPa), and blood flow was continuous in the ventral aorta throughout diastole. Dorsal aortic pressure (mean 6.3±1.28kPa; 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 kPaml−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 −1.1rad) meaning that flow leads pressure at all harmonics. In swimming yellowfin tuna (N=5), heart rate averaged 108.8±12.1 beatsmin−1 and mean ventral and dorsal aortic pressures were 11.6±0.5 and 6.8±1.2kPa, respectively, so gill resistance was 42% of total peripheral resistance. Average stroke volume in three swimming kawakawa was 0.54±0.2mlkg−1 at a mean heart rate of 128±48 beatsmin−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.