Ventricular and Arterial Dynamics of Anaesthetised and Swimming Tuna

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 1 1 2 S l beats min-l (mean 2 s.E.) and stroke volume was 0.67M.24mlkg-' 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 . I5 Wa), and blood flow was continuous in the ventral aorta throughout diastole. Dorsal aortic pressure (mean 6.3k1.28 P a ; 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 0.17M.4 P a ml-' kg-' min-] of which gill resistance averaged 48k15 % (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 .I rad) meaning that flow leads pressure at all harmonics. In swimming yellowfin tuna (N=5), heart rate averaged 108.8+12.1 beatsmin-' and mean ventral and dorsal aortic pressures were 11.6iO.5 and 6.8*1.2kPa, respectively, so gill resistance was 42% of total peripheral resistance. Average stroke volume in three swimming kawakawa was 0.54k0.2 ml kg-l at a mean heart rate of 128k48 beats min-I. 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.