BLOOD-VOLUME, PLASMA-VOLUME AND CIRCULATION TIME IN A HIGH-ENERGY-DEMAND TELEOST, THE YELLOWFIN TUNA (THUNNUS ALBACARES)

We measured red cell space with Cr-51-labeled red blood cells, and dex tran space with 500kDa fluorescein-isothiocyanate-labeled dextran (FIT C-dextran), in two groups of yellowfin tuna (Thunnus albacares). Red c ell space was 13.8 +/- 0.7 ml kg(-1) (mean +/- S.E.M.) Assuming a whol e-body hematocrit equal to the hematocrit measured at the ventral aort ic sampling site and no significant sequestering of Cr-51-labeled red blood cells by the spleen, blood volume was 46.7 +/- 2.2 ml kg(-1). Th is is within the range reported for most other teleosts (30-70 ml kg(- 1)), but well below that previously reported for albacore (Thunnus ala lunga, 82-197 ml kg(-1)). Plasma volume within the primary circulatory system (calculated from the Cr-51-labeled red blood cell data) was 32 .9 +/- 2.3 ml kg(-1). Dextran space was 37.0 +/- 3.7 ml kg(-1). Becaus e 500 kDa FITC-dextran appeared to remain within the vascular space, t hese data imply that the volume of the secondary circulatory system of yellowfin tuna is small, and its exact volume is not measurable by ou r methods, Although blood volume is not exceptional, circulation time (blood volume/cardiac output) is clearly shorter in yellowfin tuna tha n in other active teleosts, In a 1kg yellowfin tuna, circulation time is approximately 0.4 min (47 ml kg(-1)/115 ml min(-1) kg(-1)) compared with 1.3 min (46 ml kg(-1)/35 ml min(-1) kg(-1)) in yellowtail (Serio la quinqueradiata) and 1.9 min (35 ml kg(-1)/18 ml min(-1) kg(-1)) in rainbow trout (Oncorhynchus mykiss). In air-breathing vertebrates, hig h metabolic rates are necessarily correlated with short circulation ti mes. Our data are the first to imply that a similar relationship occur s in fishes.