Short Communication Dependence of Blood Viscosity on Haematocrtt and Shear Rate in a Primitive Vertebrate

Hagfish have an unusual circulation with intravascular pressures that are lower than those in other vertebrates (Davie et al. 1987; Forster, 1989) and since peripheral resistances appear similar to those of teleost fishes (Rood, 1979; Elger, 1987), it follows that blood flow rates will also be lower. One might therefore predict that the relatively low haematocrits in hagfish (Johansen et al. 1962; Forster et al. 1989) and lack of plasma albumins (Manwell, 1963) would maintain a relatively low blood viscosity at low flow rates when blood tends to become more viscous by virtue of its non-Newtonian behaviour (McDonald, 1974; Fletcher & Haedrich, 1987). Viscosity measurements were made on the blood and plasma of the hagfish, Eptatretus cirrhatus (Forster), to determine whether blood viscosity was unusually low in a vertebrate with a circulation operating at low shear rates, and to see whether this primitive vertebrate conformed to optimal haematocrit theory (cf. Crowell & Smith, 1967). Hagfish were cannulated via the ventral aorta, and maintained in running sea water. Heparinized whole blood was drawn for measurements of viscosity, haematological parameters and total plasma protein (Biuret reaction). Packed erythrocytes were resuspended in autologous plasma to provide a series of haematocrits (Hct) from 0 to 40 %. The viscosity of 0-5 ml samples was measured at 20°C in a rotating cone-plate viscometer (model LVTD, Brookfield Engineering, Stoughton, MA) equipped with cone angle of 0-8° (Fig. 1). Haematological data are summarized in Table 1. The linear dimensions for hagfish erythrocytes were about twice those reported for teleost fishes (e.g. Hughes et al. 1982; Hughes & Kikuchi, 1984). A comparison of vertebrate erythrocytes with widely differing sizes led Chien et al. (1971) to the conclusion that erythrocyte volume was not an important determinant of blood viscosity. The mean ventral aortic Hct value reported here of 12-6% is low among vertebrates. Total plasma protein in the hagfish was about half that recorded for teleosts (Graham & Fletcher, 1985), no doubt because of the absence of albumin.