STRUCTURE AND FUNCTION OF THE FISH CARDIAC VENTRICLE - FLEXIBILITY AND LIMITATIONS

Fishes show the highest diversity among vertebrates. Defined differenc es in ventricular myoarchitecture exist in fish. There are two main ty pes of cardiac ventricle in fish: a spongy type and a mixed type. In t he spongy ventricle, the muscle trabeculae form a sponge-like network the spongiosa. In the mixed ventricle, one or more superficial layers of compact tissue (compacta) enclose an inner spongiosa. The spongiosa and compacta are respectively associated with a lacunary and a vascul arized supply of blood. Interspecies differences exist in the proporti on of compacta and the extent of vascularization. Here the mechanical limits and flexibility of the different types of ventricular organizat ion are examined. The spongy type (found only in teleosts) seems to be particularly suitable for pel forming volume work. An example is the icefish heart. The main characteristics of this fish are the absence o f hemoglobin in the blood and the very large volume of blood. The card iac ventricle of the icefish is characterized by a cardiomegaly of the spongy type with myocardial pseudohypertrophy It functions as a speci alized volume pump which moves large stroke volumes at a low heart rat e, but is not able to produce high pressures. The most active teleosts have mixed heart ventricles with different thicknesses of compacta. T he presence of compacta gives these types of heart the potential to ac t as pressure pumps: they move small volumes at a relatively high rate and high pressure. The tuna heart is an extreme example of the mixed type. It has the highest relative mass and proportion of compacta (40- 70%) among fishes. Tt can beat at a high rate and produce up to 18 kPa of pressure. The performance of this heart seems to be highly depende nt on the adequate perfusion of the coronaries. Insights into the sign ificance of coronary perfusion in terms of ventricular mechanical beha vior can be given by the comparison between hearts in which the vascul arization is limited to the compacta (such as the trout heart) and hea rts in which the vascularization extends to the spongiosa (like the el asmobranch heart).