Structure and function of the fish cardiac ventricle: flexibility and limitations.

Fishes show the highest diversity among vertebrates. Defined differences in ventricular myoarchitecture exist in fish. There are two main types of cardiac ventricle in fish: a spongy type and a mixed type. In the 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 vascularized supply of blood. Interspecies differences exist in the proportion of compacta and the extent of vascularization. Here the mechanical limits and flexibility of the different types of ventricular organization are examined. The spongy type (found only in teleosts) seems to be particularly suitable for performing volume work. An example is the icefish heart. The main characteristics of this fish are the absence of hemoglobin in the blood and the very large volume of blood. The cardiac ventricle of the icefish is characterized by a cardiomegaly of the spongy type with myocardial pseudohypertrophy. It functions as a specialized volume pump which moves large stroke volumes at a low heart rate, but is not able to produce high pressures. The most active teleosts have mixed heart ventricles with different thicknesses of compacta. The presence of compacta gives these types of heart the potential to act 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.(ABSTRACT TRUNCATED AT 250 WORDS)