Function and control of the fish secondary vascular system, a contrast to mammalian lymphatic systems

Teleost fishes and mammalian lineages diverged 400 million years ago, and environmental requirements (water versus air) have resulted in marked differences in cardiovascular function between fish and mammals. Suggestions that the fish secondary vascular system (SVS) could be used as a model for the mammalian lymphatic system should be taken with caution. Despite molecular markers indicating similar genetic origin, functions of the SVS in teleost fish are probably different from those of the mammalian lymphatic system. We determined that, in resting glass catfish (Kryptopterus bicirrhis), plasma moves from the primary vascular system (PVS) to the SVS through small connecting vessels less than 10 μm in diameter, smaller than the red blood cells (RBCs). During and following hypoxia or exercise, flow increases and RBCs enter the SVS, possibly via β-adrenoreceptor-mediated dilation of the connecting vessels. The volume of the SVS can be large and, as RBCs flow into the SVS, the haematocrit of the PVS falls by as much as 50% of the resting value. Possible functions of the SVS, including skin respiration, ionic and osmotic buffering, and reductions in heart work and RBC turnover, are discussed.