Figure 39.11 “Cold” and “Hot” Fishes (A) Blood flowing through fish gills comes into temperature equilibrium with water temperature. In most fish species, this oxygenated but cool blood flows through a large dorsal aorta to the rest of the body. Thus it is an internal cooling source. (B) The anatomy of “hot” fish species circulates most of the cool, oxygenated blood to the body through large peripheral arteries rather than through a central aorta. As this cool arterial blood flows into the swimming muscle, it is warmed by heat being carried out of the swimming muscle by the veins. Thus, the heat generated by the swimming muscles is conserved in the muscle mass. (C) The close, parallel arrangement of the arteries carrying blood into the muscles and the veins carrying blood out of the muscle makes efficient countercurrent heat exchange possible.