Some fishes conserve metabolic heat

The muscles of active fishes produce substantial amounts of metabolic heat, but most fishes cannot retain that heat. Blood pumped from the fish heart goes directly to the gills, where it comes very close to the surrounding water to exchange oxygen and carbon dioxide. Any heat the blood picks up from metabolically active muscles is lost to the water flowing across its gills. It is therefore surprising that some large, rapidly swimming fishes, such as giant bluefin tuna (see Figure 1.15A) and great white sharks, can maintain swimming muscle temperatures 10°C–15°C higher than the surrounding water. The heat comes from their powerful swimming muscles, and the ability of these “hot” fishes to conserve that heat is based on the remarkable arrangement of their blood vessels.

In the typical (“cold”) fish circulatory system, oxygenated blood from the gills collects in a large blood vessel, the aorta, that runs through the center of the fish, distributing blood to all organs and muscles (Figure 39.11A). “Hot” fishes have a small aorta, and most of their oxygenated blood is transported in large side vessels just under the skin (Figure 39.11B). The cold blood leaving the gills is therefore kept close to the surface of the "hot" fishes as it flows posteriorly to the swimming muscles. The cold blood flows into the muscle mass through small vessels branching off the large side vessels. These many small vessels run parallel to the vessels carrying warm blood from the swimming muscles back toward the heart. Because the vessels carrying the cold blood into the muscles are in close contact with the vessels carrying warm blood out of the muscles, heat flows from the warm to the cold blood by conduction. Therefore the heat produced by the active muscles is retained in the muscle mass.

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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.

Because heat is exchanged between blood vessels carrying blood in opposite directions, this adaptation is called countercurrent heat exchange (Figure 39.11C). By keeping heat within the muscles, these fishes can maintain an internal body temperature considerably higher than the water temperature. Each 10°C rise in muscle temperature increases the fish’s power output almost threefold, giving it a faster and more powerful swimming capability.