The system acts as a counter-current heat exchanger and the heat from the blood in the veins is given up to the colder arterial blood rather than being lost at the gills. The net effect is less heat loss through the gills. Fish from warmer water elevate their temperature a few degrees whereas those from cold water may raise it as much as  warmer than the surrounding sea.
The system functions as a counter-current heat exchanger, transferring heat from the veins' blood to the cooler arterial blood instead of losing it at the gills. The overall result is reduced heat loss through the gills. Fish from warmer waters increase their temperature by a few degrees, while those from colder waters may elevate it even more than the surrounding sea water.