Image: Counter Current Heat Exchange image, Image by Windover14, Sourced Under a Creative Commons 4.0 License from Wiki Commons

1 - This is the outside of the ducks foot. 2a - Warm blood is flowing from the core of the body down the vein. 2b - The blood flowing up from the cold substance is receiving heat from the vein. 3 - The transfer of heat form the vein to the vessel.

One of the adaptations you will need to know more in-depth is counter-current heat exchange, a physiological adaptation which refers to the exchange of heat between two fluid flowing in opposite directions in vessels that are in close proximity.

An excellent example of this is penguins; their feet are constantly touching the cold, snowy ground, and so in order to survive, this adaptation means that their feet don't get as freezing cold!

Heat in the blood travelling in the arteries to the foot/fin warms the blood returning to the body in the adjacent veins. The outgoing blood to the extremities is cooled in the process, but not enough to slow cellular activities. As the temperature gradient is reduced, heat loss is minimised; a temperature gradient is produced when two objects in close proximity have differing temperatures, and the the difference in heat energy between the two objects causes heat to travel in one direction, from the hotter to cooler object.