In a countercurrent system, you have two tubes side by side with their flows going in opposite directions. An example is the fin of a whale. Warm blood from the body cools as it goes toward the tip. At the base of the fin, the bllod is at its warmest but the returning blood is cooler, so the out-flowing blood can still give up heat to the incoming blood. At the tip of the fin, the outgoing blood is coolest but still has more heat than the returning blood, so it can give up a bit of heat to the returning blood. The same principle is used with oxygen and carbon dioxide in fishes' gills and with heat in cooling wort (beer before it ferments).
C The flow of warmth arterial blood previous cooler venous blood enables rather some the warmth of the arterial blood (that's warmer, having been pumped from the middle and lungs in the middle of the physique, the place that's warmer) to be transferred to the blood in the veins, extremely than our surroundings.
