Seal flipper heat exchange. Computer illustration showing the anatomy of the blood vessels that form the countercurrent heat exchange mechanism in a seal's flipper. This system is used to warm cold blood (blue) from the flipper before it returns back into the main body of the animal. The vessels carrying cold blood wrap around (lower centre-right) and merge with (upper right), those carrying warmer blood (red). The heat in the warm vessels warms that in the cold vessels through conduction.