One simplistic way to divide up the animal kingdom is to split it into "warm-blooded" (capable of regulating their internal body temperature, i.e. mammals) and "cold-blooded" (reliant on external conditions to warm or cool the body, i.e. reptiles). However, like seemingly every categorical system ever devised on earth, black and white are interspersed with shades of grey.
Like oceanic tuna. In this group, bluefin tuna have been most successful in the art of keeping one's core muscles warm for high-speed swimming in cold ocean currents. So how does an animal with no true thermoregulatory mechanism achieve this? By the sleek and efficient workings of counter-current exhange.This relies on a simple principle of physics: that heat always travels from warmer places to colder places. You experience this every day in a thousand different ways. For example, get cold then climb into a hot bath. The heat from the water will travel into your body and the surrounding cool air. Your body gets warmer, but the water in the tub gets steadily cooler.
Bluefins use this to great advantage. All of their arteries are set up so they flow from the core of the body out towards the skin and extremeties while all the veins flow from the outside in. The two sets of blood vessels run in parallel and lie close to each other. As a result, as warm arterial blood (heated by muscle activity and food metabolism) passes cool venous blood, heat leaves the arterial blood and is transferred to the venous blood. Heat exchange is greatest when the difference in temperature between the two vessels is greatest, so when the arterial blood has 100% of its warmth and it is next to venous blood that has 0% of its warmth, a lot of heat will be transferred. It looks a lot like this:
So what you get is blood steadily losing warmth as it runs out of the core. Blood traveling back into the core is steadily gaining warmth. Net result: the warmth stays close to the core and is not lost to the surrounding, heat-sucking water.
It's a life-saver. Cool muscles are slow to respond to demands for movement so if you keep your muscles warm, you have a better chance of escaping things that want to eat you and you can swim farther and faster in search of food in a giant ocean. Birds use the same system to reduce heat loss though their long skinny legs and to keep oxygen levels in their lungs and flight muscles high at altitude.
A certain song keeps popping into my head: "AH-AH-AH-AH, stayin' alive, stayin' alive..."
