Hot-blooded Sharks

Every summer as the month of August draws near, I get excited. One of my favorite times of year will soon arrive—Shark Week! The success of this popular television series is not surprising. From their special drag-reducing scales to their sophisticated equipment to detect other fish’s electrical fields, shark designs astound and inspire scientists, engineers, and the public at large.1

One feature of special note is the elegant thermal regulation system of certain Lamnoid sharks, which enables them to maintain core body temperatures ranging from 14°–38°F (8–21°C) above the surrounding water temperature. Most other fish are cold-blooded, meaning their body temperature matches the surrounding water temperature.

Some of the world’s most famous sharks, such as the common thresher, the shortfin mako, and the dreaded great white, need this elevated blood temperature to sustain their incredible speeds and spectacular maneuvers. The red muscles that give these sharks their strength and endurance require warm blood to function properly.

The water can be very cold as it flows through the gills, where oxygen is transferred to the shark’s blood. To keep this cold from reaching the blood that flows to the red muscles, these special sharks possess a mesh of blood vessels referred to as a rete mirabile.

These wonderful “nets” allow blood from the gills to flow past the blood coming from the muscles. As the two systems flow past each other, heat is transferred from the warmer internal blood to the colder outer blood.

This heat recycling system allows the internal blood to stay warm by the friction of constantly moving red muscle tissue.

Retia mirabilia are not unique to sharks and can be found in camels, giraffes, goats, and birds. The retia’s function in each of these animals is as diverse as the animals. Giraffes can bend over without the sudden increase in blood pressure causing brain damage, goats can monitor outside air humidity, and birds are better able to send oxygen to their brains.2

The hallmark of any great designer is the ability to successfully use one basic design for multiple purposes. How amazing that our God is able to use a simple network of blood vessels in such extraordinary and diverse ways.