The Shark Inside

The inside of the shark is designed for maximum efficiency, just like its outside.

Sharks have large J-shaped stomachs that can expand considerably. When prey is captured, it is usually swallowed whole or in large pieces. The stomach produces an acid that is strong enough to dissolve metal. Large bones and other indigestible objects are prevented from going past the stomach due to the small size of the opening to the intestine, but can be regurgitated through the mouth.

To rid their stomachs of indigestible material, some sharks can force their stomachs inside-out through their mouths, wash it in sea water, then pull it back to its normal location.

Shark intestines are short and compact. The surface area of the intestine is increased by internal valves, or coils, that can take any of several forms. The increased surface area slows food passage through the intestine, and speeds up the rate at which it can be digested and absorbed into the blood.

Shark livers are huge, consisting of two large lobes surrounding the digestive tract. In some sharks the liver comprises up to 30% of their body weight. The liver stores carbohydrates and fats, releases sugars for energy when needed, and contains oils that assist with buoyancy.

In marine fishes, the salt concentration of body tissue fluids is less than half that of the surrounding water, so there would normally be a tendency for water to leave the shark’s body across the gills by osmosis. But sharks compensate for this by retaining urea, a waste product of food produced by the liver. By maintaining elevated blood levels of urea, along with sodium and chloride ions and trimethylamine oxide (TMAO), sharks avoid dehydration in a saltwater environment.

Like other fish, sharks often have two types of muscle tissue, red and white (somewhat like dark and light meat on a turkey). Red muscle contains a high concentration of myoglobin, and can store a lot of oxygen. It is used for sustained swimming over long distances.

In some sharks, such as whites and makos (and in some bony fishes), the heat generated by red muscle is returned to it by networks of blood vessels. This raises the internal body temperature, and increases the efficiency of red muscle tissue. White muscle makes up the majority of muscle mass in sharks, but it doesn’t store much oxygen and can’t sustain swimming for long periods of time.