On a typical shark, five gill slits are located on the side of its head (right); however, some sharks, such as the broadnose sevengill shark and bluntnose sixgill shark on Canada's Pacific coast, have more than five gills. During breathing, water enters through the mouth and goes into the pharynx where the gills are located. Here, oxygen is extracted from the water and carbon dioxide is released. In some more bottom-dwelling sharks, the spiracle, located behind the eye, draws in additional oxygenated water.
Many sharks must continually swim to stay alive - by moving through the water they pass oxygenated water across their gills, a process known as ram-ventilation. Some species, such as tiger, nurse, lemon and wobbegong sharks, can actively pup water over their gills by inhaling water in through their mouth or spiracle – known as buccal pumping. Interestingly, sharks that possess this ability often have a better chance of survival when they become caught or entangled in fishing gear.
Morphology and Buoyancy
Body shape of individual species is influenced by habitat, predator/prey relationships and its general way of life. Most sharks have a streamlined body, a long, flattened snout, a ventral mouth, and an asymmetric caudal fin with the upper lobe often being larger than the lower lobe. In general, there are two extremes, "pelagic" or those built for speed (e.g. shortfin mako shark) and "benthic" or seabed dwellers (e.g. greenland shark); however, there are many other shapes of sharks that exist. Sharks that live near the bottom, like the spiny dogfish, have a body that is more snakelike, in order to wriggle through marine vegetation.
The fins of a shark, which usually number eight, are instrumental in swimming. The caudal fin (tail), made up of the upper and lower lobes, is used for propulsion. While the upper lobe drives the shark down, the flattened head and horizontal pectoral fins help to balance the shark in motion by generating a slight lift while swimming. In some benthic sharks the lower lobe may be absent, while in other sharks, such as the shortfin mako or salmon shark, the upper and lower lobes are of equal size, allowing the shark to achieve bursts of speed.
The shark's skin is also designed to improve hydrodynamics. Their skin is composed of "dermal denticles", very small modified teeth, that give the shark its characteristic sandpaper skin texture and help reduce friction during swimming. The shape of dermal denticles, similar to shark teeth, varies from species to species.
Due to the shark's light cartilaginous skeleton and huge oily liver, they are only slightly heavier than seawater. Density ultimately varies between species, as it is related to their habitats, but in general, pelagic species are less dense than benthic species.
Sharks have a simple circulatory system composed of an auricle and a ventricle. Blood flows from the ventricle to the ventral aorta. From there blood moves into the branchial arteries and the capillaries located in the gills. As blood becomes oxygenated, it moves onto the dorsal aorta and continues through the rest of the body by means of smaller arteries. Once blood and nutrients have been delivered to the organs, blood enters into the venous system and returns to the heart through the cardinal veins.
While most sharks have body temperatures equal to that of the surrounding seawater, some endothermic species, including the porbeagle, shortfin mako, common thresher and white shark, maintain a higher body temperature due to a heat-retaining system. These sharks have large amounts of red muscle tissue (tissue most powerful for swimming), which is connected to the circulatory system through a complicated network of arteries and veins called the rete mirabile. Heat generated in the red muscles by swimming passes into the rete mirabile where heat is transferred into parallel arteries and conserved in the body. These warm-blooded sharks have more energy at their disposal and thus are more powerful and faster than the average shark.