<P> Like other fish, sharks extract oxygen from seawater as it passes over their gills . Unlike other fish, shark gill slits are not covered, but lie in a row behind the head . A modified slit called a spiracle lies just behind the eye, which assists the shark with taking in water during respiration and plays a major role in bottom--dwelling sharks . Spiracles are reduced or missing in active pelagic sharks . While the shark is moving, water passes through the mouth and over the gills in a process known as "ram ventilation". While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water . A small number of species have lost the ability to pump water through their gills and must swim without rest . These species are obligate ram ventilators and would presumably asphyxiate if unable to move . Obligate ram ventilation is also true of some pelagic bony fish species . </P> <P> The respiration and circulation process begins when deoxygenated blood travels to the shark's two - chambered heart . Here the shark pumps blood to its gills via the ventral aorta artery where it branches into afferent brachial arteries . Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries, which come together to form the dorsal aorta . The blood flows from the dorsal aorta throughout the body . The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses . From there blood enters the heart ventricle and the cycle repeats . </P> <P> Sharks and rays typically have five pairs of gill slits that open directly to the outside of the body, though some more primitive sharks have six or seven pairs . Adjacent slits are separated by a cartilaginous gill arch from which projects a long sheet - like septum, partly supported by a further piece of cartilage called the gill ray . The individual lamellae of the gills lie on either side of the septum . The base of the arch may also support gill rakers, small projecting elements that help to filter food from the water . </P> <P> A smaller opening, the spiracle, lies in the back of the first gill slit . This bears a small pseudobranch that resembles a gill in structure, but only receives blood already oxygenated by the true gills . The spiracle is thought to be homologous to the ear opening in higher vertebrates . </P>

Where do seawater fishes get their supply of oxygen