The respiratory systems of fish must be adapted to the limited supply of oxygen available in the water and the high density of the medium. Because of the density of water and limited supply of oxygen, respiration requres large amounts of energy. The shark has evolved efficient structures to minimize energy consumption during respiration.

    The shark has evolved a way to move water steadily in one direction, instead of in and out of gill slits. During inspiration, lampreys must pump water into the external gill slits. Sharks, on the other hand, pump water through a spiracle. The spiracle evolved as a reduced branchial pouch. The branchial pouch of the lamprey lies between two gills. The spiracle is advantageous because this opening avoids inspired water over delicate gill surfaces. Also, when the spiracle opens for inspiration it reduces the pressure within the shark, allowing water to be sucked in without much loss of energy. At the same time that the spiracle is open, the parabranchial chambers close their external gill slits. This reduces pressure and allows the water to flow into the parabranchial chambers thus bathing the gills.

    As water passes through the gills in one direction, water is absorbed through lamella into the bloodstream. Sharks also have structures called secondary lamella. These secondary structures increase the surface area so that more oxygen can be absorbed into the blood stream.

    The shark obtains efficient gas exchange through counter current flow. In this system, blood and water flow in opposite directions. Well-oxygenated blood leaves the secondary lamella and flows past water that has not yet encountered the secondary lamella. The water contains more oxygen than the blood. This creates a gradient for absorption of more oxygen into the blood along the secondary lamella.

    The shark also conserves energy during expiration. The mouth and the spiracle both close causing the pressure to build in the shark. The increased pressure will cause the visceral skeleton to bend. This stored energy helps to force the septum, which is the flap of the external gill slits, open. Water then flows out of the body. Lampreys, on the other hand, have reduced septums that are not suited for this process.