Gills & Respiration
Gills allow fish to take in oxygen from the surrounding water and eliminate carbon dioxide from the blood. Sharks can have up to seven external gill openings, but most species have five.
Gill arches are considered part of the skeleton; they hold the gills in place. The arches support one or two rows of gill filaments. The filaments are designed so that water flows in one direction alongside them, while inside the filaments blood flows in the opposite direction. This countercurrent system is the most efficient method of exchanging oxygen and carbon dioxide between the water and blood.
Bony fish generally have four gill arches on each side, covered and protected by a single external bony plate. Sharks do not have a protective bony covering over their gill slits, which leaves gills more vulnerable to injury.
Many sharks, especially bottom-dwelling species, have paired openings called spiracles located between the eye and the gill slits. Spiracles are used to take in water and ventilate the gills, even while the shark may be feeding or at rest on the bottom.
Some sharks must maintain a certain swimming speed in order to ventilate their gills with water taken in through the mouth. But others have specialized muscles in their pharynx (throat) which they use to pump water over their gills even when they’re not moving.