When we humans breathe, we breathe through our lungs. The air we pull in when we inhale through our nose and our mouth moves into our bronchial tubes, then into our avaoli sacks, and from there into our blood stream.

Once in the blood stream the oxygen travels through our bodies. At the same time we are breathing in the oxygen we are exhaling the carbon dioxide. The CO² leaves our bodies when we exhale.

Here the blood stream is pulling in the Oxygen and getting rid of the Carbon Dioxide.

Once in the blood stream the Oxygen travels through our bodies. At the same time we are breathing in the Oxygen we are exhaling the Carbon dioxide. The CO² leaves the body when we exhale.

Sharks have five to seven gill slits on each side of their heads, unlike bony fish which have one gill on each side. As water passes over their gills, oxygen is absorbed by the blood in the gills and transported from there to the rest of the body.

Some sharks have small openings called spiracles behind their eyes, at the top of the head. Spiracles are sort of like baby gill slits. The more active, fast swimming sharks seem to have outgrown the need for these spiracles, and only have very tiny ones, or don't have them at all.