Lesson 1
Thin tiles on the gills
Almost every fish, from a tiny goldfish to a huge tuna, breathes with gills. They sit under the flaps on each side of the head. Each gill has rows of thin threads, and each thread is covered with even thinner plates called lamellae (say "la-MEL-ee").
Pavement cells cover almost all of these plates. They are flat and fit together like paving stones on a path. Their tops have tiny ridges, a bit like a fingerprint.
They are a lot like the super-thin cells in the tiny air sacs of your lungs!
Lesson 2
Breathing water
A fish gulps water into its mouth and pushes it out over the gills. As the water flows past the plates, oxygen slips through the thin pavement cells into the blood just underneath. Carbon dioxide, a waste gas, goes the other way, out into the water.
Because pavement cells are so thin, the oxygen does not have far to go.
Try thisScroll up to the 3D model and press Breathe water, then Grab oxygen. Next time you see a fish in a tank, watch its mouth and gill flaps open and close as it breathes.
Wow!Blood inside the gill plates flows the opposite way from the water. This clever trick helps fish grab more of the oxygen in the water.