Drawing On Science
by Stephen Yaeger
We breathe air, but do fish breathe water? All animals use oxygen for breathing, but it’s how they get the oxygen that counts. Mammals, birds, reptiles, and amphibians (except for some salamanders) have lungs to capture the oxygen that they need for breathing. Fish, on the other hand, have gills to capture the oxygen that they need. The oxygen that we breathe is in the air that surrounds us. The oxygen that a fish breathes is in the water that surrounds it. But let’s not get confused at this point. The oxygen that we are talking about is not part of the water molecule; what we know as H2O. It is dissolved oxygen that we are referring to.
A fish’s gills are perfectly designed for getting this dissolved oxygen out of the water and into its blood. So before we go on let’s take a look at the structure of a fish’s gills. A gill is composed of many, very delicate filaments each of which contains two blood vessels. One blood vessel carries blood away from the heart. This is the vein. The other carries the blood to the body and then back to the heart. This is the artery (see diagram). Each filament is composed of many small disc-shaped structures called lamellae (leh-Mel-lay). Each lamella (singular) has capillaries, which carries blood from the vein to the artery (imagine a ladder where the rungs or steps connect the two sides). The filaments are always paired and are attached to a boney gill arch , which supports and holds the filaments in place. The gills are found in the gill chambers located on each side of the fish’s head. Because of the delicate nature of gills they are protected by a gill plate or operculum (oh-PER-ka-lum). We are all familiar with the way a fish opens and closes its mouth. But not everyone realizes that the gill plates are opening and closing in reverse. That is, when the fish opens its mouth the gill plates close and when the fish closes its mouth the gill plates open. There is a good reason for this. When the fish opens its mouth water is drawn in. The gill plates must be closed at this point. When the mouth is closed water is pushed out through the open gill plates; the mouth must now be closed. Between the water entering and leaving, gas exchange has occurred.
As the water is drawn into the gill chambers and over the gills, oxygen is diffused (=passes through the thin membrane of the gill filament and blood vessels) directly into the blood. At the same time carbon dioxide diffuses in the opposite direction: passing from the blood into the water. Since water contains much less oxygen than air the fish must have a way of getting as much of it as possible. If the water and the blood moved in the same direction, the fish would only get about half the oxygen in the water. It would be better if the water moved in the opposite direction than the blood. Remember that the vein is connected to the artery by capillaries in a filament’s lamellae. And the blood is moving from the vein to the artery. As water passes over the filament it is moving in the opposite direction of the blood flow in the capillaries. As a result the blood takes in the greatest amount of oxygen possible.
When a fish is removed from the water it will drown in air. This is because of two reasons. First the gill arches collapse so that there is not enough area for oxygen to diffuse into the blood. Second the gills dry out. Water is absolutely necessary for diffusion of gases. A small fish called the mudskipper can crawl on land because it is able to close its gill chambers so tight that water is still in the chambers. The lungfish have primitive lungs that also allow it to breathe in air. Walking catfish have special lamellae, which they use to breathe air when necessary.
Questions/comments? E-mail Steve: Drawingonscience@aol.com