The fish and human circulation - a short comparision
 

Before we go further into fish circulation, let's compare a human with a fish to get a perspective on the anatomy and function.

In mammals (like us) there is a double circulation. The right side of the heart receives blood returning back from the body; this blood is low in oxygen (blue in the figure to the right). This “deoxygenated” blood enters the right atrium and then the right ventricle to be pumped to the lungs were the blood will be oxygenated. The oxygenated blood (red in the figure) from the lungs enters the left ventricle via the left atrium and is then pumped out into the larger body circulation. Note that the volume pumped by the right and left side of the heart is the same:  approximately 5 liters per minute in a 70 kg human.

In fish, the heart only has one atrium and one ventricle. The oxygen-depleted blood that returns from the body enters the atrium, and then the ventricle, and is then pumped out to the gills where the blood is oxygenated, and then it continues through the rest of the body.

On the next page, you will see that the arrangement of the circulation in the fish is based on a fine balance.