The function of a circulatory system is to distribute nutrients, oxygen, and hormones within the fish body, and remove the metabolic waste. Fish have a closed circulatory system. A closed system is when the blood is closed within vessels of different size and wall thickness. No blood is in open body cavities.

Fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle. The atrium collects blood that has returned from the body. The ventricle then pumps the blood to the gills where it is oxygenated; this is called gill circulation. The blood then continues through the rest of the body, before arriving back at the atrium; this is called systemic circulation.