Fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle (figure a). The atrium collects blood that has returned from the body, while the ventricle pumps the blood to the gills where gas exchange occurs and the blood is re-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. This unidirectional flow of blood produces a gradient of oxygenated to deoxygenated blood around the fish's systemic circuit. The result is a limit in the amount of oxygen that can reach some of the organs and tissues of the body, reducing the overall metabolic capacity of fish.
Source: Boundless. “Types of Circulatory Systems in Animals.” Boundless Biology. Boundless, 21 Jul. 2015. Retrieved 12 Dec. 2015 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/the-circulatory-system-40/overview-of-the-circulatory-system-224/types-of-circulatory-systems-in-animals-845-12090/