Why the heart of fish is called venous heart?

The heart of a fish is not called a venous heart. It is referred to as a single-circulation heart because fish have a one-way circulatory system. This means that oxygen-poor blood from the body flows directly to the heart, and oxygen-rich blood from the heart is then pumped out to the body. In contrast, humans and other mammals have a double-circulation heart, where blood flows from the body to the heart, and then back to the heart before being pumped out to the body.