Single circulatory system. In the circulatory system of a fish, the blood travels from the heart to the gills, where it absorbs oxygen and releases carbon dioxide. It then flows from the gills to the organs and tissues in the rest of the body, and back to the heart. There is just one circuit from the heart.

Advantages :

 The blood circulating in the body has already been oxygenated in the gills.

Disadvantages:

Due to lower pressure the rate of delivery of oxygen is slow. This lowers the metabolic rate. The nutrients are not properly distributed.