Since the blood does not return to the heart after flowing through the gills, pressure is not regenerated within the system before it flows to the systemic circulation (the rest of the body). A single circulation would experience a loss of pressure after flowing through the gas change organs (gills of a fish) due to the high cross-sectional area presented by the capillaries of the gills. For ectotherms like fish, this is efficient enough to support their activity level. A fishes behavior would change in a few ways if it had an open circulatory system. However, the most common would be the fish would become extremely active.
