circulatory system: 

fish

In the fish circulatory system the ventricle pumps deoxygenated blood into the blood capillaries in the gills, where a gaseous exchange occurs. Then the oxygenated blood leaves the gills and flows through the body tissues. This is called systemic circulation. In the systemic capillaries oxygen diffuses in the tissue and carbon dioxide diffuses out of the tissues. Then the deoxygenated blood then flows back to the heart. The bad things are the blood has to flow through 2 network capillaries (gills and systemic) causing a reduction in the flow of blood and the diameter of the capillaries are small causing a high resistance to the blood flow.