Guppies, like all fish, have gills. The heart of a guppy is very different from that of a mammal, for instance there are two compartments instead of four. They are called the auricle and ventricle. When the fish breaths the ventricle contracts and blood is forced out to the gills which act as lungs. This is where the gas exchange happens. The blood receives oxygen and releases carbon dioxide. The blood that now carries oxygen goes through the dorsal artery, which divides into smaller and smaller vessels and go all over the body to then distribute the oxygen and gets carbon dioxide and waste. Then, the blood is carried to the heart by veins and lymph vessels. Then the cycle repeats over and over.