what is double circulation?

Answer 1

Double circulation means that the blood passes the heart twice during one circulation around the body. Humans have double circulation. Blood is first pumped to lungs from the heart but then it returns to it to be pumped around the body. On the other hand fish have single circulation. Blodd is pmped to gills then to the body and back. Double circulation is thought to be better than single because blood can be pumped to lungs at a lower pressure which makes gas exchange more efficient.