In animals such as fish, blood flows through the heart once during each circuit of the body. Fish are said to have a single circulation(Figure 8.22). From the heart, arteries carry the blood to the gills where the blood takes up oxygen. The arteries that leave the gills carry oxygen-rich blood to various parts of the body. The veins collect blood from the body cells. By the time the blood enters the veins, it is poor in oxygen content. The veins carry the deoxygenated blood back to the heart.