You may wish to use a transparency of the relay to help in explaining. Divide the students into teams. Explain to students that the red balloons will represent oxygenated blood cells. Meanwhile, the blue balloons will represent carbon dioxide loaded blood cells that have given away their oxygen and are now carrying away the cells' waste. Demonstrate the path with one student.
Students begin in the Left Ventricle as an oxygenated blood cell.
They travel through the Aorta. After passing through the aorta students carry their oxygenated blood to the muscles. From the muscles, students carry carbon dioxide loaded blood to the Right Atrium. From the Right Atrium students travel into the Right Ventricle. Students travel through the Pulmonary Artery.