Have children locate their hearts by placing their hands on their chests. If possible, allow each child to listen to his heartbeat through a stethoscope. Ask if anyone knows what the heart does.
Describe the heart. Make a fist and hold it up for all the students to see. Explain that the heart is a muscle about the size of your fist that pumps blood through the body. Open and close your fist to demonstrate the pumping of the heart. Have the students copy your motions.
Distribute a handful of Play-doh to each student, and instruct them to place it in one hand. When everyone has their Play-doh, show the students how to make the heart-pumping motion with their hand again, only this time with the Play-doh in their palm. When they squeeze their fingers into a fist, the Play-doh will squirt out between them. Explain that this is what happens with the heart and blood. When the heart pumps, it squeezes blood out into the body.
Hold up a straw for all the students to see. Explain that the heart has a lot of tubes (like straws) attached to it to carry the blood to all parts of the body. Veins carry blood to the heart, and arteries carry blood away from the heart. (Hint: students can remember this by remembering that arteries and away both begin with the letter "a.")
Distribute straws and cups of water to the students. Instruct them to suck lightly on the straws to demonstrate the function of veins. Demonstrate how to blow water from their mouths through the straws and into their cups, illustrating arteries.
Display the body chart, and explain that blood takes a trip through the body. Trace the blood's path through the body on the chart as you explain. The heart pumps the blood into the lungs so that it can be mixed with oxygen. The lungs send the blood back to the heart. Then, the heart sends the blood through the entire body. When blood is running low on oxygen, it travels back to the heart and starts the trip all over again.