syringe with a one-way valve to represent the heart, some clear plastic

tubing to represent blood vessels, some colored water to represent blood

and a t-tube connector. Connect the plastic tubing to two ends of the

t-tube, make sure the tubing is filled with colored water then connect the

syringe to the other end of the t-tube. When you squeeze the syringe

(heart) you will be able to observe the colored water (blood) circulating

through the plastic tubing (blood vessels).

more complex use 4 one way valves and 4bulb syringe tops connected to tubing top 2 pulling liquid in through tube inserted in bulb directly with 1 way valve on other end when you squeeze flows into other bulbs release and liquid flows from tube. bottom chamber same thing in only comes from other chamber and into tubes. supplies might be hard to find but are made. could use baster tops as well. good luck