This interactive model demonstrates a conventional scheme of blood circulation of fishes, reptiles and amphibia, mammals and birds. Arterial blood is marked by red colour while venous blood is shown by blue one. A selection of the animal kind may be done by a group of switches ("Fishes", "Amphibia, reptiles", "Birds, mammals"). It should be noticed that despite a similarity between various groups of animals inside individual class there is some minor variability in the texture of blood circulation inside an individual order. This variability is not shown for simplicity. To view a scheme of heart functioning bring the flag "Heart (zoom)" forward. The model is initiated by the "Start" button. The "Stop" button interrupts the animation, while the "Reset" button returns it into initial state.