Leeches have an open circulatory system, where blood flows at minimal velocity and pressure due to an absence of smooth muscles to push the blood around. Th direct contact between the leech's body cells and blood allows them to exchange nutrients with each other. For an in depth description of how an open cardiovascular system works, please refer back to the Red Scorpion, Maine Lobster, and House Centipede tabs of the Arthropoda phylum.