The vessels in more sophisticated animals may have valves , which are flaps of endothelium shaped like a hammock with one edge attached to the vessel wall. Two of these flaps on opposing walls catch the blood and prevent its return. Most invertebrates have open circulatory systems. In this systems hearts pump blood into arteries which distribute blood to different parts of the body. Then the vessels open up to the interstitial space and the blood becomes lymph.
Open systems have lower pressures and less of an ability to distribute blood to needy areas.
Closed Circulatroy Systems Cephalopods, large annelids and vertebrates have closed circulatory systems. In these systems blood is always contained in blood vessels. Pressures are higher in these systems, circulation is faster, blood volume less and control of blood flow more precise. Since fluid is lost to the interstitial system, there must be a means of getting fluid back into the circulatory system. Electrical Activity of the Heart Cardiac muscle is composed of two cell types.