Vertebrates Rete Mirabile Hemodynamics Starling's Law of the Capillary Regulation of Blood Flow Through the Capillaries Application Discussion Reflection Lecture General Plans for Circulation Many animals, like earthworms, move blood by peristaltically contracting blood vessels. Peristalsis is a contraction preceded by relaxation so that the internal fluid moves along in one direction. 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.