An artery branches into a blood vessel of smaller diameter called a(n) , which conducts blood into a network of capillaries. 2. blood travels along the circuit when it passes from the heart to the lungs and then back to the heart. 3. invertebrates with an open circulatory system have a body cavity called a(n) , which contains a blood-like fluid that bathes body tissues directly. 4. a(n) , or white blood cell, is larger than a red blood cell but is able to fit through much narrower spaces. 5. a(n) is a red blood cell. its red color is produced by the iron-containing protein , which transports most of the blood's oxygen. 6. the heart's valves open when the ventricles contract, allowing blood to enter the pulmonary artery and the aorta. 7. when the heart's valves open, blood can flow from the atria into the ventricles.