Today's electrical wizardry can monitor the faintest signals and analyze them in minute detail. Blood vessels are tubes of various diameters through which the blood is transported. The heart is a muscular pump that generates pressure to keep the blood flowing. Blood is returned to the heart at a leisurely rate.
Vertebrates have a closed system.
All the vessels and the heart are connected so that blood remains enclosed. The lymphatic system picks up excess fluids, solutes, and disease agents from the interstitial fluid. This lymph is cleansed by exposure to the infection-fighting cells before being returned to general circulation. It carries oxygen and nutrients to cells, and it carries secretions and wastes away from them. It contains phagocytic cells that fight infection.