This document summarizes the key functions and components of circulatory systems across different organisms. It describes how circulation occurs through diffusion in simple organisms like hydra, while more complex invertebrates use open circulatory systems with hearts and hemolymph. Vertebrates have closed circulatory systems with distinct pulmonary and systemic circuits supported by four-chambered hearts. The circulatory system transports nutrients, gases, wastes, immune cells, hormones and more to maintain homeostasis throughout the body.