The document discusses various topics related to circulatory systems, including: 1) How invertebrates have either a gastrovascular cavity or circulatory system for internal transport, while vertebrates have more advanced cardiovascular systems adapted for their phylogeny. 2) The human cardiovascular system involves a four-chambered heart that uses double circulation to separate oxygenated and deoxygenated blood, and variations in vessel structure related to their different functions in transporting blood. 3) Substances are transferred between blood and tissues through the thin walls of capillaries via diffusion or bulk transport, and cardiovascular diseases are a leading cause of death.

16. Double Circulation in Mammals Depends on the Anatomy and Pumping Cycle of the Heart

23. Structural Differences of Blood Vessels Correlate with Their Different Functions