THE CIRCULATORY SYSTEM
Besides transporting oxygen and nutrients that sustain body processes, the circulatory system transports waste materials produced by cells.

The thick-walled vessels that carry blood away from the heart are called arteries; those that carry blood toward the heart are called veins. Arteries form an extensive network in every organ. Within organs, they divide into smaller branches, arterioles, and eventually into microscopic capillaries. Within the capillaries (as well as directly from arterioles into the venous system), diffusion enables exchange of materials between the bloodstream and interstitial fluids. Blood eventually flows from capillaries into larger venules and into larger veins that return it to the heart. The heart initiates pumping force to propel blood throughout the body. Its four chambers include two upper, thin-walled atria that receive blood entering the heart, and two lower, thick-walled ventricles whose strong muscular action forces blood through the circulatory system.

The lymphatic system is a subsystem of the circulatory system. Its functions are to return excess tissue fluid to the blood and to defend the body against disease.

Unlike blood, which contains red blood cells and platelets, lymph is a watery, yellowish liquid transported from capillary networks within many organs to the heart through a network of vessels separate from the veins and arteries.