This document discusses the components of blood and their functions, as well as the structure and role of the heart in circulation. It explains that red blood cells carry oxygen from the lungs to tissues, white blood cells fight infection, and platelets form clots to repair damaged blood vessels. The heart is divided into four chambers that blood passes through - from the right and left atria to the ventricles and back out again. The heart pumps blood continuously around the body via the circulatory system to deliver oxygen and nutrients to tissues and remove waste.