The document discusses the circulatory system and blood circulation in humans. It defines circulation as the movement of body fluids like blood and lymph through the body. It describes the closed circulatory system in vertebrates which involves blood flowing within blood vessels and not exiting them. The major components of circulation are identified as the heart, blood vessels (arteries, veins, capillaries), and blood. In humans, the four-chambered heart pumps oxygenated blood to the body through the left side and deoxygenated blood to the lungs through the right side. The mechanisms of gas, nutrient, and waste exchange between blood and body tissues are also summarized.