The document summarizes the circulation of blood in the human body. It describes how blood carries oxygen from the lungs to tissues and carbon dioxide back to the lungs, passing through the heart's four chambers - the right and left atria and ventricles. Blood is composed of red blood cells, white blood cells, plasma and platelets. There are different blood types which determine blood donation compatibility. The circulatory system includes arteries which carry oxygenated blood away from the heart, and veins which carry deoxygenated blood back to the heart.