Just as a plumbing system carries water through a series of pipes to different parts of a house, the circulatory system carries blood through a series of blood vessels to different parts of the body. Blood is a type of connective tissue containing both dissolved substances and specialized cells. Blood collects oxygen from the lungs, nutrients from the digestive tract, and waste products from tissues. Blood helps to regulate factors in the body's internal environment, such as body temperature. In addition, components in blood help to fight infections. Blood can even form clots to repair damaged blood vessels.