The body is a system of interacting subsystems, each made of groups of cells. The lungs have two main jobs: to get oxygen from the air into the body and to remove carbon dioxide, a gas the body produces when it uses oxygen. The circulatory system, a network of vessels and a pump, carries oxygen from the lungs to every tissue and brings carbon dioxide back. Muscle cells use oxygen to release energy from sugar for work such as contracting. When muscles work harder, the heart beats faster and breathing speeds up together, because the whole system is meeting one demand.