The lungs are the primary organs of the respiratory system. The two lungs are located in the chest on either side of the heart. Their principal function is to transport oxygen from the atmosphere into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere. They also help in regulating the concentration of hydrogen ion (pH) in your blood. This exchange of gases is accomplished in the network of specialized cells that form millions of tiny, exceptionally thin-walled air sacs called alveoli. The lungs are made of spongy, elastic tissue that stretches and constricts during respiration. The airways that bring air into the lungs (the trachea and bronchi) are made of smooth muscle and cartilage, allowing the airways to constrict and expand.