Anterior view of the lungs and heart. The cone-shaped lungs occupy most of the thoracic cavity. Each lung is suspended in its own pleural cavity and connected to the mediastinum, which houses the heart, by its root, which is made up of vascular and bronchial attachments. The anterior, lateral and posterior surfaces of the lung are in close contact with the ribs and form a continuously curving surface called the costal surface. De-oxygenated red blood cells are sent by the right side of the heart through the pulmonary artery into the vessels of the lungs to be refilled with oxygen for their next circuit through the body. The blood is carried through the lung tissues, where it exchanges its carbon dioxide for oxygen in the alveoli. It is then returned through the pulmonary veins to the left side of the heart and sent out to the rest of the body.