The lungs are the most important respiratory organs (see Image. Relationship Of Thoracic Contents And Thoracic Cage Linings). The bony thorax and pleurae protect this organ pair. The lung apices project superiorly through the supraclavicular fossae and their inferior margins slope from the 6th rib at the midclavicular line to the 8th rib at the midaxillary line and the 10th rib at the scapular line. Each lung is divided into lobes and has the alveolus as the basic gas exchange structural unit. Alveolar tissue comprises the lung parenchyma.  The respiratory airways deliver oxygen to the lungs and tissue carbon dioxide to the environment. The tracheobronchial tree is the branching system of the respiratory airways (see Image. Tracheobronchial Tree, Anterior View). The trachea bifurcates into the right and left main bronchi—one for each lung—at the level of the sternal angle. The shorter and wider right main bronchus runs more vertically than the left. Each main bronchus divides into lobar bronchi, which supply the lung lobes. Each lobar bronchus gives rise to the segmental bronchi, which supply the bronchopulmonary segments. The bronchopulmonary segments divide into terminal bronchioles, which branch into respiratory bronchioles. Respiratory bronchioles give rise to both alveolar ducts and alveoli. Alveoli may extend from the respiratory bronchioles or alveolar ducts. Alveolar sacs are common spaces between alveolar clusters. The lungs are supplied by the right and left pulmonary arteries, which originate from the pulmonary trunk at the sternal angle. Pulmonary arteries branch into lobar arteries, then segmental arteries. These arteries carry deoxygenated blood and carbon dioxide to the lungs. The blood-air barrier—the site of gas exchange—is composed of the alveolar epithelium and the interfacing pulmonary capillary endothelium. Pulmonary veins receive oxygenated blood from the bronchopulmonary segments on their way to the hilum before returning to the heart. Distal visceral pleural and bronchial veins also drain into the pulmonary veins but carry deoxygenated blood.  Bronchial arteries supply oxygenated blood to the root of the lungs (bronchi) and visceral pleurae. The bronchial veins drain the proximal capillaries branching out from the bronchial arteries. Lung lymphatic vessels drain into the tracheobronchial lymph nodes.  A pulmonary contusion is a lung parenchymal injury that does not lead to lung or pulmonary vascular lacerations. Blunt chest trauma, shock waves from penetrating chest injuries, and explosions can cause this condition. Pulmonary contusions were first described in 1761 by Morgagni, an Italian anatomist. The term "pulmonary contusion" was coined in the 19th century by French military surgeon Dupuytren. Widespread explosive use during World Wars I and II led to increased recognition of pulmonary contusion due to blast injuries. Many soldiers with blast injuries developed pulmonary bleeding without apparent external injuries.