Anatomy I -- mediastinum and lungs

The parietal and visceral pleura meet at the root of the lung. If the pleural cavity is breached, the potential space becomes a real space as the lung collapses due to its intrinsic elastic recoil. Air (pneumothorax) or blood (hemothorax) can then enter this space. A needle can be inserted into the costodiaphragmatic recess to clear any fluid accumulated.

The mediastinum can be divided into superior and inferior divisions. The dividing line intersects the sternal angle (inferior edge of manubrium) and the T4/T5 junction. The inferior division is further subdivided into anterior, middle, and posterior subdivisions. The superior mediastinum contains the big tubes: trachea, esophagus, aorta. The trachea branches into the left and right bronchus at this level. It contains the thymus, which is atrophied in adults. It contains the phrenic nerves, which innervate the diaphragm, as well as the great vessels. The azygos vein and thoracic duct are found here as well. The anterior mediastinum is pretty boring, containing only lymph nodes, fat, connective tissue, and perhaps the inferior edge of the thymus. Heme-oncs look here for lymphomas (both HL and NHL) and other malignant masses. The middle mediastinum contains the heart, pericardium, great vessels, and phrenic nerves. The heart is not directly centered under the sternum but protrudes more to the left. The posterior mediastinum contains the descending aorta, which is quite posterior, behind the esophagus. It also contains the thoracic duct, azygos vein, splanchnic nerves, and vagus nerves.

The heart is quite analogous to the lung, as the pericardium has as its two inner layers the visceral and parietal serous layers. However there is now a third layer, the fibrous pericardium, which essentially has the parietal layer as its inner lining.

The heart is attached at its bottom surface to the diaphragm, so it moves up and down with inspiration and expiration.

The lung apex extends over the clavicle and first rib. The parietal pleura pretty much extends over the entire rib cage laterally and posteriorly, tapering up the xiphoid process anteriorly. The left lung is more circumscribed along its medial edge anteriorly due to the heart pressing into it. The lung extends past the xiphoid process in the front and down past the T10 vertebra in the front.

The left lung has two lobes; the right, three. The left lung has the cardiac, diaphragm, and aortic impressions. The right lung has the cardiac, diaphragm, esophageal, and tracheal impressions. The right main bronchus is steeper, wider, and shorter than the left; it is more likely to trap and lodge foreign objects. It branches into the right superior lobar bronchus and intermediate bronchus before entering the hilum, where the intermediate branches into the right middle lobar bronchus and right lower lobar bronchus. The left main bronchus branches into the left superior lobar bronchus and left inferior lobar bronchus. The lobar bronchi on both sides then branch into tertiary (segmental) bronchi. The ridge between the right main bronchus and left main bronchus is called the carina; it is distorted and immobile if the tracheobronchial lymph nodes are enlarged (i.e. bronchogenic carcinoma).