Lungs — MRCS Part B Anatomy

Two lung specimens, each to be sided and read: the lobes and fissures, the structures at the hilum, and the impressions on the mediastinal surface, which are the surest way to tell one lung from the other. Then the pulmonary arteries.

Telling right from left

The right lung has three lobes and two fissures, the oblique and the horizontal, and is larger, shorter and broader. The left has two lobes and one fissure, a lingula projecting below its cardiac notch, and is longer and narrower. Hold the specimen with the hilum towards you and the apex up: the sharp anterior border is the side with the cardiac notch on a left lung, and the rounded posterior border faces the spine. On either lung the bronchus lies at the back of the hilum, the pulmonary artery above and in front, and the pulmonary veins lowest and most anterior; on the right, the bronchus to the upper lobe leaves above the artery.

Impressions on the right lung

The heart makes a shallow hollow in front of the hilum. Above it runs the groove of the superior vena cava and the right brachiocephalic vein, and the azygos vein arches over the hilum from behind to join the cava. Behind the hilum lie the grooves for the oesophagus and, near the apex, the trachea and right subclavian artery. The first rib grooves the apex.

Impressions on the left lung

The heart makes a deep hollow, the cardiac impression, with the cardiac notch cut into the anterior border above the lingula. The arch of the aorta grooves the lung above the hilum and the descending aorta runs down behind it; near the apex are the grooves for the left subclavian artery and the left brachiocephalic vein, and the oesophagus touches the lower part behind the pulmonary ligament.

The root and the hilum

The structures that enter and leave the lung together at the hilum, wrapped in a sleeve of pleura, are collectively the root of the lung: the main bronchus, the pulmonary artery, the two pulmonary veins, the bronchial arteries and veins, the pulmonary plexus of nerves, and the bronchopulmonary lymph nodes. The pleural sleeve hangs below the root as the pulmonary ligament, which gives the root room to move as the lung expands. The phrenic nerve passes in front of the root and the vagus behind.

The pulmonary arteries

The pulmonary trunk divides under the arch of the aorta at the level of the sternal angle. The right pulmonary artery is the longer: it runs horizontally to the right behind the ascending aorta and the superior vena cava and in front of the right main bronchus and the oesophagus, then divides at the hilum into a branch to the upper lobe, which passes in front of the eparterial bronchus, and a larger branch to the middle and lower lobes. The left pulmonary artery is shorter and higher: it arches over the left main bronchus, tied to the aortic arch by the ligamentum arteriosum with the left recurrent laryngeal nerve hooking beneath, and divides at the hilum into upper and lower lobe branches. Each artery then follows the bronchial tree, branch for branch, down to the bronchopulmonary segments, which is why a segment can be removed with its own artery and bronchus.

Inside the lung

Each main bronchus divides into lobar bronchi, then into segmental bronchi, one to each of the ten bronchopulmonary segments, then through smaller and smaller branches to the terminal bronchioles, beyond which the respiratory bronchioles, alveolar ducts and alveoli make up the acinus where gas exchange happens. The pulmonary arteries carry deoxygenated blood alongside the bronchi to the alveolar capillaries, and the pulmonary veins run separately between the segments, carrying oxygenated blood back to the left atrium. The lung itself is fed by the bronchial arteries from the aorta.