Thoracic Cavity Anatomy: Lung Lobes, Mediastinum, and Pleural Spaces Explained

The thoracic cavity is divided into two pleural cavities (housing the lungs) and the mediastinum (the central compartment containing the heart, great vessels, trachea, and esophagus). The right lung has three lobes separated by the oblique and horizontal fissures, while the left lung has two lobes separated by a single oblique fissure and features the cardiac notch and lingula. Understanding this layout is the foundation for reading chest imaging, localizing pathology, and performing procedures like chest tube insertion.

The right lung is larger and has three lobes: superior, middle, and inferior. The oblique fissure separates the inferior lobe from the upper two, running from roughly the T3 spinous process posteriorly to the sixth costochondral junction anteriorly. The horizontal fissure separates the superior and middle lobes, running along the fourth rib anteriorly. An easy way to remember: the horizontal fissure only exists on the right side. The left lung has just two lobes — superior and inferior — divided by its oblique fissure. The left superior lobe includes the lingula, a tongue-shaped projection that sits anterior to the cardiac notch and is the anatomic equivalent of the right middle lobe. This matters clinically because lingular pneumonia on a chest X-ray can mimic left middle lobe disease, and forgetting that the left lung has no middle lobe leads to embarrassing mislabeling on exams. Each lobe is further divided into bronchopulmonary segments — 10 on the right, 8-10 on the left depending on how you count variant anatomy. Each segment has its own segmental bronchus, artery, and venous drainage, which is why a surgeon can resect a single segment without destroying the rest of the lobe. Segment anatomy shows up on practicals as cross-sectional images where you need to name which segment a lesion sits in.

The mediastinum is the central thoracic compartment between the two pleural cavities. It extends from the thoracic inlet superiorly to the diaphragm inferiorly, and from the sternum to the vertebral bodies. The traditional teaching divides it into four compartments: superior, anterior, middle, and posterior. The superior mediastinum sits above an imaginary line from the sternal angle to the T4/T5 vertebral body. It contains the aortic arch and its three branches (brachiocephalic trunk, left common carotid, left subclavian), the SVC, the trachea, the esophagus, the thoracic duct, and the vagus and phrenic nerves. The classic exam question: what structures are found at the level of the sternal angle? The answer is the aortic arch, bifurcation of the trachea (carina), and the T4/T5 disc. The anterior mediastinum contains the thymus (largest in children, involutes with age), fat, and lymph nodes. The middle mediastinum houses the heart in its pericardial sac, the ascending aorta, the SVC, and the main bronchi. The posterior mediastinum contains the descending aorta, the esophagus (below the carina), the thoracic duct, the azygos venous system, and the sympathetic trunks. Here is a practical mnemonic for mediastinal masses by compartment: anterior gets the four Ts (Thymus, Teratoma, Terrible lymphoma, Thyroid goiter), middle gets lymphadenopathy and vascular lesions, posterior gets neurogenic tumors. This shows up constantly on board-style questions.

Each lung sits within a pleural cavity lined by two layers: the visceral pleura (stuck to the lung surface, cannot be separated) and the parietal pleura (lines the chest wall, diaphragm, and mediastinum). Between them is the pleural space, which normally contains just a few milliliters of serous fluid that reduces friction during breathing. The parietal pleura has named portions based on what it contacts: costal (chest wall), diaphragmatic (diaphragm), mediastinal (mediastinum), and cervical (extends above the first rib into the root of the neck — this is why a subclavian line insertion can cause a pneumothorax). The visceral pleura has no somatic innervation and cannot feel pain. The parietal pleura, especially the costal portion, is richly innervated by intercostal nerves — this is why pleurisy hurts so much and why you feel the chest tube going in. The costodiaphragmatic recess is the lowest point of the pleural cavity, where the costal and diaphragmatic pleura meet. The lung does not fully fill this space during quiet breathing, which means fluid collects here first. This recess extends to roughly the 10th rib in the midaxillary line and the 12th rib posteriorly. Thoracentesis targets this recess — you insert the needle just above a rib (to avoid the neurovascular bundle that runs along each rib's inferior border) in the midaxillary line, typically at the 8th or 9th intercostal space.

Chest tube placement uses the safe triangle: bordered by the anterior border of the latissimus dorsi, the lateral border of the pectoralis major, a horizontal line at the level of the nipple (5th intercostal space), and the base of the axilla. You go through the 4th or 5th intercostal space in the midaxillary line. The tube goes just above the rib to avoid the intercostal nerve, artery, and vein that run in the costal groove along the inferior margin of each rib. A tension pneumothorax shifts the mediastinum toward the opposite side because air under pressure in one pleural cavity pushes everything away. Needle decompression targets the 2nd intercostal space at the midclavicular line — you are entering the anterior chest where the chest wall is thinnest. After decompression, a formal chest tube goes into the safe triangle. Right-sided heart catheterization passes through the SVC into the right atrium, right ventricle, and then the pulmonary trunk. On imaging, the right heart border on a PA chest X-ray is formed by the SVC superiorly and the right atrium inferiorly. The left heart border is the aortic knob, the pulmonary trunk, the left atrial appendage, and the left ventricle from top to bottom. Being able to trace these borders is a frequently tested practical skill. AnatomyIQ has labeled cross-sectional images that let you drill mediastinal structure identification at every vertebral level.