4.2 Lungs, Trachea, Bronchi & Pericardium

Trachea and Bronchial Tree

The trachea is a 10–12 cm fibrocartilaginous tube beginning at the cricoid cartilage (level of C6 vertebra) and descending to bifurcate at the carina (level of the sternal angle, T4/T5) into the right and left main (primary) bronchi. It is held open by 16–20 C-shaped cartilage rings closed posteriorly by the trachealis muscle (smooth muscle) and is lined with pseudostratified ciliated columnar epithelium. The posterior membranous wall abuts the esophagus.

The right main bronchus is shorter, wider, and more vertical than the left — this is why aspirated foreign bodies, inhaled objects, and endotracheal tubes advanced too far preferentially enter the right bronchus (PA-CAT Bulletin of Information, rev. 20240815, Table 3 lists "Lungs/Trachea/Bronchi" as a testable leaf). The left main bronchus is longer, narrower, and more horizontal as it arches under the aortic arch and crosses the esophagus anteriorly.

The bronchial tree divides: main → lobar (secondary) → segmental (tertiary) bronchi → subsegmental bronchi → bronchioles. The smallest conducting airways are terminal bronchioles; beyond them, respiratory bronchioles mark the start of the gas-exchange region and give rise to alveolar ducts, sacs, and alveoli.

Bronchopulmonary Segments

Each lung is divided into bronchopulmonary segments, each with its own segmental bronchus, segmental artery, and vein, surrounded by connective tissue — making each surgically resectable independently. There are 10 segments in the right lung and 8–10 in the left lung (segments may be fused). The PA-CAT expects recognition of segmental organization rather than memorization of every name, but commonly referenced segments include the apical (S1), posterior (S2), and anterior (S3) segments of the right upper lobe, the lateral and medial segments of the right middle lobe, and the superior and inferior lingular segments (S5) of the left upper lobe.

Lobes, Fissures, and Hilum

The right lung has three lobes (superior, middle, inferior) separated by a horizontal fissure (between superior and middle) and an oblique fissure (between the upper/middle lobes and the inferior lobe). The left lung has two lobes (superior and inferior) separated only by an oblique fissure; the lingula (tongue-like projection of the left upper lobe) is the homologue of the right middle lobe. The right lung is slightly larger than the left and has 10 bronchopulmonary segments; the left is indented by the cardiac notch to accommodate the heart's apex and typically has 8–10 segments.

The hilum (root) of each lung is on the medial surface and transmits the pulmonary artery, main bronchus, pulmonary veins, bronchial vessels, lymphatics, and autonomic nerves, all enclosed in a sleeve of pleura. The reliable relationship at both hila is bronchus posterior, pulmonary artery anterosuperior, pulmonary veins inferior. The pulmonary ligament is a double fold of mediastinal pleura extending below the hilum and is a useful surgical plane.

Pericardium

The pericardium encloses the heart and roots of the great vessels in the middle mediastinum. It has two layers:

1. Fibrous pericardium — tough outer dense connective tissue, anchored to the diaphragm via pericardiacophrenic ligaments and to the adventitia of the great vessels, preventing overdistension of the heart.
2. Serous pericardium — a double-layered membrane with a parietal layer lining the inner surface of the fibrous pericardium and a visceral layer (epicardium) reflected onto the heart surface. Between them is the pericardial cavity containing ~15–50 mL of serous fluid, which reduces friction during cardiac contraction.

Two pericardial sinuses are clinically important recesses formed by serous reflections. The transverse sinus lies posterior to the aorta and pulmonary trunk and anterior to the SVC and pulmonary veins — a finger or clamp can be passed through it to occlude the great vessels during cardiac surgery. The oblique sinus is a blind cul-de-sac posterior to the left atrium, bounded by the reflection of serous pericardium around the pulmonary veins and IVC; fluid or pus can pool here.

Pericarditis (inflammation) produces a friction rub and may progress to pericardial effusion; rapid accumulation of fluid in the restricted fibrous sac causes cardiac tamponade — impaired filling, muffled heart sounds, distended neck veins, and pulsus paradoxus. The pericardium is supplied by the phrenic nerves (C3–C5), so pericarditis can refer pain to the shoulder (the C3–C5 dermatomal overlap).

A chart comparing lobar anatomy is provided below.

Pleura and Surface Projections

Each lung is covered by visceral pleura (continuous with the lung root) and surrounded by parietal pleura lining the chest wall (costal), diaphragm (diaphragmatic), and mediastinum (mediastinal). The costodiaphragmatic recess is the lowest point of the pleural cavity where costal and diaphragmatic pleura meet; fluid accumulates here and it is the target of thoracentesis. The parietal pleura is pain-sensitive (supplied by intercostal and phrenic nerves) whereas the visceral pleura is not, which is why pleuritic pain arises from the parietal layer. The lower border of the lung crosses rib 6 at the midclavicular line, rib 8 at the midaxillary line, and rib 10 at the paravertebral line; the parietal pleura lies two ribs lower (ribs 8, 10, 12), creating a bare area for safe needle insertion. The pleural recesses also explain why a pneumothorax preferentially collects apically in an upright patient while a pleural effusion collects dependently in the costodiaphragmatic recess — a chest tube for fluid is placed in the costodiaphragmatic recess.