Thorax

The thoracic wall is a flexible cage formed by thoracic vertebrae, ribs, costal cartilages, sternum and intercostal muscles. It protects the heart and lungs while changing volume during respiration. Ribs articulate posteriorly with vertebrae and, depending on level, connect directly or indirectly to the sternum anteriorly. Intercostal spaces contain muscle layers and a neurovascular bundle. In the costal groove, the main vein, artery and nerve run in a characteristic superior-to-inferior order.

Breathing requires coordinated movement of ribs and diaphragm. Elevation of ribs can increase transverse and anteroposterior thoracic dimensions, while diaphragmatic contraction increases vertical dimension. The thoracic wall is therefore both protective and dynamic. Procedures through an intercostal space must respect the location of the neurovascular bundle.

Each lung is covered by visceral pleura, while parietal pleura lines the thoracic wall, diaphragm and mediastinum. Between them is a potential pleural cavity containing a thin lubricating film. The pleural layers normally remain closely opposed as the lungs follow thoracic movement. The lungs are divided into lobes by fissures, and their roots contain bronchi, pulmonary vessels, bronchial vessels, lymphatics and nerves.

The right and left lungs are not mirror images because mediastinal structures alter their shapes. Bronchopulmonary segments are functionally useful subdivisions supplied by segmental bronchi and arterial branches. Pleural recesses provide potential spaces into which the lungs expand during deep inspiration. Air or excess fluid entering the pleural cavity can impair normal mechanical coupling.

The mediastinum is the central compartment between the pleural cavities. Dividing it into superior and inferior regions, with the inferior region further described as anterior, middle and posterior, provides a map for organizing many structures. The middle mediastinum contains the heart and pericardium; the superior and posterior regions transmit major vessels, trachea or esophagus and important nerves according to level.

Boundaries should be learned before contents because compartment location helps interpret imaging and masses. A structure that enlarges can compress its neighbours, so symptoms may arise from anatomical relationships rather than from the diseased organ alone. The phrenic and vagus nerves, for example, take different courses through the thorax and have different functional targets.

The heart lies obliquely within the middle mediastinum rather than vertically in the center of the chest. Its chambers contribute differently to its surfaces and borders. The right ventricle forms much of the anterior surface, while the left ventricle contributes strongly to the left border and apex. The fibrous and serous pericardium enclose the heart and roots of great vessels, limiting excessive movement while permitting near-frictionless cardiac motion.

Valves direct one-way flow, but auscultation sites are chosen according to where sound is best transmitted, not simply where the valve physically lies. Coronary arteries arise from the ascending aorta and supply myocardium. Coronary dominance refers to the artery giving rise to the posterior interventricular branch, not to which artery supplies 'most of the heart.'

The ascending aorta continues into the arch and descending thoracic aorta. The arch gives major branches to the head, neck and upper limbs. The superior vena cava returns venous blood from above the diaphragm, while the azygos system provides an important venous pathway along the posterior thoracic wall. Pulmonary arteries carry deoxygenated blood from the right ventricle to the lungs, whereas pulmonary veins return oxygenated blood to the left atrium.

The thoracic duct carries lymph from most of the body and ascends through the thorax before draining near the left venous angle. These vessels are learned best by tracing origin, course, relations and destination. Their enlargement or injury can affect neighbouring nerves, airway or esophagus.

Chest radiography compresses three-dimensional anatomy into a two-dimensional projection, so interpretation depends on systematic orientation. Identify patient side, projection, lung fields, diaphragms, mediastinal contours, heart silhouette and visible bones. CT restores cross-sectional localization and makes relationships among lung, mediastinum, vessels and chest wall much clearer.

Applied anatomy should always be derived from normal structure. A collapsed lung, pleural collection, mediastinal shift or enlarged cardiac silhouette changes expected relationships. The student should ask: which compartment is abnormal, which border has changed, and what structure normally occupies that location? This approach is safer than memorizing isolated radiological signs without anatomical reasoning.