Lung

The lungs are a pair of spongy, air-filled organs located in the thoracic cavity, separated by the mediastinum and enclosed by a double-layered pleural membrane. The right lung has three lobes (upper, middle, lower) separated by fissures; the left lung has two lobes (upper and lower) with a single oblique fissure. Each lung sits on the diaphragm with a convex base and a rounded apex near the collarbone. The hilum is the doorway where the main bronchi, pulmonary arteries, veins, nerves, and lymphatics enter and leave the lung.

Air is conducted through the trachea, which divides into main bronchi and progressively finer conducting airways ending in microscopic alveoli. Gas exchange occurs across the thin alveolar-capillary membrane. Oxygen from inhaled air diffuses into the pulmonary capillaries and binds to hemoglobin, while carbon dioxide diffuses from blood to be exhaled.

The lungs receive blood through the pulmonary circulation, which brings deoxygenated blood to the lungs via the pulmonary arteries and returns oxygenated blood to the left atrium via the pulmonary veins. Systemic bronchial arteries supply the airways with oxygenated blood from the aorta. Venous drainage includes pulmonary veins and bronchial veins to the systemic circulation.

Innervation is provided by autonomic nerves; the vagus nerve mediates bronchoconstriction, while sympathetic input causes bronchodilation. The airway lining consists of ciliated, mucus-secreting epithelium that traps particles and uses the mucociliary escalator to clear debris.

Development begins with the foregut endoderm forming the respiratory diverticulum around week 4, followed by branching morphogenesis that creates the bronchial tree; alveoli continue to mature after birth.

Clinically, the lungs can be affected by infections such as pneumonia, chronic conditions like asthma and COPD, interstitial diseases such as pulmonary fibrosis, and cancer. Trauma or disease can lead to pneumothorax or pleural effusion.