The Lungs

Core Concepts

Gross anatomy of the lungs

Each lung is a cone-shaped organ with an apex (the rounded top, reaching above the clavicle) and a base (the concave bottom, resting on the diaphragm). The costal surface faces the ribs; the mediastinal surface faces the midline and contains the hilum — the "doorway" where the main bronchus, pulmonary vessels, bronchial vessels, nerves, and lymphatics enter and leave. Together these structures form the lung root.

• Right lung: three lobes — superior, middle, and inferior — separated by the horizontal (transverse) and oblique fissures.
• Left lung: two lobes — superior and inferior — separated by the oblique fissure. The cardiac notch is an indentation on its medial side where the heart sits, which is why the left lung is smaller and has no middle lobe.

Bronchopulmonary segments and lobules

Each lobe is divided into bronchopulmonary segments, each served by its own tertiary bronchus and its own blood supply — commonly cited as roughly ten per lung, with some variation between texts. Because each segment is functionally independent, disease can be localized to a segment and surgeons can remove one without disturbing the rest of the lung. The smallest structural unit, the lobule, contains a bronchiole, an arteriole, a venule, and lymphatics, and is served by a cluster of alveoli arranged into alveolar sacs.

Pleura and the pleural cavity

Each lung is enclosed in a double-walled sac:

• The visceral pleura clings tightly to the lung surface.
• The parietal pleura lines the thoracic wall and diaphragm.
• Between them lies the pleural cavity, containing a thin film of pleural fluid that lubricates the surfaces so the lungs slide against the chest wall during breathing.

The pleural space normally has negative pressure relative to the atmosphere, which keeps the lungs stretched open against the chest wall. If air enters the space (pneumothorax), that negative pressure is lost and the lung collapses on that side — like a stretched balloon whose seal has been broken.

Blood supply: two separate circuits

The lungs have a famously unusual arrangement of two blood supplies:

• Pulmonary circulation (functional): the pulmonary arteries carry deoxygenated blood from the right ventricle to capillary networks around the alveoli, where gas exchange occurs; the pulmonary veins carry oxygenated blood back to the left atrium. Note the reversal: in the lungs, arteries carry deoxygenated blood and veins carry oxygenated blood.
• Bronchial circulation (nutritive): the bronchial arteries, branches of the aorta, carry oxygenated blood to nourish the airway walls and lung tissue themselves; bronchial veins drain some of this blood.

Alveoli and the respiratory membrane

The alveoli are tiny, thin-walled sacs — commonly estimated at roughly 300 million in an adult pair of lungs, giving a huge surface area for exchange. Three cell types matter:

• Type I alveolar cells (pneumocytes): extremely thin squamous cells that cover most of the alveolar surface and are the site of gas exchange.
• Type II alveolar cells (pneumocytes): rounded cells that secrete surfactant, a phospholipid-protein mixture that reduces surface tension — without it, the moist alveolar walls would tend to stick shut (a key concept behind respiratory distress in premature infants, whose type II cells are immature).
• Alveolar macrophages (dust cells): phagocytes that patrol the alveolar surface, engulfing inhaled particles and pathogens.

The respiratory membrane is the barrier gases must cross: the thin cytoplasm of a type I cell, the fused basement membranes of the alveolus and capillary, and the capillary endothelium. Its total thickness is only a fraction of a micrometer in places, so oxygen and carbon dioxide diffuse across rapidly.

Common Confusions