Lung Anatomy - Overview

General Facts

• Two lungs occupy the thoracic cavity, separated by the mediastinum (heart, great vessels, trachea, esophagus).
• Each lung weighs 450-500 g (combined 900-1000 g), with ~40-50% of that weight being blood.
• Gas volume ranges from ~2.5 L at end-expiration (FRC) to ~6 L at total lung capacity (TLC).
• Lung density is highest (~1 g/mL) near the hilum and lowest (~0.1 g/mL) peripherally - hence the peripheral lung looks "empty" on chest X-ray.

Lobes and Fissures

• The left lung has a cardiac notch to accommodate the heart, and the lingula (tongue-like projection of the upper lobe) corresponds roughly to the right middle lobe.
• Each lobe is divided into bronchopulmonary segments - the smallest independent, surgically resectable units (10 right, 8-10 left).

Airways (Conducting Zone)

• The trachea bifurcates at the carina (T4-T5 level, angle of Louis) into right and left main bronchi.
• Right main bronchus is wider, shorter, and more vertical - so aspirated foreign bodies preferentially enter the right lung.
• Bronchi have cartilage rings in their walls; bronchioles do not.
• Airways branch dichotomously for ~23 generations (Weibel model).
• The conducting zone (generations 0-16) warms, filters, and humidifies air but does not participate in gas exchange - this is anatomical dead space (~150 mL).

Respiratory Zone (Gas Exchange)

• Approximately 300-500 million alveoli in the adult lung.
• Total alveolar surface area: ~70-80 m² (size of a tennis court).
• Alveolar walls are extremely thin (<0.5 µm) to minimize diffusion distance for O₂ and CO₂.

• 50% of all alveoli lie in the outer 30% of the lung (hilum to chest wall).

Vasculature

Pulmonary Circulation (gas exchange)

• Pulmonary artery (right ventricle → lungs): carries deoxygenated blood.
• Pulmonary veins (lungs → left atrium): return oxygenated blood.
• Runs alongside bronchi in the peribronchovascular sheath.
• Vascular pressure increases from apex to base at ~1 cm H₂O per cm height - so lower lung zones are better perfused.

Bronchial Circulation (nutrition)

• Bronchial arteries arise from the aorta (systemic circulation) and supply the airway walls down to the terminal bronchioles.
• Drain via bronchial veins into the azygos system or pulmonary veins.

Lymphatics

• Form two networks: subpleural and peribronchial/perivascular.
• Drain toward hilar and mediastinal lymph nodes.
• Parietal pleura contains lymphatic stomata (1-12 µm openings) that drain pleural fluid.
• Lymphatic drainage follows a zone-based pattern: lower lobe → subcarinal nodes; upper lobe → superior mediastinal nodes.

Pleura

• Visceral pleura: covers the lung surface (including fissures); no pain fibers.
• Parietal pleura: lines the thoracic wall, diaphragm, and mediastinum; has pain fibers.
• Pleural space: a real (not potential) space containing 0.1-0.2 mL/kg of fluid, keeping the two layers separated by only 10-20 µm.
• Negative intrapleural pressure keeps the lung expanded against the elastic recoil tendency to collapse.

Innervation

• Parasympathetic (vagus nerve): bronchoconstriction, mucus secretion, pulmonary vasodilation.
• Sympathetic (T2-T5 sympathetic ganglia): bronchodilation, vasoconstriction.
• Sensory: C-fibers (J-receptors) in alveolar walls detect stretch and irritants; rapidly adapting receptors (RARs) in airways respond to chemical stimuli and trigger cough.

Key Cellular Components

Lung Anatomy - Diagrams & Images

1. Lung Position in the Chest (Gross Anatomy)

2. Lung Tissue Cross-Section (Blood Oxygenation)

3. Bronchial Wall - Histology

4. Terminal vs Respiratory Bronchioles

• Terminal bronchiole (TB): Single layer of ciliated cuboidal epithelium + smooth muscle; no alveoli in wall - conducting zone only
• Respiratory bronchiole (RB): Partially lined by ciliated epithelium, partially by squamous; alveoli begin opening in walls here - this is where gas exchange starts
• Pulmonary artery (PA) travels alongside

5. Pulmonary Artery Running Alongside Bronchi

6. Terminal Respiratory Unit (Acinus)

7. Alveolar Structure - Scanning Electron Microscopy

• (A-C): Normal alveoli - polygonal, thin-walled, interconnected via pores of Kohn (arrows)
• (D): Collapsed/atelectatic alveoli - walls fold and distort
• Scale: 20 µm each panel - shows just how thin the gas exchange surface is

8. Pleural Space