The Lungs: Anatomy and Respiration

The lungs are paired, spongy organs responsible for gas exchange between the atmosphere and the bloodstream. Together they weigh approximately 1 kg in adults, with the right lung slightly larger and heavier than the left. They occupy the thoracic cavity, one on each side of the mediastinum, and are essential for delivering oxygen to every cell in the body while removing carbon dioxide. This guide is for educational purposes only.

## Lobes and Fissures

The right lung is divided into three lobes — upper, middle, and lower — by the horizontal fissure (separating upper from middle) and the oblique fissure (separating middle and lower). The left lung has only two lobes — upper and lower — separated by a single oblique fissure; the left upper lobe includes the lingula, which is the anatomical counterpart of the right middle lobe. The absence of a right-sided heart means the left lung is smaller and narrows to accommodate the cardiac notch on its anteromedial surface.

Each lobe is further divided into bronchopulmonary segments — self-contained anatomical units each served by its own segmental (tertiary) bronchus and segmental artery, with intersegmental veins draining at segment boundaries. The right lung has 10 segments; the left has 8–10 (nomenclature varies). Bronchopulmonary segments are surgically resectable units, enabling segmentectomy that preserves more lung tissue than lobectomy.

## Bronchial Tree

The trachea bifurcates at the carina (at the level of the sternal angle, T4–T5) into the right and left main bronchi. The right main bronchus is shorter, wider, and more vertical than the left, making it the preferential site for aspirated foreign bodies. Each main bronchus divides into lobar (secondary) bronchi — three on the right, two on the left — and then into segmental (tertiary) bronchi. Further branching produces smaller bronchi, bronchioles (diameter < 1 mm, no cartilage), terminal bronchioles, respiratory bronchioles, alveolar ducts, and finally alveolar sacs.

From the trachea to the alveolar sacs there are approximately 23 generations of branching. The first 16 generations (trachea to terminal bronchiole) form the conducting zone — responsible for moving air but not involved in gas exchange. Generations 17–23 form the respiratory zone, where gas exchange occurs. The total cross-sectional area of the airways increases enormously at each branching level, so airflow velocity falls dramatically as air approaches the alveoli, allowing time for diffusion.

## Alveoli and Gas Exchange

The lungs contain approximately 300–500 million alveoli, providing a total gas exchange surface area of 70–100 m² in an adult — roughly the size of a tennis court. The alveolar wall (the respiratory membrane) is extremely thin (0.2–0.5 µm) to facilitate rapid diffusion. It consists of type I pneumocytes (flat cells covering ~95% of the alveolar surface; the primary gas exchange cells), type II pneumocytes (cuboidal cells that secrete surfactant — a mixture of phospholipids and proteins that reduces surface tension, preventing alveolar collapse on expiration), and alveolar macrophages (dust cells that phagocytose particles and pathogens).

Oxygen and CO₂ cross the respiratory membrane by simple diffusion down partial pressure gradients (Fick's law). Oxygenated blood in pulmonary capillaries equilibrates with alveolar gas within 0.25 seconds — one-third of the ~0.75 seconds a red blood cell spends in a capillary at rest. Ventilation-perfusion (V/Q) matching is critical: the apices of upright lungs are relatively over-ventilated and under-perfused (V/Q > 1), while the bases are relatively under-ventilated and over-perfused (V/Q < 1). V/Q mismatch is the most common cause of hypoxaemia in lung disease.

## Pleura and Pleural Space

Each lung is enclosed by a double-layered serous membrane, the pleura. The visceral pleura closely adheres to the lung surface, including within the fissures; the parietal pleura lines the thoracic wall, diaphragm, and mediastinum. The two layers are continuous at the lung hilum. Between them is the pleural cavity — a potential space containing a thin film (~15 mL) of serous fluid that lubricates lung movement and generates a subatmospheric pressure (approximately −5 cmH₂O at end-expiration) that keeps the lung expanded against the chest wall.

Pneumothorax occurs when air enters the pleural space, abolishing the negative pressure and causing lung collapse. A tension pneumothorax (one-way valve effect allowing progressive air accumulation) is a medical emergency: it compresses the heart, great vessels, and contralateral lung, causing obstructive shock and requiring immediate needle decompression.

Pleural effusion — excess fluid in the pleural space — may be a transudate (caused by hydrostatic or oncotic pressure imbalance, as in heart failure or hypoalbuminaemia) or an exudate (caused by increased capillary permeability, as in infection or malignancy). Light's criteria distinguish the two.

## Pulmonary Vasculature

The pulmonary circulation carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium. The pulmonary trunk arises from the right ventricle and divides into the right and left pulmonary arteries. These branch in parallel with the bronchial tree, accompanying each bronchus. Unlike systemic circulation, pulmonary arteries carry deoxygenated blood and the pulmonary veins carry oxygenated blood. There are four pulmonary veins (right superior, right inferior, left superior, left inferior) draining into the left atrium.

Pulmonary vascular resistance is approximately one-seventh that of the systemic circulation, enabling the right ventricle — a thin-walled, low-pressure pump — to circulate the same cardiac output as the more muscular left ventricle. Hypoxic pulmonary vasoconstriction redirects blood away from poorly ventilated regions of the lung, optimising V/Q matching. Chronic hypoxia, as in altitude or obstructive lung disease, causes sustained vasoconstriction, pulmonary hypertension, and eventually right heart failure (cor pulmonale).

## Clinical Correlations

Chronic obstructive pulmonary disease (COPD) — encompassing emphysema (alveolar wall destruction, loss of elastic recoil, air trapping) and chronic bronchitis (productive cough for at least 3 months in 2 consecutive years) — is the third leading cause of death worldwide, predominantly caused by smoking. Asthma involves reversible airway obstruction from bronchospasm, mucosal oedema, and mucus hypersecretion driven by type 2 inflammation. Pulmonary embolism (PE) — obstruction of a pulmonary artery by thromboembolus — causes V/Q mismatch, hypoxaemia, and if massive, haemodynamic collapse; CT pulmonary angiography is the diagnostic gold standard.