Lung Anatomy and Bronchial Tree

The lungs are the paired organs of gas exchange, suspended in the thoracic cavity and attached to the mediastinum at the hilum, where the bronchi and pulmonary vessels enter and leave. Each lung is divided into lobes by fissures, and air reaches the gas-exchanging surface through the bronchial tree, a repeatedly branching system of airways descending from the trachea to the terminal and respiratory bronchioles. The right lung has three lobes and the left has two, reflecting the space occupied by the heart.

Definition

The lungs are the paired respiratory organs occupying the pleural cavities; the bronchial tree is the branching system of conducting airways, from the main bronchi at the hilum through lobar and segmental bronchi to the bronchioles, that delivers air to the gas-exchanging regions of the lung.

Scope

This topic covers the external form and lobar organisation of the lungs, the hilum and root, the architecture of the tracheobronchial tree down to bronchopulmonary segments, and the developmental process of branching that generates this pattern, together with common anatomical variation. It is reference-educational and does not provide clinical guidance.

Core questions

• How are the right and left lungs divided into lobes by fissures?
• What structures enter and leave at the hilum and how are they arranged?
• How does the bronchial tree branch from trachea to bronchopulmonary segments?
• How does branching morphogenesis establish the airway pattern, and how variable is it?

Key concepts

• Right lung (three lobes) and left lung (two lobes)
• Oblique and horizontal fissures
• Hilum and root of the lung
• Main, lobar, and segmental bronchi
• Bronchopulmonary segments
• Bronchioles and the respiratory zone
• Branching morphogenesis

Mechanisms

The trachea bifurcates at the carina into right and left main bronchi; the right main bronchus is wider and more vertical than the left. Within each lung the main bronchus divides into lobar then segmental bronchi, each segmental bronchus supplying a bronchopulmonary segment, a structurally and functionally semi-independent unit. Branching continues through conducting bronchioles to terminal and respiratory bronchioles, beyond which lies the gas-exchanging respiratory zone. This dichotomous pattern is laid down during development by branching morphogenesis, in which epithelial buds repeatedly extend and divide under reciprocal epithelial-mesenchymal signalling (Goodwin 2020; Schittny 2017; Standring 2020). The branching pattern and the position of fissures vary between individuals, including accessory fissures and incomplete fissures (Wooten 2014; Abdu 2026).

Clinical relevance

Segmental and lobar anatomy underpins the localisation of lesions, the planning of resections, and the interpretation of bronchoscopy and cross-sectional imaging, while the more vertical right main bronchus is the anatomical reason aspirated material tends to enter the right lung. This entry describes the relevant normal anatomy and its variation for orientation; it is not a diagnostic or procedural guide.

Evidence & guidelines

Lobar, hilar, and bronchial anatomy is described in standard reference texts (Standring 2020; Moore 2018). The frequency and types of fissural and bronchial variation are summarised in anatomical reviews and a systematic review and meta-analysis (Wooten 2014; Abdu 2026), and the developmental basis of airway branching is reviewed in the developmental-biology literature (Goodwin 2020; Schittny 2017).

Seminal works

• schittny-2017
• goodwin-2020
• standring-2020

Frequently asked questions

How many lobes does each lung have?

The right lung has three lobes (upper, middle, lower) separated by oblique and horizontal fissures, and the left lung has two lobes (upper and lower) separated by an oblique fissure (Standring 2020).

What is a bronchopulmonary segment?

It is the portion of lung supplied by a single segmental bronchus and its accompanying artery, forming a semi-independent structural and functional unit that can be considered separately in surgery and imaging (Standring 2020).