Anatomy of the Lung

Overview

Gross Anatomy

External Features

• An apex projecting into the root of the neck (~2.5 cm above the medial third of the clavicle)
• A base resting on the diaphragm (concave)
• Costal surface (convex, related to ribs)
• Mediastinal (medial) surface containing the hilum
• Anterior, posterior, and inferior borders

Lobes and Fissures

• The oblique (major) fissure is present in both lungs; it separates the inferior lobe from the superior (and middle on the right).
• The horizontal (minor) fissure (right lung only) separates the superior from the middle lobe.

The Bronchial Tree

Main Bronchi

• Right main bronchus: wider, shorter (~2.5 cm), more vertical — foreign bodies preferentially enter here
• Left main bronchus: narrower, longer (~5 cm), more horizontal, passes inferior to the aortic arch

Lobar Bronchi

• Right: 3 lobar bronchi (superior, middle, inferior)
• Left: 2 lobar bronchi (superior, inferior)
• Right superior lobar bronchus arises 1–2.5 cm from the carina; middle and inferior arise ~5 cm from it
• On the left, division into superior and inferior lobar bronchi also occurs ~5 cm from the carina

Segmental Bronchi → Bronchopulmonary Segments

• Right lung: 10 segmental bronchi → 10 bronchopulmonary segments
• Left lung: 9 segmental bronchi (segments 1 & 2 fuse as the apicoposterior segment)

• An irregular cone, apex toward the hilum, base at the lung surface
• Supplied by one segmental bronchus and an accompanying pulmonary artery branch (bronchoarterial unit)
• The smallest functionally independent unit of the lung — can be surgically resected without damaging adjacent segments
• Pulmonary veins run intersegmentally, demarcating segment boundaries

Bronchopulmonary Segments — Detailed Map

Right Lung (10 segments)

Left Lung (9 segments)

Lung Lobules

• Bronchioles (no cartilage) → supply pulmonary lobules
• Lobules are polygonal, 0.5–3 cm on each side, bounded by connective tissue septa (which appear blue-black from inhaled carbon)
• Each bronchiole divides 3–4 times → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli

The Hilum

• Main bronchus
• Pulmonary artery (1 per lung)
• Pulmonary veins (2 per lung — superior and inferior)
• Bronchial arteries and veins
• Lymphatics
• Autonomic nerve plexus

Arrangement at the Hilum (from anterior to posterior, right side):

• Left: pulmonary artery is most superior; bronchus is posterior

Pulmonary Vasculature

Pulmonary Arteries (Low-pressure, deoxygenated blood)

• Arise from the pulmonary trunk, bifurcates at TIV/V
• Right pulmonary artery: longer, passes horizontally across mediastinum anterior to tracheal bifurcation, posterior to ascending aorta and SVC
• Left pulmonary artery: shorter, curves over the left main bronchus, connected to aortic arch by the ligamentum arteriosum
• Branches follow the bronchial tree (intrasegmental)

Pulmonary Veins (Oxygenated blood → left atrium)

• 4 pulmonary veins (2 right, 2 left) drain into the left atrium
• Run intersegmentally — do not follow bronchi
• Drain into superior and inferior pulmonary veins on each side

Bronchial Circulation

• Bronchial arteries (from the thoracic aorta): supply oxygenated blood to the bronchial tree walls, supporting tissue down to the respiratory bronchioles
• Bronchial veins: drain partly into the azygos/hemiazygos system and partly into the pulmonary veins

Lymphatics

• Superficial (subpleural) plexus: drains the periphery; flows toward the hilum
• Deep (peribronchial) plexus: follows the airways and vessels; drains toward hilar lymph nodes
• Right lung and left lower lobe → right bronchomediastinal trunk
• Left upper lobe → left bronchomediastinal trunk

Pleura

• Visceral pleura: intimately covers the lung surface (cannot be separated from lung parenchyma)
• Parietal pleura: lines the chest wall, diaphragm, and mediastinum
• Pleural cavity: potential space containing only a few micrometers of serous fluid; creates negative pressure that keeps the lung expanded
• Costophrenic recess: the lowest part of the pleural cavity, between the costal and diaphragmatic pleura — site of fluid accumulation

Innervation

• Parasympathetic: vagus nerve (CN X) → bronchoconstriction, increased mucus secretion, vasodilation
• Sympathetic: T2–T5 → bronchodilation, vasoconstriction
• Visceral afferents: carried with both sympathetic and vagal fibers; cough reflex via vagus
• Pulmonary plexus surrounds the hilum and sends branches along bronchi and vessels

Microstructure of the Airways

Bronchial Wall Layers (from lumen outward):

1. Mucosa: pseudostratified columnar ciliated epithelium with goblet cells
2. Lamina propria: loose CT with lymphoid tissue
3. Smooth muscle (spiral bands)
4. Submucosa: seromucous glands
5. Cartilaginous plates (present in bronchi; absent in bronchioles)
6. Adventitia

Key Epithelial Cells:

The Alveoli (Gas Exchange Unit)

• Total ~300–500 million alveoli per lung
• Total surface area for gas exchange: ~70–140 m²
• Alveolar wall (blood-air barrier) is extremely thin to facilitate O₂/CO₂ diffusion
• Type I pneumocytes (~95% of alveolar surface): thin, flat; gas exchange
• Type II pneumocytes (~5% of surface but 60% of cells): produce surfactant (reduces surface tension); serve as progenitors for Type I cells
• Alveolar macrophages: first-line defense; phagocytose inhaled particles

Connective Tissue Framework

1. Peribronchial/periarterial CT: surrounds bronchial tree and pulmonary artery branches → allows movement against surrounding gas-exchanging tissue; forms the peribronchovascular connective tissue space (important in pulmonary edema)
2. Subpleural CT: lines lobe surfaces; forms interlobular septa and intersegmental boundaries; acts as sliding layer and prevents overexpansion

• Murray & Nadel's Textbook of Respiratory Medicine (Chapter 1: Anatomy)
• Gray's Anatomy for Students (Chapter 3: Thorax)
• Color Atlas of Human Anatomy, Vol. 2: Internal Organs