The Pleura

1. Overview

• Gray's Anatomy for Students, p. 201
• Color Atlas of Human Anatomy Vol. 2

2. Anatomy

Visceral Pleura

• Covers the lung surface almost entirely and cannot be stripped from it
• Dips into the interlobular fissures
• Continuous with the parietal pleura at the hilum of the lung
• Contains three connective tissue layers that support lung shape:
  • Elastic fibers wrapping the lung lobes (following the mesothelium)
  • Fine fibers outlining individual alveoli
  • An intermediate layer of connective tissue interspersed with cells
• Its neurovascular supply and lymphatic drainage mirror those of the lung
• Ganong's Review of Medical Physiology; Color Atlas of Human Anatomy

Parietal Pleura

• Gray's Anatomy for Students; Color Atlas of Human Anatomy

Pleural Recesses

• Costodiaphragmatic recess - between the costal and diaphragmatic pleura; the most clinically significant recess (most dependent - fluid collects here first)
• Costomediastinal recess - between the costal and mediastinal pleura; wider on the left at the level of the cardiac notch

Root of the Lung

3. Nerve Supply

4. Blood Supply & Lymphatics

• Visceral pleura: branches of the pulmonary vasculature
• Parietal pleura: posterior intercostal arteries, internal thoracic artery, musculophrenic artery; venous drainage via thoracic wall veins
• Lymphatic drainage of the parietal pleura follows the thoracic wall; visceral pleura drains into bronchopulmonary nodes

5. Pleural Surface Borders

6. Physiology

• Medical Physiology (Boron & Boulpaep); Ganong's Review of Medical Physiology

7. Pathology (Robbins Pathologic Basis of Disease)

Pleural Effusion

• Increased hydrostatic pressure - congestive heart failure (most common)
• Increased vascular permeability - pneumonia
• Decreased osmotic pressure - nephrotic syndrome, cirrhosis
• Increased intrapleural negative pressure - atelectasis
• Decreased lymphatic drainage - mediastinal carcinomatosis

Inflammatory effusions (Exudates)

• Serous/serofibrinous pleuritis - TB, pneumonia, lung infarction, abscess, bronchiectasis; also rheumatoid arthritis, SLE, uremia, radiotherapy
• Empyema - purulent exudate from bacterial/fungal seeding; characterized by loculated yellow-green pus (masses of neutrophils). If unresolved, organizes into dense fibrous adhesions restricting pulmonary expansion
• Hemorrhagic pleuritis - hemorrhagic diatheses, rickettsial infections, neoplasms

Noninflammatory effusions (Transudates)

• Hydrothorax - clear straw-coloured fluid; most commonly heart failure, also renal failure, cirrhosis
• Hemothorax - blood in the pleural cavity; usually traumatic; aortic aneurysm rupture is almost invariably fatal
• Chylothorax - milky fluid from lymphatic disruption (thoracic duct trauma or malignant obstruction)

Pneumothorax

Pleural Tumors

• Soft tissue tumor, propensity for the pleura; highly associated with NAB2::STAT6 fusion gene (cryptic inversion of chromosome 12)
• Grossly: dense fibrous tissue, often pedunculated; microscopically: whorls of reticulin/collagen with fibroblast-like spindle cells
• Immunostaining: CD34+, STAT6+, keratin- (distinguishes from mesothelioma)
• No association with asbestos exposure

• Arises from visceral or parietal pleura
• Strongly associated with asbestos exposure (up to 90% in high-exposure regions); 25-45 year latent period
• Lifetime risk in heavily exposed individuals: 7-10%
• Smoking does NOT increase mesothelioma risk (contrast with lung carcinoma)
• Most common cytogenetic abnormality: homozygous deletion of the CDKN2A locus (encoding p16/INK4a and p14/ARF)
• Immunostaining: keratin+ (opposite to solitary fibrous tumor)
• Tends to encase the lung as a thick rind; spreads locally to mediastinum and diaphragm

• Gray's Anatomy for Students (Gray's)
• Color Atlas of Human Anatomy Vol. 2
• Ganong's Review of Medical Physiology, 26th Edition
• Medical Physiology (Boron & Boulpaep)
• Robbins, Cotran & Kumar - Pathologic Basis of Disease

Right Lung vs Left Lung - Key Differences

Fig. 3.44 - Right and Left Lungs. (Gray's Anatomy for Students)

1. Size & Shape

2. Lobes & Fissures

Fig. 60.1 - Lobar and segmental divisions. Right lung (above), Left lung (below). (Bailey and Love's Surgery)

3. Bronchopulmonary Segments

4. Main Bronchus

Fig. 3.46 A & B - Right Lung and its mediastinal relations. (Gray's Anatomy for Students)

5. Mediastinal Relations (Impressions on Medial Surface)

Summary Table

The Bronchial Tree

Overview: Conducting vs. Respiratory Portions

• Conducting portion: nasal cavities → pharynx → larynx → trachea → bronchi → bronchioles → terminal bronchioles. Conditions and conducts air; no gas exchange.
• Respiratory portion: respiratory bronchioles → alveolar ducts → alveolar sacs → alveoli. Gas exchange occurs here.
• Junqueira's Basic Histology, 17th Ed.

1. Trachea

• Extends from vertebral level CVI (lower neck) to TIV/V (mediastinum), where it bifurcates
• Held open by C-shaped hyaline cartilage rings (16-20); open part faces posteriorly, closed by the trachealis smooth muscle
• Lined by pseudostratified ciliated columnar (respiratory) epithelium with goblet cells, on a thick basement membrane
• The lowest tracheal ring has the carina - a hook-shaped ridge projecting between the origins of the two main bronchi; highly sensitive area (cough reflex)

2. Primary (Main) Bronchi

Fig. 17-6 - Bronchial Tree. Green = primary bronchi, Blue = secondary bronchi, Yellow = tertiary bronchi, Red = smaller bronchi. (Junqueira's Basic Histology)

• Respiratory (pseudostratified ciliated columnar) epithelium
• Prominent spiral bands of smooth muscle
• Irregular plates of hyaline cartilage (not rings - become discontinuous as bronchi narrow)
• Submucosal mucous and serous glands
• Elastic fibers

3. Lobar (Secondary) Bronchi

• Right lung: 3 lobar bronchi (superior, middle, inferior)
• Left lung: 2 lobar bronchi (superior, inferior)
• Diameter: 8-12 mm
• Each supplies one lobe
• Right superior lobar bronchus arises ~1-2.5 cm from the tracheal bifurcation; the right middle and inferior, and all left lobar bronchi arise ~5 cm from the bifurcation

4. Segmental (Tertiary) Bronchi

• Each lobar bronchus divides into segmental bronchi
• Right lung: 10 segmental bronchi; Left lung: 9 segmental bronchi (some fused)
• Each segmental bronchus, with its accompanying branch of the pulmonary artery, defines a bronchopulmonary segment

Bronchopulmonary Segments

Fig. 3.48 A & B - Bronchial tree and bronchopulmonary segments. (Gray's Anatomy for Students)

• An irregular cone with its apex toward the hilum and base on the lung surface
• The smallest functionally independent region of lung
• The smallest area that can be surgically resected without affecting adjacent lung
• Contains a central segmental bronchus + pulmonary artery branch (bronchoarterial unit)
• Pulmonary vein tributaries travel intersegmentally (between segments), demarcating boundaries

• Upper lobe: apical (1), posterior (2), anterior (3)
• Middle lobe: lateral (4), medial (5)
• Lower lobe: superior (6), medial basal (7), anterior basal (8), lateral basal (9), posterior basal (10)

• Upper lobe: apico-posterior (1+2 fused), anterior (3), superior lingular (4), inferior lingular (5)
• Lower lobe: superior (6), anteromedial basal (7+8 fused), lateral basal (9), posterior basal (10)

5. Smaller Bronchi → Bronchioles

Bronchioles

• No cartilage (key distinction from bronchi)
• No submucosal glands
• Epithelium: simple ciliated columnar to cuboidal with club cells (formerly Clara cells)
• Prominent circular layer of smooth muscle (proportionately thicker than bronchi)
• Elastic fibers in lamina propria

Terminal Bronchioles

• Last part of the conducting portion (no alveoli, no gas exchange)
• Epithelium: simple cuboidal, ciliated cells + club cells predominate
• Club cells secrete: surfactant components, protease inhibitors, cytokines for local immune regulation; also serve as stem cells for bronchiolar epithelium
• Thin, incomplete smooth muscle layer; no cartilage

Clinical note - Asthma: Chronic inflammation causes mast cell degranulation → sudden smooth muscle constriction (bronchospasm) in bronchioles. Epinephrine/sympathomimetics relax the muscle and relieve obstruction. - Junqueira's Basic Histology

6. Respiratory Bronchioles

• Similar to terminal bronchioles but interrupted by scattered alveolar outpouchings (gas exchange begins here)
• Epithelium: simple cuboidal (club cells) with simple squamous cells at alveolar openings
• Smooth muscle and elastic connective tissue in lamina propria
• Alveoli become progressively more numerous moving distally

7. Alveolar Ducts → Alveolar Sacs → Alveoli

Fig. 17-11 - Pulmonary circulation, bronchioles, and alveoli. (Junqueira's Basic Histology)

• Alveolar duct: a respiratory bronchiole leads to an alveolar duct, which is lined by a continuous series of alveoli in its walls
• Alveolar sac: cluster of alveoli at the blind end of an alveolar duct
• Alveoli: small, air-filled, saclike structures; sites of actual O₂/CO₂ exchange; separated by interalveolar septa containing elastic/reticular fibers and dense capillary networks
• Alveolar pores of Kohn: allow air pressure to equilibrate between adjacent alveoli; provide collateral ventilation if a local airway is blocked

Alveolar Cell Types

Blood-Air Barrier

1. Type I alveolar cell (extremely thin cytoplasm)
2. Fused basal laminae of alveolar epithelium and capillary endothelium
3. Capillary endothelial cell (thin)

Summary: Divisions of the Bronchial Tree