Basic anatomy of the lungs

• The lungs are paired respiratory organs on either side of the mediastinum, each enclosed by a pleural cavity.
• Air reaches them through the trachea, right and left main bronchi, lobar bronchi, segmental bronchi, bronchioles, and finally alveoli.
• The pulmonary arteries carry deoxygenated blood from the right ventricle to the lungs; pulmonary veins return oxygenated blood to the left atrium.
• The right lung is usually larger and shorter than the left. The left lung is relatively smaller because of the cardiac impression.

• Apex: extends into the root of the neck, above the first rib.
• Base: rests on the diaphragm.
• Costal surface: faces the chest wall and ribs.
• Mediastinal surface: faces the mediastinum and contains the hilum.
• Borders: anterior, posterior, and inferior.

Lobes and fissures

Major fissures

• Present on both sides.
• Separate the lower lobe from the upper lobe on the left, and from the upper and middle lobes on the right.
• On a lateral chest radiograph, they are often visible when the X-ray beam is tangential to them.

Minor fissure

• Present only on the right.
• Separates the right upper lobe from the right middle lobe.
• Usually runs approximately horizontally from the right hilum toward the lateral chest wall.
• It may be visible on a frontal chest radiograph.

Bronchial tree

• Trachea divides at the carina into right and left main bronchi.
• The right main bronchus is wider, shorter, and more vertical than the left. Aspirated material therefore more often enters the right bronchial tree.
• The right main bronchus gives rise to three lobar bronchi:
  1. Upper-lobe bronchus
  2. Middle-lobe bronchus
  3. Lower-lobe bronchus
• The left main bronchus gives rise to two lobar bronchi:
  1. Upper-lobe bronchus
  2. Lower-lobe bronchus

Bronchopulmonary segments

• one segmental bronchus, and
• a corresponding branch of the pulmonary artery.

Right lung: 10 segments

Left lung: usually 8 to 10 segments

Key comparison

• Right lung: conventionally 10 segments.
• Left lung: conventionally 8 segments when the apical and posterior segments, and the medial and anterior basal segments, are combined. Some descriptions still list 10 anatomical segmental bronchi.
• The lingula of the left upper lobe is the anatomical counterpart of the right middle lobe.

Practical radiology points

• Right middle lobe lies anteriorly and medially. Collapse or consolidation may obscure the right heart border on a frontal chest radiograph.
• Lingula lies anteriorly in the left upper lobe. Disease here may obscure the left heart border.
• Lower lobes are predominantly posterior, so lower-lobe disease may be more conspicuous on a lateral chest radiograph.
• The superior segment of the lower lobe is posterior and may be involved in aspiration in a supine patient.
• Dependent basal segments are common sites for aspiration in upright patients.
• Azygous fissure is an anatomical variant. It does not create a true additional bronchopulmonary lobe or alter segmental anatomy. Grainger & Allison's Diagnostic Radiology, p. 35.

Short memory aid

1. a one-page radiology revision note,
2. a labeled segment table for HRCT, or
3. chest X-ray localization using lobes and fissures.

1. Title: Basic Lung Anatomy
2. Gross anatomy: apex, base, surfaces, borders, pleura
3. Hilum and root: bronchus, pulmonary artery, pulmonary veins, lymphatics
4. Lobes and fissures: right versus left lung
5. Bronchial tree: trachea to segmental bronchi
6. Right bronchopulmonary segments: S1-S10
7. Left bronchopulmonary segments: apicoposterior, lingula, basal segments
8. Radiology localization: silhouettes, fissures, aspiration-prone segments
9. Summary slide with segment tables and key labels

Normal chest X-ray anatomy

Standard views

• PA view: Patient faces the detector. This is the preferred routine view because the heart is less magnified.
• Lateral view: Helps localize disease anteriorly or posteriorly and assess the retrosternal and retrocardiac regions.
• AP portable view: Common in ill patients. The heart can appear artificially enlarged because of magnification.

PA chest radiograph: structures to identify

1. Trachea and main bronchi

• Trachea should be central or only mildly displaced.
• It bifurcates at the carina into the right and left main bronchi.
• The right main bronchus is more vertical than the left.

2. Hila

• Hilar shadows are mainly formed by the pulmonary arteries.
• The left hilum is normally slightly higher than the right.
• Hila should be assessed for size, density, shape, and position.

3. Mediastinum

• The central mediastinal shadow includes the heart, great vessels, trachea, and other mediastinal structures.
• On the right upper mediastinal border, the superior vena cava and brachiocephalic vessels contribute to the contour.
• On the left upper mediastinal border, identify the aortic knuckle.

4. Cardiac silhouette

5. Lung fields and vascular markings

• Compare the two lungs for equal transradiancy.
• Pulmonary vessels normally become smaller toward the periphery.
• Lung markings should extend almost to the pleural surface, but not beyond it.
• The apices, upper zones, hilar regions, lower zones, and retrocardiac region should all be reviewed systematically.

6. Diaphragms and costophrenic angles

• Both hemidiaphragms should have smooth, sharp contours.
• The right hemidiaphragm is normally slightly higher than the left.
• The left hemidiaphragm may be partly obscured by the cardiac silhouette.
• The lateral costophrenic angles should be acute and sharp.
• Blunting suggests pleural fluid, pleural thickening, or less commonly hyperinflation-related change.

7. Fissures

• The right horizontal fissure may be visible on the PA radiograph as a thin horizontal line extending from the right hilum toward the lateral chest wall.
• Major, or oblique, fissures are usually better seen on the lateral radiograph.
• A fissure is visible only when the X-ray beam is tangential to it.

Lateral chest radiograph: structures to identify

• Retrosternal clear space
• Tracheal air column
• Hila
• Major fissures
• Right horizontal fissure, if visible
• Thoracic vertebral bodies
• Posterior costophrenic angles
• Diaphragmatic contours

Useful normal lateral-view observations

• The thoracic spine should become progressively more radiolucent inferiorly because there is less overlying soft tissue.
• The retrosternal clear space should remain lucent.
• Posterior costophrenic angles should be sharp.
• Both major fissures run obliquely from posterior-superior to anterior-inferior direction.

Basic silhouette sign

Simple chest X-ray review order

• A - Airway: trachea, carina, main bronchi
• B - Breathing: lungs, pleura, hila, fissures
• C - Cardiomediastinum: heart size, contours, mediastinum
• D - Diaphragm: hemidiaphragms, costophrenic angles, gastric bubble
• E - Everything else: bones, soft tissues, lines and devices