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secondary pulmonary lobule HRCT anatomy diagram

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pulmonary interstitium compartments diagram lung

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A clean labeled medical educational diagram of the Secondary Pulmonary Lobule (Reid Lobule) showing: a polyhedral hexagonal lobule shape, central bronchiole with centrilobular artery in the middle, peripheral pulmonary veins and lymphatics in interlobular septa, 2 lymphatic networks labeled (central bronchovascular network and peripheral septal network), acini clusters within the lobule, and the 4 compartments of pulmonary interstitium labeled: central/peribronchovascular, centrilobular, peripheral/subpleural, and intralobular/alveolar. Use soft colors, clear arrows, and simple easy-to-read labels. White background, educational textbook style.

Here is the full content from your slides explained in a simple, easy way:
Secondary Pulmonary Lobule & Pulmonary Interstitium Diagram

HRCT Anatomy of the Lung - Made Easy

Part 1: What is the Secondary Pulmonary Lobule (SPL)?

Think of the lung like an orange. The whole orange = the lung. One segment = the lobe. One tiny slice = the Secondary Pulmonary Lobule (also called the Reid Lobule). HRCT is the only imaging that can actually show you this tiny unit.
FeatureSimple Explanation
DefinitionSmallest unit of the lung you can see separately on HRCT
ShapePolyhedral (like a little 3D polygon / diamond shape)
Size10-30 mm on each side (about the size of a small grape)
Contents3-12 acini, supplied by 3-5 terminal bronchi

What's inside the lobule?

Think of it like a building:
  • Core (center): Terminal bronchiole + centrilobular artery (the "plumbing core")
  • Walls (periphery): Pulmonary veins + lymphatics run along the walls (interlobular septa)

2 Lymphatic Networks - Easy Memory Trick

NetworkWhere it runsThink of it as...
CentralAlong the bronchovascular bundle → towards center of lobule"Inner highway"
PeripheralIn the interlobular septa + pleural linings → at the edges"Outer ring road"

Part 2: Pulmonary Interstitium

The interstitium is the scaffolding/skeleton of the lung - it holds everything together. It starts at the hilum (lung gate) and extends all the way out to the visceral pleura.
Think of it like the frame of a building: blood vessels and airways run through it.

4 Compartments - Easy Memory: "C-C-P-I"

C - Central (axial) / Peribronchovascular
C - Centrilobular
P - Peripheral / Subpleural
I - Intralobular / Alveolar
CompartmentLocationContainsMemory Aid
1. Central (Peribronchovascular)Around the big airways from hilum inwardBronchovascular sheaths + lymphatics"The core corridor"
2. CentrilobularSurrounds the small central artery + bronchiole in each lobuleConnective tissue"The little core"
3. Peripheral / SubpleuralAt the edges - pleura, subpleural tissue, interlobular septaPulmonary veins + lymphatics"The outer walls"
4. Intralobular / AlveolarInside the alveolar wallsAlveolar wall (interalveolar septum)"The inner rooms"

Part 3: Normal HRCT Appearances - What Can You Actually See?

This is the key clinical part - what do you see vs. not see on normal HRCT:
StructureVisible on HRCT?Distance from septa/pleura
Centrilobular arteriesYES5-10 mm from septa or pleura
Acinar arteriesSometimes3-5 mm from septa or pleura
Lobular arteriesEasily seen-
BronchiolesBarely visible-
Interlobular septaSometimes-
VeinsVisible 1-2 cm from pleuraNear pleura

Bronchus vs. Artery on HRCT

A useful rule on cross-section: The artery (A) is equal in size to its accompanying bronchus (B). If artery > bronchus = abnormal (suggests pulmonary hypertension or pruning). If bronchus > artery = bronchiectasis.

Quick Recall Summary

SPL = Smallest lung unit on HRCT
     → Center: bronchiole + artery
     → Periphery: vein + lymphatics
     → Shape: polyhedral, 10-30mm
     → Contents: 3-12 acini, 3-5 bronchi

Interstitium = Lung skeleton (hilum → pleura)
     → 4 parts: Central, Centrilobular, Peripheral, Intralobular

HRCT visibility:
     → Arteries (centrilobular) visible at 5-10mm from pleura
     → Veins visible 1-2cm from pleura
     → Bronchioles NOT well seen

Figure of different type of septal thickening with differential diagnosis

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interlobular septal thickening HRCT types smooth nodular irregular

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A clean medical educational diagram showing the 3 types of interlobular septal thickening on HRCT, side by side in 3 panels: Panel 1 - SMOOTH septal thickening: uniform, straight, even thickness lines at lung periphery, labeled causes: Pulmonary edema, PVOD, Pulmonary hemorrhage Panel 2 - NODULAR septal thickening: beaded/lumpy appearance along the septa with small round dots along the lines, labeled causes: Lymphangitic carcinomatosis, Sarcoidosis, Amyloidosis Panel 3 - IRREGULAR/DISTORTED septal thickening: irregular, crooked, distorted lines with fibrotic traction, labeled causes: UIP/IPF, Asbestosis, End-stage fibrosis Each panel shows a magnified cross-section view of lung lobules. Use dark lung background (like CT appearance), white lines for septa, clear bold text labels with colored boxes per panel (blue=smooth, orange=nodular, red=irregular). Add a title at top: Types of Interlobular Septal Thickening on HRCT. Educational medical textbook style.

Here is a complete, easy guide to the types of interlobular septal thickening with differentials:

Types of Interlobular Septal Thickening on HRCT

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Types of Interlobular Septal Thickening - Smooth, Nodular, Irregular

What Are Interlobular Septa?

These are the thin connective tissue walls that separate individual secondary pulmonary lobules. Normally they are invisible or barely visible on HRCT. When thickened, they appear as short, straight white lines at the lung periphery - the classic Kerley B lines on CXR.

The 3 Types - Easy Memory: "S-N-I"

🔵 Type 1: SMOOTH Septal Thickening

Lines are uniform, straight, and even in thickness - like a clean ruler line.
FeatureDescription
AppearanceUniform thin white lines, regular thickness throughout
DistributionTypically bilateral, basal predominant, subpleural
Key sign on CXRKerley B lines (short, horizontal, peripheral)
Differential Diagnosis - "OLPH"
CauseClue
Pulmonary Edema (most common)Cardiomegaly + pleural effusion + bilateral
PVOD (Pulmonary Veno-Occlusive Disease)No cardiomegaly, PAH features
Lymphangitic carcinomatosisCan be smooth early on
Pulmonary hemorrhageGround glass + hemoptysis
Key point: Smooth thickening = think fluid or early infiltration. Pulmonary edema is #1 cause.

🟠 Type 2: NODULAR Septal Thickening ("Beaded Septa")

Lines have small bumps/nodules along them - like a string of pearls or a beaded necklace.
FeatureDescription
AppearanceIrregular beaded/lumpy septa - nodules sit ON the lines
DistributionOften unilateral or asymmetric
Pattern"Beaded septum" sign
Differential Diagnosis - "SLAB"
CauseClue
SarcoidosisPerilymphatic nodules + upper lobe + bilateral hilar LAD
Lymphangitic CarcinomatosisKnown malignancy (breast, lung, stomach, colon)
AmyloidosisRare, systemic features
BerylliosisOccupational history (aerospace/electronics workers)
Key point: Nodular/beaded septa = think lymphatic disease (tumor or granuloma tracking along lymphatics).

🔴 Type 3: IRREGULAR Septal Thickening

Lines are distorted, broken, non-uniform, and pulled out of shape - like crumpled paper.
FeatureDescription
AppearanceIrregular, coarse, distorted lines with traction
DistributionBasal, subpleural, associated with honeycombing
Associated findingsTraction bronchiectasis, honeycombing, volume loss
Differential Diagnosis - "AURIS"
CauseClue
UIP/IPFBasal + subpleural + honeycombing, no trigger
AsbestosisOccupational exposure + pleural plaques
Rheumatoid lung (UIP pattern)RA history + joint disease
Interstitial fibrosis (any cause)End-stage fibrosis
Scleroderma lungSkin tightening, Raynaud's
Key point: Irregular thickening = think fibrosis - the lung architecture is being destroyed and distorted.

Real HRCT Comparison - From Textbooks

Normal lobular anatomy + septal veins: (Murray & Nadel, Figure 20.18 - showing normal HRCT secondary pulmonary lobule anatomy)
Normal HRCT secondary lobule anatomy showing normal interlobular septa and centrilobular arteries
Smooth septal thickening in pulmonary edema (Kerley B lines): (Murray & Nadel, Figure 108.3 - pulmonary edema CXR + CT with smooth septal thickening)
Pulmonary edema showing smooth interlobular septal thickening (Kerley B lines) on CXR and CT
Panel C (CT) shows the classic smooth peripheral interlobular septal thickening of pulmonary edema.

Quick Summary Table

TypeAppearanceTop 3 Causes
SmoothUniform, straight linesPulmonary edema, PVOD, Lymphangitic Ca
NodularBeaded (string of pearls)Sarcoidosis, Lymphangitic Ca, Amyloidosis
IrregularDistorted, broken, fibroticUIP/IPF, Asbestosis, Scleroderma

One-Line Memory Rule

"Smooth = Wet (fluid), Nodular = Dirty (lymphatics/tumor/granuloma), Irregular = Scarred (fibrosis)"
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