Terminal Respiratory Unit (Pulmonary Acinus)

From: Fishman's Pulmonary Diseases and Disorders, 5th Edition

1. Definition

"The acinus is defined as the parenchymal unit in which all airways have alveoli attached to their wall and thus participate in gas exchange. Along the airway tree, the acinus begins with a transitional bronchiole."

• Fishman's, Chapter 2

2. Position in the Airway Tree

3. Components of the Acinus

a. Transitional Bronchiole (z' = 0)

• The entry point of the acinus (located at approximately generation 14-15 of the airway tree)
• Marks the boundary between purely conducting airways and gas-exchanging airways
• Inner diameter: ~0.50 mm; length: ~1.4 mm

b. Respiratory Bronchioles (z' = 1-3)

• The first three generations of acinar airways following the transitional bronchiole
• Have only a few alveolar openings in their walls - so named because they participate partially in gas exchange
• Walls are lined by cuboidal epithelium with cilia, interspersed with alveolar patches
• The first three acinar generations (respiratory bronchioles) account for 3.4 mm of the total 8.3 mm acinar path length

c. Alveolar Ducts

• Deeper generations of the acinus where alveoli line the entire circumference of the airway wall
• Duct lumen is bounded on all sides by alveolar mouths
• Segment lengths decrease progressively from ~1330 μm to ~640 μm in peripheral generations

d. Alveolar Sacs

• The terminal blind-ending structures
• Clusters of alveoli opening into a common space
• The path length from the transitional bronchiole through respiratory bronchioles, alveolar ducts, to the end of alveolar sacs = 8.3 ± 1.4 mm (average longitudinal path length of the human acinus)

4. Morphometric Data - Typical Path Model of Human Acinus

• Mean acinus volume: 187 mm³ (SD = 79 mm³)
• Number of branching generations within acinus: ~8
• Terminal alveolar sacs per acinus: 256
• Total longitudinal path length (transitional bronchiole to alveolar sac): 8.3 ± 1.4 mm

5. Secondary Lobule vs Acinus

• The secondary lobule (~1 cm³, visible on lung surface, delineated by connective tissue septa) comprises approximately a dozen acini
• Secondary lobules are supplied by airway branches ~1 mm in diameter
• The secondary lobule is the unit visible on HRCT as the "secondary pulmonary lobule"

6. Ventilation Within the Acinus - Two-Step Mechanism

1. Bulk convective flow: Upon inspiration, O₂-rich air flows through the airways into the acinus, carrying O₂ by convection
2. Diffusion in air phase: In the peripheral acinar airways, flow velocity slows because airway cross-section increases enormously. O₂ now moves toward the periphery by molecular diffusion in the gas phase, driven by the PO₂ gradient established as O₂ is absorbed at the alveolar surface

7. Gas Exchange Units Within the Acinus

• Is individually perfused - corresponds to the unit capillary network spanning between a pulmonary arteriole and venule
• Is roughly disk-shaped, ~500 μm in diameter
• Has a surface area approximately equivalent to one alveolus
• Spans over several alveoli (and each alveolus is in contact with several capillary units)

8. Alveolar Wall (Cellular Composition)

• Type I and Type II cells occur in a numeric ratio of ~1:2 (Type II are more numerous by number, but Type I cover far more surface area)
• The air-blood barrier has a thin part (Type I cell + fused basement membrane + endothelium) and a thick part (containing nuclei and connective tissue fibers)
• About half of the barrier surface is optimized (thin parts); the other half contains structural support elements

9. Alveolar Numbers and Surface Area (Morphometric Summary)

• Total number of alveoli in adult human lung: ~480 million (some older estimates ~300 million, Fishman's cites ~400 million gas-exchange units)
• Total alveolar surface area: ~143 m² (size of a tennis court)
• Mean alveolar diameter: ~200 μm

10. Clinical Relevance

• Emphysema: Panacinar emphysema destroys the entire acinus (alveolar ducts, sacs, and respiratory bronchioles); centrilobular/centriacinar emphysema preferentially destroys the proximal respiratory bronchioles (acinar center), sparing distal alveolar ducts initially
• Gas exchange impairment: The serial-ventilation design of the acinus means diffusion limitation at high acinar path lengths contributes to alveolar-arterial PO₂ gradients even in normal lungs
• Diffusing capacity (DL): Morphometric DLO₂ (~60 mL O₂/min/mmHg at rest) has ~2× reserve; only fully exploited during heavy exercise or hypoxia