The Respiratory Unit of the Lungs
The acinus is the respiratory unit of the lungs, defined as the portion of lung parenchyma distal to a terminal bronchiole, consisting of respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli where gas exchange occurs. 1
Structural Definition
The acinus represents the functional gas-exchanging unit with the following characteristics:
- Begins at the first respiratory bronchiole (the first airway generation that carries alveoli on its walls) and extends distally to include all subsequent branching airways and alveoli 1
- Measures approximately 5 mm in diameter in human lungs 1
- Contains approximately 300 million alveoli distributed across all acini in the adult human lung 2
Components of the Acinus
The acinar airways have distinct structural features:
- Intra-acinar airways continue to branch but their walls become progressively reduced to networks of alveolar entrance rings as part of the axial fiber system 1
- Alveolar ducts are airways that continue to divide without reaching the terminal alveolar sac, with walls consisting entirely of alveolar openings 1
- The gas exchange apparatus forms a sleeve of alveoli on the surface of approximately eight generations of the most distal airways 3
- Terminal alveolar sacs represent the final generation where airways terminate 1
Functional Significance
The acinus is where actual gas exchange occurs:
- The air-blood barrier within the acinus consists of alveolar epithelium, capillary endothelium, and their shared basement membrane 3
- Gas exchange surface area ranges from 40 to 80 square meters in adult human lungs, depending on lung size 2
- Alveolar capillaries form a mesh surrounding each alveolus, composed of two specialized endothelial cell types (aerocytes for gas exchange and gCap cells for vasomotor regulation) 4
Hierarchical Context
Understanding the acinus requires recognizing its position in the airway tree:
- Conducting airways (generations 0-16 approximately) have multilayered walls with mucous membrane, smooth muscle, and cartilage but do not participate in gas exchange 3, 5
- The terminal bronchiole is the last purely conducting airway before the acinus begins 1
- Respiratory bronchioles mark the transition zone where alveoli first appear on airway walls, defining the start of the acinus 1
Clinical Relevance
The acinus is the primary site affected in many lung diseases:
- Centriacinar emphysema destroys respiratory bronchioles, alveolar ducts, and alveoli at the center of the acinus, with surrounding alveoli remaining relatively intact initially 6
- Panacinar emphysema involves destruction of the entire acinus uniformly, affecting all components from respiratory bronchioles through alveoli 6
- Morphometric assessment of acinar structure requires serial section reconstruction or acinar casts to maintain connectivity of the units 1