The secondary pulmonary lobule is the functional unit of the lung and is the key to HRCT terminology.
Secondary pulmonary lobules measure between 1 and 2.5 cm across. They are polyhedral in shape bounded by fibrous septa (the interlobular septa) which are themselves continuous with the peribronchovascular interstitium (axial connective tissue) and pleura (peripheral connective tissue).
Each secondory pulmonary lobule is supplied by a lobular bronchiole and a pulmonary artery branch. They are drained by pulmonary veins which form in the periphery of the lobule and pass through the interlobular septa.
Within the secondary lobule, separating adjacent acini is a much less pronounced network of supporting connective tissue which forms the intralobular septa.
There are two sets of lymphatics:
- central: runs with the arteries in the peribronchovascular interstitium
- peripheral: in the interlobular septa and drains to the subpleural plexus
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