Development and Anatomy
Alveolar parenchyma--The terminal bronchiole is the last membranous branch before the respiratory bronchioles, which have alveoli around part of their circumference. In turn, they give rise to alveolar ducts, which are completely surrounded by alveoli. The respiratory bronchioles, alveolar ducts, and alveoli distal to a terminal bronchiole comprise an acinus . A group of acini delimited by interlobular septa is known as a lobule. Interlobular septa are most prominent at birth, but in the adult are present only in a narrow subpleural zone.
The interlobular septum with veins (V) is at the bottom right. Two acini indicated by centriacinar airways (B) and pulmonary arteries (A) lie above. Two or more acini comprise a lobule.
Alveoli (below) are lined by flat type I cells, which cannot be seen with the light microscope, and plump type II cells, which can be seen . Normally, each alveolus has one type I cell and 2 type II cells. The type II cell secretes surfactant and is the cell that undergoes proliferation after injury, having the capacity to differentiate into a type I cell. The alveolar wall has capillaries and connective tissue, including elastic fibers. The capillary basement membrane is focally fused with that of the epithelial cell to facilitate gas exchange. The alveoli contain a few alveolar macrophages, which represent the first line of defense against foreign particles.
The photo shows an alveolar wall with a capillary containing a red blood cell.
The electron micrograph (below) shows a cuboidal type II cell with multiple lamellated, surfactant-containing bodies (arrow and inset). Note the short surface microvilli.
Vessels--Normal postmortem angiogram: Note the size of the main pulmonary artery and the gradual tapering and uniform filling of the fine branches. In pulmonary hypertension the fine branches are lost, giving a pruned effect. (The white band at the mid right is the cannula.)