Bronchioles are defined as being 1 millimeter in diameter or less; they come off the tertiary bronchi and end in terminal bronchioles. There are more than a thousand terminal bronchioles per lung. They have muscular walls that can increase or decrease their size, affecting airflow.
THE RESPIRATORY ZONE These structures are directly involved in the exchange of respiratory gases. This zone starts at the respiratory bronchiole, which is the smallest bronchiole. The bronchiole leads to an alveolar duct, which opens into a cluster of various numbers of alveoli. Figure 6 shows what the alveoli and their circulation look like:
Figure 6.
The alveolar duct is a tube made from connective tissue and alveoli. One alveolus is a small, grape-like structure that is involved in gas exchange. Many alveoli together form an alveolar sac. One alveolus is about 200 micrometers in diameter and stretch because
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