The
pulmonary lobule is the portion of the
lung ventilated by one bronchiole. Bronchioles are approximately 1 mm or less in diameter and their walls consist of
ciliated cuboidal
epithelium and a layer of
smooth muscle. Bronchioles divide into even smaller bronchioles, called
terminal, which are 0.5 mm or less in diameter. Terminal bronchioles in turn divide into smaller respiratory bronchioles which divide into
alveolar ducts. Terminal bronchioles mark the end of the conducting division of air flow in the
respiratory system while respiratory bronchioles are the beginning of the respiratory division where
gas exchange takes place. The diameter of the bronchioles plays an important role in air flow. The bronchioles change diameter to either increase or reduce air flow. An increase in diameter is called
bronchodilation and is stimulated by either
epinephrine or
sympathetic nerves to increase air flow. A decrease in diameter is called
bronchoconstriction, which is the tightening of the smooth muscle surrounding the bronchi and bronchioles due to and stimulated by
histamine,
parasympathetic nerves, cold air, chemical irritants, excess mucus production, viral infections, and other factors to decrease air flow. Bronchoconstriction can result in clinical symptoms such as wheezing, chest tightness, and dyspnea, which are common features of asthma, chronic obstructive pulmonary disease (COPD), and chronic bronchitis.
Bronchioles The
trachea divides into the left main
bronchus which supplies the left lung, and the right main bronchus which supplies the right lung. As they enter the lungs these primary bronchi branch into secondary bronchi known as
lobar bronchi which supply each lobe of the lung. These in turn give rise to tertiary bronchi (
tertiary meaning "third"), known as
segmental bronchi which supply each
bronchopulmonary segment. These bronchioles mark the end of the
conducting zone, which covers the first division through the sixteenth division of the
respiratory tract.
Alveoli only become present when the conducting zone changes to the
respiratory zone, from the sixteenth through the twenty-third division of the tract.
Terminal bronchioles The terminal bronchioles are the most distal segment of the conducting zone. They branch off the lesser bronchioles. Each of the terminal bronchioles divides to form respiratory bronchioles which contain a small number of alveoli. Terminal bronchioles are lined with
simple ciliated cuboidal epithelium containing
club cells. Club cells are non-ciliated, rounded protein-secreting cells. Their secretions are a non-sticky, proteinaceous compound to maintain the airway in the smallest bronchioles. The secretion, called
pulmonary surfactant, reduces
surface tension, allowing for bronchioles to expand during inspiration and keeping the bronchioles from collapsing during expiration. Club cells are a
stem cell of the
respiratory system, and also produce
enzymes that detoxify substances dissolved in the respiratory fluid.
Respiratory bronchioles The respiratory bronchioles are the narrowest airways of the lungs, 0.5 mm across. The
bronchi divide many times before evolving into the bronchioles. The respiratory bronchioles deliver air to the exchange surfaces of the lungs. They are interrupted by
alveoli which are thin walled
evaginations.
Alveolar ducts are side branches of the respiratory bronchioles. The respiratory bronchioles are lined by ciliated cuboidal epithelium along with some non-ciliated cells called
club cells. ==Clinical significance==