The conducting division has thicker walls, and in most places it has smooth muscle, a glandular, ciliated pseudostratified columnar epithelium, and cartilage. These structures are functionally important; the smooth muscle allows for the diameter of the airway to be adjusted to the body’s state of activity and need for ventilation; the glandular ciliated epithelium allows for the secretion and movement of mucus to trap debris in the air stream; and the cartilage ensures that the airway does not collapse under the negative pressure produced during inspiration. The respiratory division, by contrast, has alveoli, which are minute air sacs lined mainly by a very thin simple squamous epithelium. The thinness of this epithelium allows for rapid exchange of gases between the air and blood. It would be poorly suited to the more proximal parts of the respiratory tract because it would not provide adequate supportive and other functions, whereas the thick epithelium and subepithelial tissues of those parts of the tract would be an obstacle to gas exchange if they continued into the alveoli.
The highest ratio of ciliated to goblet cells occurs in the lower trachea. Mucus secreted by the goblet cells tends to drain downward by gravitational pull, and a high density of ciliated cells in the lower trachea serves to “catch” this mucus and propel it back upward to the pharynx.
The arteries ultimately serve to deliver blood to the capillaries and the airway serves ultimately to deliver air to the alveoli; the capillaries and alveoli are where the most physiologically important events in these two systems occur. It is important to be able to regulate the supply of blood and air reaching these extreme points. The arterioles and bronchioles are the most effective points for controlling the flow of these fluids because (a) they are the most numerous divisions of the respective systems preceding the “business end,” so they provide the most points of control, and (b) both are capable of dilation and constriction, so they can vary the fluid flow from moment to moment.
Elevated blood pressure in the alveolar capillaries causes serous fluid to filter out of the capillaries and into the alveoli. As the alveoli fill with liquid, oxygen cannot diffuse as quickly as normal from the inhaled air to the blood stream. Blood leaving the lungs is therefore oxygen-deficient (hypoxemic), and deoxygenated blood has a relatively dark color that imparts a bluish cast (cyanosis) to the skin and mucous membranes.
In this incident (from Morbidity and Mortality Weekly Reports), the nasoenteric tube was misdirected down the woman’s trachea and bronchus instead of her esophagus. It pierced the lung surface and admitted air into the pleural cavity. The patient exhibited pneumothorax and atelectasis, and soon died.
