This page outlines/reviews the basics about the airways leading into the lungs.
Inhaled air first flows through the:
The nasal passages help moisten, warm and clean the air flowing through. Note that the conchae increase the surface area. Also, vigorously suck some air in through your nasal passages. Notice how the conchae create turbulence? Here it is desirable, unlike in the circulatory system, since turbulence creates more contact between the air and the epithelium. And little energy is required to breathe at sea level. The epithelium has cilia, which move the mucus with any trapped particles in the direction of the pharynx.
The pharynx (nasopharynx, oropharynx, laryngopharynx) includes substantial MALT (tonsils).
The trachea has cartilage bands that do not completely encircle the airways, leaving the posterior flexible. The larger bronchi have a similar structure to the trachea. Moving into the smaller bronchi, the cartilage is in the form of plates (as you can observe in the lung slide #48 in lab). In all these airways, smooth muscle and submucosal glands in the walls are an important issue, and they are discussed below.
QUESTION: Unlike the stiffening of a vacuum cleaner hose, the cartilage in the trachea and large bronchi does not completely circle the airways, leaving the posterior flexible. What is the advantage of this? Answer
Then inhaled air moves into airways without cartilage:
After several further branchings these are called terminal bronchioles. Smooth muscle is important through these airways.
A few alveoli branch off the next several branches. These airways are termed respiratory bronchioles. The epithelium becomes cuboidal and lacks cilia.
Alveolar ducts have no airway epithelium. Their structure is purely from the alveoli, which is the subject of the next page.
A CT scan is computed from a series of x-ray images to show a slice through the body. The CT scan shown to the right shows a cross section through upper lobes of lungs.
A similar CT scan, but slightly lower.
The branching of the pulmonary artery follows the branching of the bronchi. In this pulmonary angiogram, a radio-opaque dye was injected from a catheter into the right pulmonary artery. The branches of the artery are clearly visible. They parallel the airways.
At the left is a histological section showing smallest airways cut longitudinally.
Again, when airways no longer have any cartilage in their walls, they are called bronchioles. The last of those without alveoli are called terminal bronchioles (TB). These lead into respiratory bronchioles (R), which have a few of the thin-wailled alveoli (A) branching from them. After the respiratory bronchioles, the inhaled air enters a cluster of alveoli. An open space leading down through the cluster of alveoli is called an alveolar duct (AD).
The airways are lined by a ciliated epithelium.
The diagram to the right illustrates the organization of the epithelial cells and glands in the trachea, where the epithelium is classified as a pseudostratified, simple columnar ciliated epithelium. The basal cells regenerate the other cells.
Goblet cells within the epithelium release mucus. Under the epithelium are submucosal glands that secrete both mucus and fluid.
The factors controlling mucus secretion by the goblet cells are
not well understood.
To the left is a micrograph of the epithelium, but with a different histological stain than the slides we use in lab. Note the lamina propria too.
Parasympathetic nerves cause contraction of the smooth muscle.
Sympathetic nerves cause relaxation of the smooth muscle and thus dilation of the airways.
Inflammatory paracrines stimulate contraction. Here we will emphasize histamine and especially leukotrienes .
An increase in carbon dioxide leads to dilation of the airways. This increases airflow into poorly ventilated regions of the lung.
Neural reflexes occur through stimulation of sensory afferents in the airways, which can lead to stimulation of parasympathetic neurons and thus constriction of the airways. The afferents in these neural reflexes are stimulated by irritating stimuli or inflammation. The reflexes may pass through the central nervous system or take place purely in the walls of the airways.
QUESTION: Review: What substance is cleaved from a membrane phospholipid by phospholipase A2? Then, what enzyme acts on this substance to begin the synthesis of leukotrienes? Answer
The submucosal glands are regulated especially by:
The level of hydration of the mucus in the airways is determined by the amount of water secreted across the epithelium lining the airways. The amount of water secreted is determined by the net movement of ions across the epithelium. When there is a greater net movement of ions, more water follows by osmosis.
In general, the net movement of chloride ions from the interstitial fluid into the lumen tends to be the regulatory factor promoting secretion of fluid into the lumen.
Chloride ions enter the epithelial cells from the interstitial fluid via a cotransporter, which also carries sodium ions and potassium ions. Chloride ions enter the lumen via a regulated chloride channel. A second messenger, cyclic AMP, activates a kinase that phosphorylates the chloride channel, which leads to its opening.
Note that the transporters and ion channels are completely different in the plasma membrane facing the lumen and the plasma membrane facing the interstitital fluid. This is usually the case in epithelia.
What happens in cystic fibrosis?