This page outlines/reviews the basics about the airways leading into the lungs.

Large Airways

Inhaled air first flows through the:

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:

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).

Structure of Epithelium

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.

Regulation of Airway Smooth Muscle

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

Regulation of Submucosal Glands

The submucosal glands are regulated especially by:

Fluid Transport in Epithelium

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?

Quick Quiz

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Correct Answer

What type of airway receives air from a terminal bronchiole?

What is the name of the smallest airway? (It has no airway epithelium.)

Do leukotrienes cause contraction or relaxation of smooth muscle in airways?

Do sympathetic nerves cause contraction or relaxation of smooth muscle in airways?

Do parasympathetic nerves stimulate or inhibit secretion of submucosal glands in airways?

What ion moves through the ion channel that is opened by phosphorylation to increase fluid secretion in an airway?

(Spelling must be correct)