Adult trachea, horizontal section healthy and crowded trachea. Simplified horizontal section through the trachea of an adult. This illustration compares the diameter of the tracheal lumen on a healthy trachea and on a congested or edematous trachea. To the left of the drawing, the tracheal lumen of a healthy adult trachea measures approximately 25 mm. The cartilaginous part of the trachea is shown in purple. The pink part represents the mucous membrane. We can clearly see on the right section of an edematous trachea that the tracheal lumen is narrowed. Nevertheless, it remains open enough to let air through. In the event of inflammation, the respiratory resistance is increased in a more moderate way compared to that of a child.