From The Cell p.1274

Figure 22-18 Respiratory epithelium

The goblet cells secrete mucus, which forms a blanket over the tops of the ciliated cells. The regular, coordinated beating of the cilia sweeps the mucus up and out of the airways, carrying any debris that is stuck to it. The mechanism that coordinates the ciliary beating is a mystery, but it seems to reflect an intrinsic polarity in the epithelium. If a segment of rabbit trachea is surgically reversed, it carries on sweeping mucus, but in the wrong direction, back down toward the lung in opposition to adjacent unreversed portions of trachea.