The mammalian respiratory system.

(a) In this overview, the ribs have been removed in front, and the major airways have been drawn in the lungs. The thoracic cavity is bounded by the ribs and intercostal muscles, and the muscular diaphragm. (b) Ciliated epithelial cells and mucus-producing goblet cells line the trachea. The mucus traps inhaled particles, and the cilia help move the mucus toward the mouth, where it can be swallowed. (c) The bronchioles deliver air to clusters of alveoli. Note the smooth muscle cells around the bronchioles, which can cause the bronchioles to constrict or dilate. Capillaries surround the alveoli. Red represents oxygenated blood; blue represents deoxygenated blood. (d) Cross-section through a cluster of alveoli. Note the single layer of alveoli cells and their close proximity to the adjacent capillaries.