Secondary bronchi (also known as lobar bronchi) arise from the primary bronchi, with each one serving as the airway to a specific lobe of the lung. They have relatively large lumens that are lined by respiratory epithelium. There is a smooth muscle layer below the epithelium arranged as two ribbons of muscle that spiral in opposite directions. This smooth muscle layer contains seromucous glands. Irregularly arranged plates of hyaline cartilage surround the smooth muscle. These plates give structural support to the bronchus and maintain the patency of the lumen. The human lungs are the human organs of respiration. ... In zootomy, epithelium is a tissue composed of a layer of cells. ... Smooth muscle is a type of non-striated muscle, found within the walls of hollow organs; such as blood vessels, bladders, uteri. ... Cartilage is type of dense connective tissue. ... A bronchus (plural bronchi, adjective bronchial) is a caliber of airway in the respiratory tract that conducts air into the lungs. ...

References

• Gartner, Leslie P. and James L. Hiatt. Color Atlas of Histology, 3rd ed. (2000). ISBN 0-7817-3509-2
• Gartner, Leslie P. and James L. Hiatt. Color Textbook of Histology, 2nd ed. (2001). ISBN 0-7216-8806-3