This chapter discusses various aspects of the respiratory system, which consists of the lungs, and the passages through which air reaches them. Receptors for smell are located in the olfactory mucosa which is confined to a relatively small area on the superior nasal concha, and on the adjoining part of the nasal septum. Blood flowing through the network warms inspired air and Variations in blood flow can cause swelling or shrinkage of the mucosa. Subepithelial aggregations of lymphoid tissue are present especially on the posterior wall of the nasopharynx, and around the orifices of the auditory tubes, forming the nasopharyngeal and tubal tonsils. The lumen of the trachea is lined by mucous membrane that consists of a lining epithelium and an underlying layer of connective tissue. The amount of muscle in the bronchial wall increases as the bronchi become smaller. The presence of muscle in the walls of bronchi is of considerable clinical significance.