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Respiratory System The respiratory system The nostrils open into the nasal cavities, which are lined with moist eiliateol epithelium. Whether one breathes through the nose or the mouth, a breath of air enters the body and flows through the nasal cavity to the pharynx (throat). The back of the nasal cavities is continuous with the throat region, or pharying. Air flows through the larynx (voice box) and into the trachea, which divides into two bronchi, one going to each lung. The trachea and bronchi are supported by C-shaped rings of cartilage that prevent the tubes from collapsing as air is drawn into the body. The bronchi divide into thousands of bronchioles. The smallest bronchioles end in clusters of microscopic air sacs or alveoli. From them, oxygen diffuses into the blood of the multitude of capillaries enveloping each air sac. At the same time, carbon dioxide diffuses from the capillaries i