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 into the air sacs and is exhaled through the respiratory tubes. Both the trachea and bronchi are lined by a mucous membrane containing ciliated cells. Gas exchange occurs into the alveoli of the lungs. The lungs are large paired spongy organs in the thoracic cavity. The right lung is divided into three lobes, and the left into two lobes. Each lung is covered with the pleural membrane that forms a sac enclosing the lung and the lining of the chest cavity. The lungs consist largely of air tubes and elastic tissue. They are spongy, elastic organs with a very large surface area for gas exchange. Breathing is a process of moving air from the environment into the lungs and expelling air from the lungs. Inhaling air is referred to as inspiration, exhaling is referred as expiration. An adult breathes about 12 times each minute. During inspiration, the chest cavity is expanded by the contraction of the diaphragm, the muscle of the thoracic cavity. When the diaphragm contracts, it moves downward, increasing the volume of the thoracic cavity.