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ESSAY Respiration is achieved through the mouth, nose, trachea, lungs, and diaphragm. Oxygen enters the respiratory system through the mouth and the nose. The oxygen then passes through the larynx and the trachea. The trachea is a tube that enters the chest cavity. In the chest cavity, the trachea splits into two smaller tubes called the bronchi. Each bronchus then divides again forming the bronchial tubes. The bronchial tubes lead directly into the lungs where they divide into many smaller tubes and connect to small sacs called alveoli. The average adult's lungs contain about six hundred million of the air filled sacs that are surrounded by capillaries. The inhaled oxygen passes into the alveoli and then go