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Respiration is breathing, i.e. inhaling and exhaling. More specifically, it is the exchange of gases between the external environment and an organism's cells. [1][2]

Respiration is achieved through the mouth, nose, trachea, lungs, and diaphragm.[3] Inhalation is initiated by the diaphragm[4] and supported by external intercostal muscles; oxygen enters the respiratory system through the mouth and the nose.

The oxygen then passes through the larynx (where speech sounds are produced) and the trachea, which 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 to form the bronchial tubes. The bronchial tubes lead directly into the lungs where they divide into many smaller tubes which connect to tiny sacs called alveoli. The average adult's lungs contain about 600 million of these spongy, air-filled sacs that are surrounded by capillaries.

The inhaled oxygen passes into the alveoli and then diffuses through the capillaries into the arterial blood. Waste-rich blood from the veins releases carbon dioxide into the alveoli. The carbon dioxide follows the same path out of the lungs during exhalation.[5]


  1. McGraw-Hill Concise Dictionary of Modern Medicine (2002). Respiration. McGraw-Hill. Retrieved on 2008-10-01.
  2. unk. (n.d.). Respiratory Disorders Glossary. Retrieved on 2008-10-01.
  3. Franklin Institute (n.d.). Respiratory System: Oxygen Delivery System. The Franklin Institute. Retrieved on 2008-10-02.
  4. UCFSD (n.d.). Respiratory System. Retrieved on 2008-10-02.
  5. various (n.d.). Respiratory System. Wikipedia. Retrieved on 2008-10-02.