Introductory Anatomy: Respiratory System The word respiration describes two processes. Internal or cellular respiration is the process by which glucose or other small molecules are oxidised to produce energy: this requires oxygen and generates carbon dioxide. External respiration (breathing) involves simply the stage of taking oxygen from the air and returning carbon dioxide to it. The respiratory tract, where external respiration occurs, starts at the nose and mouth. (Description of respiratory tract from nose to trachea here from overheads) (There is a brief complication where the airstream crosses the path taken by food and drink in the pharynx: air flows on down the trachea where food normally passes down the oesophagus to the stomach. ) The trachea (windpipe) extends from the neck into the thorax, where it divides into right and left main bronchi, which enter the right and left lungs, breaking up as they do so into smaller bronchi and bronchioles and ending in small air sacs or alveoli, where gaseous exchange occurs. The lungs are divided first into right and left, the left being smaller to accommodate the heart,
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