172 



ELEMENTS OF BIOLOGY 



carbon dioxide from, the blood. The fundamental processes of respi- 

 ration whereby energy is transformed occur in the tissue cells of the 

 entire body. While at rest the demand of the body cells for oxygen 

 and their output of carbon dioxide are fairly constant. But with 



HYOID BONE 



fVfiiwj^v^ 



TRACHEA 



EPIGLOTTIS 



LARYNX- 



LEFT PLEURA 

 CA VITY 



\ 



BRONCHI 



ALVEOLI 



DIAPHRAGM 



Fig. 115. — Diagrammatic representation of the human respiratory system. The 

 wall of the thorax has been removed and a portion of the left lung cut away. 

 (Partly after Sobotta: Atlas and Textbook^ oj Human Anatomy, jiublished by W. B. 

 Saunders Co.) 



exercise the oxygen demand and the carbon dioxide output are 

 increased. The rate of inhaling and exhaling is controlled by a nerve 

 centre located in the medulla oblongata, the most posterior region 

 of the brain. The increase in carbon dioxide output and the oxygen 

 deprivation during exercise alters the blood chemically and thus 

 stimulates this centre, which in turn by stimulating the proper 



