Respiratory Mechanisms

 In practically all living organisms, cellular respiration uses oxygen and produces carbon dioxide as waste. These gases enter or leave the bodies of all organisms by diffusion at the gaseous exchange surfaces.

      In a simple multicellular animal, lime the hydra, gaseous exchange occurs directly between the external environ and the individual body cells. In a complex animal, however, gaseous exchange occurs by diffusion at two sites:

 • the surfaces in respiratory organs like the lungs and gills , and

• The surfaces of individual body cells.

     In many multicellular organisms, air or water from the external environment is continuously circulated over the gaseous exchange surfaces. This process is known as ventilation. Mechanisms which brings about ventilation differ according to the complexity of the organisms and the medium in which the organism lives.

   I am going to discuss the various mechanisms which brings about ventilation and gaseous exchange in the respiratory systems of animals and plants.

 Note: Cellular respiration is sometimes referred to as tissue or internal respiration, while ventilation and gaseous exchange are referred to as external respiration.