Sentence:  The intra nasal instillation of a fluid suspension of infectious agent in an anesthetized animal is far different from exposure , through natural <WSD>respiration</WSD> , to aerosolized organisms . 
Meaning of the word marked with <WSD> tags:  the metabolic processes whereby certain organisms obtain energy from organic molecules; processes that take place in the cells and tissues during which energy is released and carbon dioxide is produced and absorbed by the blood to be transported to the lungs