Organization of the Respiratory System

For millennia, people have regarded life as being synonymous with breathing. Life begins and ends with breathing. The Bible states that God " breathed into [Adam's] nostrils the breath of life, " and then later used part of Adam's ventilatory apparatus — a rib — to give life to Eve. In the fourth and fifth centuries BC, writings attributed to Hippocrates suggested that the primary purpose of breathing is to cool the heart. It was not until the 18th century that the true role of breathing began to emerge as several distinguished investigators studied the chemistry of gases. At the time, chemists recognized similarities between combustion and breathing, but thought that both involved the production of a " fire-essence " principle called " phlogiston. " According to their theory, neither combustion nor life could be supported once air became saturated with phlogiston. In the 1750s, the Scottish scientist Joseph Black found that heating calcium carbonate produces a gas he called " fixed air, " now known to be carbon dioxide (CO 2). This work revolutionized chemistry. It showed that a chemical reaction can involve a gas, and it further demonstrated that other gases exist besides ordinary " air. " Shortly thereafter, Cavendish, working in England, showed that fermentation and putrefaction produce " fixed air. " His countryman Priestley discovered several new gases between the late 1760s and mid-1770s, including " dephlogistonated air, " co-discovered by the Swedish chemist Scheele. Priestley found that combustion, putrefac-tion, and breathing all consume " dephlogistonated air, " and all reduce the volume of room air by approximately 20%. Conversely, he found that green plants produce " dephlogistonated air, " which he could quantitate by reacting it with nitric oxide (a colorless gas) to produce nitrogen dioxide (a red gas). In the mid-1770s, Priestley toured the Continent and presented his findings to the Frenchman Lavoisier, who is often regarded as the father of modern chemistry. Lavoisier quickly put Priestley's empirical observations into a theoretical framework that he used to demolish the phlogiston theory, which Priestley held to his death. Lavoisier recognized that dephlo-gistonated air, which he named oxygen (O 2), represents the 20% of room air consumed by combustion in Priestley's experiments, leaving behind " non-vital " air, or nitrogen. Furthermore, he proposed that O 2 is consumed