Lavoisier’s Respiration, 1784 – The Animal Machine

Scientists now know that respiration produces the energy the body needs through oxidation of substances such as proteins and carbohydrates. 

Important chemical reactions in this process occur in mitochondria within cells that use oxygen during respiration. 

1. Breathing brings oxygen into our lungs.
2. Haemoglobin in blood cells carries oxygen to the body’s cells.
3. We need oxygen to produce energy from food.
4. Food is broken down into small molecules for energy production.
5. These molecules are processed by structures called mitochondria inside our cells.
6. Chemical reactions needed for energy production occur in the mitochondria.
7. The process produces energy-storing molecules called ATP.
8. ATP supplies our cells’ energy needs.
9. Carbon dioxide produced during energy generation is breathed out.

Body Heat and Respiration

One of the first people to connect respiration, body heat, perspiration and digestion was French chemist Antoine Lavoisier. He described the human body as an “animal machine” based on respiration, which produces body heat alongside perspiration and digestion, and stated that the system operates as a cycle.

In 1784, Lavoisier and mathematician-astronomer Simon de Laplace invented a device to measure the heat and carbon dioxide produced by living guinea pigs. Lavoisier found that animals produced measurable amounts of heat and “fixed air” (now known as carbon dioxide) while converting oxygen in air into carbon dioxide. He also connected the gases involved in respiration with those produced and consumed when a candle burns.

Changing Oxygen Consumption

Lavoisier also showed that the human body consumes more oxygen during exercise than at rest. He suggested that the energy cost of almost any human activity, such as giving a speech or playing a musical instrument, could be determined. Even the mechanical energy expended while a philosopher thinks could, in his view, be assessed.

Sadly, Lavoisier was guillotined during the French Revolution. According to scientist C. S. Minot: “Compared with the development of science, changes of government are minor events. There is no true measure of values until it is clearly understood that the French Revolution’s gravest crime was not the execution of the king, but that of Lavoisier.”

Lavoisier is considered one of the founders of modern chemistry and made important contributions to the discovery of oxygen and hydrogen.

Oxygen: Through his contribution to discovering oxygen, Lavoisier helped bring about an important change in 18th-century chemistry: collapse of phlogiston theory. The theory sought to explain combustion by proposing that substances lose a material called “phlogiston” when burned. In experiments in the 1770s, Lavoisier showed that substances gain weight during combustion through interaction with air. He named the new element “oxygen”, disproving phlogiston theory.

Hydrogen: Lavoisier’s contribution to discovering hydrogen involved understanding its role in the composition of water. In 1783, he and fellow scientist Pierre-Simon Laplace performed experiments showing that burning hydrogen produces water. These demonstrated that hydrogen is a principal component of water and that water consists of hydrogen and oxygen—an important step in understanding chemical compounds.

Lavoisier revolutionised chemistry through this work, profoundly influencing the discovery and understanding of elements. His contributions to discovering oxygen and hydrogen played an important role in establishing the foundations of chemistry.

Article image caption: A portrait of Antoine Lavoisier and his wife, chemist Marie-Anne Pierrette Paulze, painted by French artist Jacques-Louis David.

Historical figures mentioned: Antoine-Laurent de Lavoisier (1743–1794), Pierre-Simon, Marquis de Laplace (1749–1827)

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