In 1789, Antoine Lavoisier, a French nobleman and chemist,
underscored the significance of oxygen and carbon dioxide in the process of
breathing. Despite his groundbreaking contributions to modern chemistry,
Lavoisier faced the guillotine's blade in 1794.
It wasn't until the twentieth century that the intricate roles of
these gases in metabolizing energy-rich compounds like carbohydrates and fats,
as well as their involvement in cellular respiration, became fully elucidated.
Across the spectrum of living organisms, from unicellular bacteria
to mammals, respiration entails the exchange of gases between the organism's
interior and the external environment. Oxygen is taken in, while carbon
dioxide, a byproduct of metabolism, is expelled. This gas exchange relies on
diffusion, wherein gases move from areas of higher concentration to lower
concentration across a respiratory surface. While the process remains
fundamentally similar, the respiratory surfaces vary among species.
In single-celled organisms like bacteria, gases traverse the cell
membrane readily. Earthworms and amphibians conduct gas exchange through their
skin, while insects utilize spiracles leading to tracheal tubes on their body
surface. Fish, meanwhile, extract dissolved oxygen from water through their
gills, which boast extensive surface areas rich in capillaries. Oxygen and
carbon dioxide are exchanged in opposite directions as water flows over the
gills.
In mammals, blood serves as the carrier for oxygen and carbon
dioxide exchange. This exchange occurs across the alveoli, expansive
respiratory surfaces resembling tennis courts, within the lungs.
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| A goldfish actively engaged in oxygen–carbon dioxide gas exchange across its gills. |
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