Internal vs. external respiration

Written by liz veloz
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Internal vs. external respiration
Lungs are the main organs involved in respiration for most animals. (Image by, courtesy of Rob and Stephanie Levy)

Dividing the process of respiration into two categories, internal and external respiration, can help clarify some of the complexities of this biologically important process. Together, the different parts of the respiration process allow organisms to exchange gases with their environments and convert energy into a form cells can use.


All living organisms perform some form of respiration to make energy and exchange gases.

External Respiration

External respiration in most organisms is inhalation and exhalation, or breathing.

Internal Respiration

Internal respiration has two parts: gas exchange and cellular respiration, a chemical reaction where sugar is converted into chemical energy.

Gas Exchange

Gas exchange, or the "exchange" of carbon dioxide and oxygen, occurs in animals across a respiratory surface. In humans this surface is the fine, thin capillaries in our lungs. In fish, it occurs across the gills, and some organisms exchange gases through the skin. In plants, gas exchange occurs through pores in the leaves called stomata.

Cellular Respiration

Cellular respiration, the second phase of internal respiration, is a chemical reaction that requires oxygen to convert sugar into chemical energy cells need to function. Carbon dioxide is produced during this conversion and is then exhaled.

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