External Respiration takes place in the Trachea. When oxygen and co2 "switch" its where they connect and making this external respiration!
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Nitrogen fixation is a process in which nitrogen (N2) in the Earth's atmosphere is converted into ammonia (NH3). Atmospheric nitrogen or molecular dinitrogen (N2) is relatively inert: it does not easily react with other chemicals to form new compounds.
The answer is 4. Atria. The left and right atria are smaller chambers that pump blood into the ventricles.