Where does the external respiration take place in pets?

In mammals (like cat and dog pets), external respiration takes place by the exchange of gases in the alveoli of the lungs  

Explanation:

External respiration refers to the process of exchange of gases, i.e., oxygen (O2) and carbon dioxide (CO2), with the environment. In mammals like humans, dogs and cats, external respiration occurs in the millions of alveoli in the lungs and the capillaries that are located in the walls of the alveoli. This process (gas exchange) happens due to simple diffusion. The O2 is taken from the environment (by inhalation) and travels from the alveoli to the blood in the capillaries that surround the alveoli, while and CO2 diffuses from the blood capillaries to the alveoli (where this gas is exhaled).

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organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.