Question

Explain external and internal breathing

Answer 1

Gas exchange occurs by diffusion. ...
External respiration occurs in the lungs where oxygen diffuses into the blood and carbon dioxide diffuses into the alveolar air.
Internal respiration occurs in the metabolizing tissues, where oxygen diffuses out of the blood and carbon dioxide diffuses out of the cells.

Simply

Internal respiration is the transfer of gas between the blood and cells.
External respiration also known as breathing refers to a process of inhaling oxygen from the air into the lungs and expelling carbon dioxide from the lungs to the air. Exchange of gases both in and out of the blood occurs simultaneously.