External respiration is primarily responsible for the exchange of oxygen and carbon dioxide in the lungs. This process occurs in the alveoli, which are tiny air sacs within the lungs where oxygen from inhaled air passes into the blood, and carbon dioxide from the blood is expelled out of the lungs to be exhaled. This gas exchange is vital for maintaining the body’s oxygen supply and removing waste gases, thus playing a critical role in the respiratory system’s function.
While the movement of blood through the circulatory system, absorption of nutrients in the digestive system, and filtration of air in the nasal cavity are all important biological processes, they do not directly pertain to external respiration. Instead, they belong to the circulatory, digestive, and respiratory functions respectively, indicating the specialized roles that different systems play in maintaining overall homeostasis in the body.