What occurs during external respiration?

During external respiration, the primary function is the exchange of gases between the air and the bloodstream. This process takes place in the alveoli of the lungs, where oxygen (O2) present in the inspired air diffuses across the alveolar membrane into the blood. This diffusion occurs because of the difference in partial pressures of oxygen between the alveoli and the blood—oxygen moves from an area of higher concentration (in the alveoli) to an area of lower concentration (in the blood).

This key aspect of external respiration highlights its role in ensuring that the body receives adequate oxygen necessary for cellular metabolism while simultaneously facilitating the removal of carbon dioxide (CO2) from the blood, which is then expelled from the body during expiration. Thus, the statement about O2 entering the body through contact with the alveoli accurately describes the fundamental mechanism of external respiration.