At this stage oxygen along with the air enters the lungs from the environment. Inspiration occurs in two steps.
(A) First step: At this time the nasal apertures remain open but the mouth aperture and glottis remain closed.
Simultaneously the lungs contract and the floor of the buccal cavity are lowered. As a result, the volume of the buccal cavity increases. The oxygenated air from outside enters the buccal cavity. At this time exchange of oxygen and carbon-dioxide takes place in the blood of capillaries situated in the walls of the buccal cavity and pharynx.
(B) Second step: At this stage the nasal apertures are closed and the floor of the buccal cavity is raised. As a result, oxygenated air enters the alveoli of the lungs. By the diffusion process oxygen from the cavities of the alveoli enters the blood capillaries close to the walls of the alveoli. In a similar process carbon-dioxide is released.