Oxygen enters the blood from the alveoli primarily through the process of diffusion. This process occurs across the thin alveolar wall, which is essential for efficient gas exchange. The oxygen diffuses from an area of higher concentration in the alveoli to an area of lower concentration in the blood within the capillaries that surround the alveoli.
In addition to diffusion, the initial transfer of oxygen also involves dissolving in the moisture lining the alveolar epithelium, which facilitates its ability to move into the bloodstream. However, while the oxygen needs to dissolve in the water to some extent, it does not enter the blood exclusively or primarily in that manner; rather, the primary mechanism is still diffusion through the alveolar wall.
Understanding this interplay is vital because it illustrates the efficiency of the respiratory system, where the design and function of the alveoli allow for rapid gas exchange with minimal barriers.