Diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration. In the case of oxygen passing into the blood from the alveoli, this process is responsible for the movement of oxygen molecules across the respiratory membrane.

Alveoli:
The alveoli are tiny air sacs located at the end of the bronchioles in the lungs. They are surrounded by a dense network of capillaries, which are small blood vessels. The walls of the alveoli are very thin, allowing for efficient gas exchange between the air and blood.

Difference in Concentration:
Inside the alveoli, the concentration of oxygen is higher compared to the concentration in the blood capillaries. This difference in concentration sets up a concentration gradient, which drives the movement of oxygen molecules across the respiratory membrane.

Respiratory Membrane:
The respiratory membrane is the barrier between the air in the alveoli and the blood in the capillaries. It consists of the alveolar epithelium, the capillary endothelium, and their basement membranes. This membrane is very thin, allowing for easy diffusion of gases.

Diffusion of Oxygen:
When we breathe in, oxygen from the air enters the alveoli. The oxygen molecules then diffuse across the respiratory membrane into the capillaries. This diffusion occurs because the oxygen molecules move from an area of higher concentration (alveoli) to an area of lower concentration (blood capillaries).

The oxygen molecules move through the thin walls of the alveoli, then through the basement membranes, and finally through the walls of the capillaries. This movement is driven by the concentration gradient and the random motion of oxygen molecules.

Role of Diffusion in Oxygen Transport:
Once the oxygen molecules diffuse into the blood capillaries, they bind to the hemoglobin molecules in red blood cells. This forms oxyhemoglobin, which is then transported to the body tissues where oxygen is released for cellular respiration.

In summary, oxygen passes into the blood from the alveoli by the process of diffusion. The concentration gradient between the alveoli and the blood drives the movement of oxygen molecules across the respiratory membrane. Diffusion is a passive process that does not require energy expenditure.