Why is the concentration gradient for oxygen and carbon dioxide in the lungs?

The concentration gradient for oxygen and carbon dioxide in the lungs is established due to differences in partial pressures of these gases between the alveoli and the blood. Oxygen has a higher partial pressure in the alveoli compared to the deoxygenated blood in the pulmonary capillaries, facilitating its diffusion into the blood. Conversely, carbon dioxide, which has a higher partial pressure in the blood, diffuses into the alveoli to be exhaled. This gradient is essential for effective gas exchange during respiration.