Final answer:
Gas diffusion in the lungs is mainly dependent on the partial pressure difference between the air in the alveoli and the blood, a gradient enabling oxygen and carbon dioxide to efficiently diffuse across the respiratory membrane.
Step-by-step explanation:
The gas diffusion at the lung, also known as external respiration, is principally dependent on the partial pressure difference between the air within the alveoli and the blood in the pulmonary capillaries. This difference in partial pressure creates a gradient that allows oxygen to diffuse from the alveoli, where its pressure is higher, into the blood where its pressure is lower. Conversely, carbon dioxide diffuses from the blood into the alveoli because its partial pressure is higher in the pulmonary capillaries than in the alveolar air. This exchange of gases is governed by Dalton's law, which states that each gas in a mixture exerts pressure independently, and Henry's law, which relates the amount of gas that dissolves in a liquid to its partial pressure. When the gradient is sufficient, as in normal breathing conditions, oxygen diffuses efficiently across the respiratory membrane thanks to the steep difference between the partial pressure of oxygen in the alveoli (about 104 mm Hg) and in the oxygenated blood of the pulmonary veins (about 100 mm Hg).