Explain the diffusion process of gas exchange in the lung during respiration

Oxygen and carbon dioxide, dissolved in water, are exchanged by diffusion in the lungs. Blood travelling to the lungs is deoxygenated and instead of oxygen, carbon dioxide is bound to the red blood cells. The alveoli in the lungs are filled with oxygen during respiration.  As there is a higher concentration of oxygen in the alveoli compared to in the blood, oxygen will pass down the concentration gradient across the alveoli into the blood via diffusion. Via the same process, carbon dioxide will pass down the concentration gradient from the blood into the alveoli.