gas exchange

• Gas exchange surfaces in animals (like the alveoli) are ventilated, so air moves in and out
• Alveoli is one cell thick, which provides a shorter diffusion pathway
• The lungs have a rich blood supply, which maintains the steep diffusion gradient
• There is constant air movement in the lungs, which removes carbon dioxide and brings more oxygen, and maintains the steep diffusion gradient
• There is millions of alveoli in the lungs, which gives it a large surface area, and more molecules can diffuse at once
• Alveoli have a thin layer of moisture, helping gases diffuse quicker