AP BIOLOGY

Detailed explanation-1: -Moist walls-gases dissolve in the moisture helping them to pass across the gas exchange surface. Permeable walls-allow gases to pass through. Extensive blood supply-ensuring oxygen rich blood is taken away from the lungs and carbon dioxide rich blood is taken to the lungs.

Detailed explanation-2: -a) The alveoli wall (and the capillary wall) is only one cell thick so there is only a short distance over which diffusion takes place (short diffusion pathway) so there is a faster rate of diffusion of oxygen from alveoli into the blood.

Detailed explanation-3: -Alveoli have very thin walls so the diffusion pathway is very short, this means oxygen can diffuse through the wall quickly and easily. Alveoli have a very high surface area meaning there is more space for oxygen to move across from the alveoli to the blood.

Detailed explanation-4: -Efficient gas exchange systems must: have a large surface area to volume ratio • be thin • have mechanisms for maintaining steep concentration gradients across themselves • be permeable to gases.

Detailed explanation-5: -Thin lining: the lining of the alveoli is very thin so that gases can quickly diffuse through it. Large surface area: human lungs contain about 500 million alveoli, which creates a surface area around half the size of a tennis court. This speeds up diffusion because gases have more area over which to diffuse.