Features of human gas exchange surfaces

The alveoli are tiny air sacs in the lungs, surrounded by blood capillaries. Oxygen diffuses from the air in the alveoli into the blood, while carbon dioxide diffuses from the blood into the alveoli. Diffusion is the net movement of particles from a higher concentration to a lower concentration.

Features that make gas exchange efficient

• Large surface area: The lungs contain many alveoli, providing a large total surface over which gases can diffuse.
• Short diffusion distance: The walls of the alveoli and surrounding capillaries are very thin, so oxygen and carbon dioxide have only a short distance to travel between the air and the blood.
• Good blood supply: Blood flowing through the capillaries carries oxygen away and brings carbon dioxide to the alveoli. This helps maintain the concentration differences that drive diffusion.
• Good ventilation: Breathing replaces air in the alveoli, bringing in oxygen and removing carbon dioxide. This also helps maintain the concentration differences between the air and the blood.

Together, these features allow oxygen and carbon dioxide to diffuse efficiently in opposite directions.