Gas exchange relies on Fick’s Law of Diffusion, which states that the rate of diffusion is directly proportional to the surface area of the membrane. Oxygen cannot enter the bloodstream fast enough to meet the body's demands, and carbon dioxide cannot exit efficiently. The alveolar walls contain the dense network of capillaries where the actual exchange takes place. When the walls are destroyed, the capillaries are destroyed with them. This creates a "dead space" effect where there is air in the lungs, but insufficient blood flow to pick up the oxygen. Healthy alveoli are elastic—they snap back to push air out during exhalation. Smoking destroys the elastin fibers that provide this recoil. When the smoker inhales fresh air, it mixes with this stale, trapped air.10 This lowers the partial pressure of oxygen (PO2) within the alveoli, reducing the driving force that pushes oxygen into the blood.