How is gas exchange in a human facilitated?

The gas exchange of O2 and CO2 must occur between the air-filled alveoli of the lungs and our blood circulation running through our vessels. Near our lung alveoli lie capillaries coat the alveoli. The capillaries will be facilitating the movement of CO2 from the blood stream into the alveoli for expiration, whilst picking up O2 from the alveoli to carry back into circulation, using diffusion. Diffusion is the movement of a gas from an area of high concentration to an area of low concentration. 3 main features are required to ensure the exchange of gases at this location is efficient:1) Thin diffusion barrier - the diffusion pathway consists of the alveoli wall and the capillary wall. The alveoli wall, as well as the capillary wall are both one-cell layer thick. This must be thin to make diffusion faster and easier. 2) Steep concentration gradient - The concentration gradient must be maintaned across the diffusion barrier, to ensure the correct direction of movement for the two gases. As CO2 is constantly being expired, in a healthy individual, the concentration in the alveoli is kept low, thus CO2 can continue to diffuse from the blood stream (high conc) into the alveoli (low conc). The blood by the capillaries will often be deoxygenated, meaning the level of O2 will be lower than that inspired into the lungs and alveoli, so O2 can continue to move from alveoli (high conc) into the blood (low conc).3) Large surface area - The millions of alveoli present in our lungs provide a large surface area, to allow the mass diffusion of gases to occur, fast. Ventilation must be normal and occurring for all this to be possible at a microscopic level.