Ｗｈａｔ ｉｓ Dｉｆｆｕｓｉｏｎ？ Ever wonder how substances move around the body? Or how a metabolite gets in and out the cells? Or how perfumes spray spread the aroma around in a matter of seconds from the corner you spray it them to the rest of your room? Diffusion is to be blamed. Yes, it is the kind of transport in which dissolved substances of particles move from region of higher concentration to lower concentration with no external energy involved. Since this transport does not require energy from external source, it is also called passive transport. Diffusion occurs when particles, gases, or dissolve substances are mobile or free to move as they spread from where they are in high concentration, to the region where they are in low concentration throughout the concentration gradient. The concentration gradient is an environment in which particles are distributed in different concentrations, keeping high concentration region on one side and low concentration on the other side. In the human body the blood takes oxygen using this simple mode of passive transport, in which oxygen moves from alveoli or air spaces, in which it happens to be in higher concentration, to the blood where it is used to be in lower concentration. It can also be conceived as a mode of transport in which substances move from region of abundance to the region of demand where their concentration is lower. Cells use a bit different, yet sophisticated kind of diffusion. They use some carrier proteins to take in the required substance of nutrients, and since this mode involves facilitation of these proteins to allow these substances inside the cell through the membrane. This kind of diffusion is called facilitated diffusion. An important example is that of blood. Oxygen binds with red blood cells in the bloodstream through facilitated diffusion, which gives us oxygenated blood.