diffusion The name applied to a physical process by which individual particles move randomly, within a fluid, from areas of high concentration to lower concentration until the particles are evenly distributed throughout the space. The particles referred to can be molecules or ions and the fluid can be either gaseous or liquid. For example, a person wearing perfume can be noticed on entering a room because molecules of the odiferous substances contained within the perfume diffuse in the air and are detected by the nose. Similarly, a drop of dye dropped into a glass of water will eventually colour the whole volume as the molecules of dye diffuse evenly throughout the volume. Because of thermal motion and vibration, a molecule, ion, or small particle undergoes 10¹³ to 10¹⁵ collisions per second with molecules of the fluid. The particle is said to undergo a random walk, in which the result of a particular collision is independent of the effects of previous collisions. The statistical consequence of this is that the displacement of a particle from its original position will depend on the elapsed time. Where the diffusing particles are at high concentration, many collisions will be between the diffusing species, so that they will move away from each other, thus creating a flux from high concentration to low until the concentration is even throughout the volume. Diffusion is influenced by many factors, such as viscosity and electric charge. Thus diffusion in treacle will be slower than diffusion in water, and particles carrying the same charge will exert mutual repulsion.

Diffusion is an important process for bodily function. Important substances, such as oxygen and carbon dioxide, not only need to diffuse within a particular fluid volume but need to diffuse from one bodily compartment to another across barriers, where generally diffusion will be slower than within a fluid. Consider the position in the lungs; oxygen from the air has to diffuse across the walls of the alveoli and of the capillaries within the lungs to reach the blood, and then across the walls of the red cells to reach the haemoglobin. Fortunately it combines with the haemoglobin to form oxyhaemoglobin, thus maintaining a steep concentration gradient. At the same time carbon dioxide, released from the venous blood, needs to diffuse in the opposite direction, down its concentration gradient, into the lungs, in order to be exhaled. As the transit time of blood in the lungs is just a few seconds, the diffusive process needs to be rapid. (If, for example, the lungs are filled with thick, viscid mucus, as in bronchiectasis or cystic fibrosis, then the diffusive process is impaired and full oxygenation will not occur). Conversely, when oxygenated blood arrives at the tissues the conditions are such that oxygen is released and needs to diffuse into the cells to maintain tissue respiration, whilst carbon dioxide, a product of tissue respiration, needs to be loaded into the blood for conveyance back to the lungs.

Diffusion is similarly important in the absorptive processes in the gut. The purpose of digestion is to break down complex molecules into simple ones such as sugars, fats, and peptides. The lining of the gut wall has many specialized transporters to take the breakdown products into the cells and to transfer them to the blood, for delivery to tissues where they can be used as a source of energy, stored, or used for growth and repair. However, to arrive at the transporters the products of digestion need to diffuse from the gut contents, through the aqueous stationary layer closest to the lining of the gut, to reach the transporters. Diffusion therefore is a universally important process affecting every aspect of living tissue. A single living cell is a hive of activity. Substances produced within the cell which are important for its normal functioning may be produced at one site but then interact with another part of the cell, which they reach by diffusion, moving from high concentration to low by way of thermal motion, an inherent physical property.

Alan W. Cuthbert