Diffusion is when liquids or gases mix by means of Brownian molecular motion. In the body, diffusion occurs for the exchange of substances between cells and for the exchange of gases in the lungs. Diffusion disorders in the lungs cause respiratory insufficiency.
What is diffusion?
Diffusion occurs in the body for the exchange of substances between cells and for the exchange of gases in the lungs. Diffusion involves the mixing of substances of different concentrations that are in direct or indirect contact with each other through random proper motions. The proper motions of diffusion are also called Brownian molecular motions. They are jerky and irregular thermal movements of tiny particles in gases and liquids that are visible only microscopically. The motions are named after the botanist Robert Brown, who first observed them in the 19th century. Diffusion takes place either passively or actively. In the active form, there is also talk of active mass transfer. It differs from passive diffusion in that energy is expended. In the human body, diffusion plays a role in the exchange of substances between cells and the environment. Passive diffusion rarely occurs in cells. The active form is the physiological diffusion form of substances through a cell membrane. A selective form of diffusion also occurs in organs such as the kidneys or at the blood–brain barrier.
Function and purpose
If you add individual drops of ink to a glass of water and do not stir, you can observe streaking. After some time, however, the water turns a relatively uniform blue and the streaks disappear. This process is also called passive diffusion, which compensates for the differences in concentration of two liquids. The intrinsic motions of the particles alone cancel out the concentration differences. Diffusions like this can also occur between separated fluids. This is the case in the human body, where membranes separate substances. Membranes with permeability to either substance migrate from a more concentrated solution to a less concentrated one. Their migration takes place along the so-called concentration gradient. The number of particles in the first liquid shows differences along the diffusion path at the beginning. In the course of diffusion, the number along the diffusion path equalizes and a state of equilibrium occurs. After equilibrium occurs, the same number of molecules diffuse through the membrane in each direction. Thus, at the end of diffusion, the concentration is the same on both sides of the membrane. For passive diffusion, the pore size of the membrane is most important, with smaller particles more likely to diffuse passively than larger ones. In addition to pore size, the diffusion rate is also determined by solvent viscosity and temperature. In the body, diffusion is crucial for the exchange of substances between cells. The body’s cells must maintain the concentration of certain substances. Complete concentration equilibrium in the context of passive diffusion would initiate cell death. Substances therefore tend to diffuse actively in the body against the concentration gradient. A well-known example of the active form can be traced in the K+/Na+ pumps. In selective diffusion, as found in the human body in various organs, the diffusion membrane shows permeability only to selected substances.
Diseases and ailments
A permanent gas exchange takes place in the human lungs. O2 uptake and CO2 release occur in the alveoli of the lungs. In addition to ventilation, perfusion, and distribution, diffusion also plays an important role in the exchange. In respiratory insufficiencies, there are disturbances of one or more of the four processes involved in gas exchange. Causes of respiratory insufficiency can be varied. For example, airway obstruction and central or peripheral respiratory depression as well as altered lung morphology are all possible triggers. If the partial diffusion step is disturbed, this is often due to a prolonged diffusion distance. Such prolongation may be present in the setting of pneumonia, aspiration, or ARDS. Fibrosis, pulmonary edema, or sarcoidosis may also be the cause. Sometimes diffusion defects and associated respiratory failure are also caused by a shortening of the blood contact time.Such shortening occurs, for example, due to emphysema, fibrosis, or anemia. However, sepsis or increase in cardiac output may also be primary causes. When pulmonary diffusion failure is not due to either a shortening of blood contact time or a prolonged diffusion distance, the cause of respiratory failure is probably due to a reduced exchange surface. The surface area for gas exchange may be reduced by fibrosis, for example. Resections may also be considered as a primary cause. The same is true for atelectasis, pleural effusion, and a hema or pleumotothorax. In some circumstances, an elevation of the diaphragm also reduces the surface area. Using a diffusion test, the physician can track diffusion in the lungs and identify any abnormalities. Steady-state, intra-breath and single-breath methods are all possible tests. Diffusion capacity is either determined by the concentration of gases in the exhaled air alone or combined with a measurement of the volumes of breaths. Diffusion disorders can affect not only gas exchange in the lungs, but also the metabolic exchange of body cells or selective diffusion in organs such as the kidneys. Selective diffusion occurs, for example, at the blood-brain barrier, which is designed to protect the central nervous system from harmful or useless substances. In various neurologic diseases, the blood-brain barrier gives up this selective function and allows spurious or even all substances to pass through.
