diffusion

From diffusion is when liquids or gases mix by means of Brownian molecular movements. Diffusion takes place in the body for the exchange of substances in the cells and for gas exchange in the lungs. Diffusion disorders in the lungs cause respiratory insufficiency.

What is the diffusion?

During diffusion, random movements of their own mix differently concentrated substances that are in direct or indirect contact with one another. The intrinsic motions of diffusion are also known as Brownian molecular motions. These are jerky and irregular heat movements of tiny, only microscopically visible particles in gases and liquids. The movements are named after the botanist Robert Brown, who first saw them in the 19th century.

Diffusion takes place either passively or actively. In the case of the active form, there is also talk of an active mass transport. It differs from passive diffusion in that it uses energy.

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 takes place in organs such as the kidneys or at the blood-brain barrier.

Function & task

If you put individual drops of ink into a glass of water and do not stir, you can see streaking. After a while, however, the water turns a relatively uniform blue and the streaks disappear. This process is also known as passive diffusion, which compensates for the differences in concentration between two liquids. The self-movements of the particles alone cancel out the differences in concentration.

Diffusions like this can also take place between separated liquids.This is the case in the human body, where membranes separate substances from one another. Membranes with permeability for one of the two substances migrate from a more concentrated solution to a less concentrated one. Your migration takes place along the so-called concentration gradient. The number of particles in the first liquid still shows differences along the diffusion path at the beginning. In the course of diffusion, the number balances out on the diffusion path and a state of equilibrium occurs. After equilibrium has been reached, the same number of molecules diffuses through the membrane in each direction. At the end of the diffusion, the concentration is the same on both sides of the membrane.

The pore size of the membrane is particularly decisive for passive diffusion, with smaller particles diffusing more passively than larger ones. The diffusion rate is determined not only by the pore size but also by the solvent viscosity and the temperature.

In the body, diffusion is crucial for the exchange of substances between cells. The body's cells have to maintain the concentration of certain substances. The complete concentration equalization 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 seen in the K + / Na + pumps. With selective diffusion, as can be found in various organs in the human body, the diffusion membrane only shows permeability for selected substances.

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Illnesses & ailments

There is a permanent gas exchange in the human lungs. O2 uptake and CO2 release take place in the alveoli of the lungs. In addition to ventilation, perfusion and distribution, diffusion also plays an important role in the exchange. In respiratory insufficiency, there are disorders of one or more of the four processes that are involved in gas exchange.

There are many causes of respiratory failure. An obstruction of the airways and a central or peripheral respiratory depression as well as a changed lung morphology, for example, are possible triggers. If the partial step of diffusion is disturbed, this is often due to an extended diffusion path. Such an extension can be present in the context of pneumonia, aspiration, or ARDS. Fibrosis, pulmonary edema, or sarcoid can also be the cause.

Sometimes diffusion disorders and an associated respiratory failure are also caused by a shortening of the blood contact time. Such shortenings occur, for example, through emphysema, fibrosis or anemia. However, sepsis or an increase in cardiac output are also possible primary causes.

If a diffusion disorder of the lungs is neither due to a shortening of the blood contact time nor to a longer diffusion distance, the cause of the respiratory failure is probably due to a reduced exchange surface. The surface for the guest exchange can be reduced, for example, by fibrosis. Resections are also a possible primary cause. The same is true of atelectasis, pleural effusion, and a hema or pleumotothorax. Under certain circumstances, a raised diaphragm also reduces the surface area.

The doctor can use a diffusion test to understand the diffusion in the lungs and identify any disturbances. Both the steady-state, intra-breath and single-breath methods can be used as tests. The diffusion capacity is either only determined via the concentration of the gases in the exhaled air or combined with a measurement of the tidal volumes.

Diffusion disorders can affect not only gas exchange in the lungs but also the exchange of substances in the body's cells or selective diffusion in organs such as the kidneys. A selective diffusion takes place, for example, at the blood-brain barrier, which is supposed to protect the central nervous system from harmful or unusable substances. In various neurological diseases, the blood-brain barrier gives up this selective function and allows false or even all substances to pass through.