Body processes

Diffusion – function, task and diseases

Diffusion

We speak of diffusion when liquids or gases mix by means of Brownian molecular movements. In the body, diffusion takes place 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, substances of different concentrations, which are in direct or indirect contact with one another, are mixed by random movements of their own. The proper motions of the diffusion are also referred to as Brownian molecular motions. 

These are jerky and irregular thermal movements of tiny, only microscopically visible particles in gases and liquids. The movements are named after the botanist Robert Brown, who first observed them in the 19th century.

Diffusion takes place either passively or actively. In the case of the active form, there is also talk of 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 occurs only rarely in cells. The active form is the physiological form of diffusion 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 in a glass of water and do not stir, you can observe streaking. After a while, however, the water turns relatively evenly blue and the streaks disappear. This process is also known as passive diffusion, which equalizes the difference in concentration of two liquids. The particles’ own movements 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 a permeability for one of the two substances 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 still shows differences along the diffusion path at the beginning. In the course of diffusion, the number equalizes 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 important for passive diffusion, with smaller particles more likely to diffuse passively than larger ones. In addition to the pore size, the diffusion rate is also determined by the solvent viscosity and the temperature.

In the body, diffusion is crucial for the exchange of substances between cells. The body cells have to maintain the concentration of certain substances. The complete equalization of the concentration within the framework of passive diffusion would initiate cell death. Substances therefore diffuse more actively in the body against the concentration gradient.

A well-known example of the active form can be seen in the K+/Na+ pumps . In the case of selective diffusion, as can be found in various organs in the human body, the diffusion membrane only shows permeability for selected substances.

Diseases & 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 exchange. In respiratory failure, one or more of the four processes involved in gas exchange are disrupted.Respiratory failure can have many causes. Both an obstruction of the airways and a central or peripheral respiratory depression as well as a changed lung morphology can be considered as triggers. If the partial step of the diffusion is disturbed, this is often due to an extended diffusion path. Such a prolongation can be present in the context of pneumonia , aspiration or ARDS . Fibrosis , pulmonary edema or sarcoidosis can also be the cause.

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

If a diffusion disorder in the lungs is neither due to a shortening of the blood contact time nor to an extended diffusion distance, the cause of the respiratory insufficiency is probably a reduced exchange surface. The surface area for guest exchange can be reduced by fibrosis, for example. Resections are also possible primary causes. The same applies to atelectasis, pleural effusion and a hematothorax or pleumothorax. Under certain circumstances, an elevated position of the diaphragm also reduces the surface.

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

In addition to the gas exchange in the lungs, diffusion disorders can also affect the exchange of substances in the body 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 the case of various neurological diseases, the blood-brain barrier gives up this selective function and allows the wrong or even all substances to pass through.

Lisa Newlon
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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.