What is the pressure gradient for plump cells


English: filtration

1 definition

As Filtration This is a mechanical separation process in which particles or molecules are separated from a suspension or an aerosol by means of a filter. Depending on the pore size of the filter, even small molecules can selectively penetrate the filter membrane.

2 physiology

2.1 Capillary Filtration

Filtration is the basis of the mass transfer between capillaries and interstitium. The fenestrated capillary endothelium functions as a filter membrane. This allows the liquid phase of blood plasma and small molecules to pass through. The capillary membrane is impermeable to macromolecules due to its small pore diameter.

The filtration is determined by the effective filtration pressure. In the arterial limb of the capillaries, the hydrostatic pressure outweighs the sum of the colloid osmotic capillary pressure and interstitial pressure. Due to the plasma loss caused by the filtration (= limiting factor of the filtration), the colloid-osmotic pressure increases more than linearly compared to the hydrostatic pressure. The effective filtration pressure drops to 0, the filtration comes to a standstill (filtration equilibrium).

Contrary to what is often described in textbooks, there is no greater reabsorption of the interstitial fluid by the capillary itself. If the colloid-osmotic pressure in the capillary exceeds the hydrostatic pressure, a slight reabsorption takes place - but this is immediately prevented by the inflowing fluid generates a locally higher colloid osmotic pressure in the interstitium, which balances out the pressure difference again. Most of the interstitial fluid is removed via the lymphatic vessels.

2.2 Glomerular Filtration

Within the renal glomeruli, the primary urine is pressed into the tubular system of the kidney by filtration. The filter membrane forms the 3-layer blood-urine barrier. The driving effective filtration pressure is calculated from the hydrostatic pressure of the capillaries, the pressure of the Bowman capsule and the colloid osmotic pressure of the capillaries. It corresponds to about 11 mmHg in the afferent leg.

Like capillary filtration, glomerular filtration is also limited by setting a filtration equilibrium as a result of plasma loss.

3 pharmacy

Filtration is a basic pharmaceutical operation that is used to separate solid particles. Liquids in particular are filtered, but also air in the context of clean room technology. The aim is to separate suspended particles.

3.1 Classification

3.2 ... according to filter type

  • Surface filtration: The particles are separated by a membrane that contains small pores. The particle size serves as the separation principle: If the particle diameter is larger than the diameter of the pores, they are retained. Often a firm filter cake is formed.
  • Depth filtration: The particles are adsorbed on fibers by Van der Waals forces within the filter and thus retained. Depth filters have a limited filter capacity which is exhausted as soon as all adsorption sites are occupied.

3.3 ... according to the size of the particles to be separated

3.4 ... according to the filtration process

A distinction is made here primarily between conventional filtration, in which the liquid flows through the filter due to a pressure difference, and reverse osmosis, in which water is pressed through a semipermeable membrane at increased pressure. There are low pressure and high pressure reverse osmosis systems. They are mainly used to make aqua ad injectabilia (water for injections).

3.5 types of filters

Membrane filters are mainly used in the pharmaceutical industry. They are available in a pore-like and foam-like structure:

  • Pore ​​filters: Pore-like filters consist of a membrane with pores that ensure the filtration performance. It is therefore a surface filter. It is produced by bombarding a polycarbonate membrane with neutrons and then releasing the radiation channels with hydrochloric acid.
  • Foam-like filters: They are highly porous and are therefore used as depth filters. It is produced by dissolving the polymer (usually cellulose acetate) and then precipitating it by adding a nonsolvent.

In industrial production, filter cartridges are used to filter larger amounts of liquid. With these, the filter is fixed in a housing. There are wound filters, in which the filter is wound several times around a central cylinder, and pleated filters, in which the filter is folded around the central cylinder in an accordion-like manner. Wrapped filters are used in reverse osmosis because they can withstand a higher pressure than folded filters.

The filter cartridges are mounted in a housing through which the liquid to be filtered flows ("candle filter"). The liquid to be filtered is fed into the housing at high pressure and thus pressed through the filter, through which the liquid leaves the housing.

3.5.1 HEPA filter

Air is filtered with HEPA filters (High efficiacy particulate arresting) cleaned. This is especially necessary in clean rooms. HEPA filters consist of micro glass fiber filter mats. They filter out at least 99.97% of the suspended matter particles with a diameter of 0.3 µm. A HEPA filter has four filtration mechanisms:

  • Sieving effect: retention of the particles due to their size, comparable to a pore filter
  • Inertia effect: retention of the particles due to their inertia. The particles collide with the filter and can no longer leave it.
  • Blocking effect: Retention of the particles when their center of weight could pass the fibers, but their volume still makes them stick to the filter.
  • Diffusion: Retention of the particles when they adsorb on the filters by passive diffusion.

Each of these mechanisms is dependent on the particle size. The minimum of the total separation capacity is a particle diameter of 0.3 µm - therefore this particle size is part of the definition of a HEPA filter.

4 clinic

The method of filtration is used therapeutically in the context of hemofiltration, in which blood plasma is filtered off by means of a transmembrane pressure gradient.

5 literature

Bauer, Frömmig, Führer: Pharmaceutical Technology. With an introduction to biopharmacy. 10th edition, Stuttgart 2017