• Saturday February 22,2020

Blood-urine barrier

Under the blood-urinary barrier, the nephrologist understands a filtration barrier consisting of the renal corpuscles and Bowman's capsule. Due to the permselectivity of the barrier, blood proteins are not filtered out by the kidneys. In inflammatory processes in the renal corpuscles, the blood-urinary barrier may be disturbed.

What is the blood-urinary barrier?

The blood-urinary barrier is a three-layered filtration barrier. As a filter membrane, it mechanically dissolves particles from a suspension. In the blood vessel of the kidneys, the primary urine is filtered out as the ultrafiltrate of the blood. This filtering process takes place in the kidney bodies, which are enclosed by the so-called Bowman capsule.

The blood-urinary barrier determines which molecules are filtered out. For this purpose, the anatomical system contains highly specialized structures. About 120 milliliters are filtered in the blood-urinary barrier per minute. The majority of the filtered primary urine is reabsorbed in the tubules of the kidneys.

About 1.5 liters of urine are formed per day. The most important property of the blood-urinary barrier is permselectivity. Only this permselectivity ensures that the kidneys only filter out pollutants, while in the blood important proteins such as albumin are retained.

Anatomy & Construction

The three layers of the blood-urinary barrier consist of the endothelial cells of the capillaries, the vascular bundle of the basal membrane and the Bowman's capsule. The first layer contains two selectivity filtering systems. Large molecular and negatively charged proteoglycans and glycosaminoglycans are located in the endothelial cells of the capillaries. In the intercellular spaces of the epithelial cells are also pores whose diameter corresponds to 50 to 100 nm.

The mechanical filter barrier of the blood-urinary barrier is formed by the vascular bundle of the basal membrane. The densely woven meshwork of this barrier is negatively charged and permeable only to molecules over 200 kDa. The cytoplasmic extensions of the Bowman capsule limit the intercellular spaces to 25 nm. A proteinaceous slit diaphragm in the intercellular spaces reduces the pores to five nm. Thanks to the slit diaphragm, only molecules can pass through this part of the blood-urinary barrier weighing 70 kDa exceed.

Function & Tasks

The blood-urinary barrier is impermeable to blood cells, anionic molecules and macromolecules. This impermeability results from the pore size and the anionic charge. The speech is also of a charge selectivity. The negative charges thus prevent that at a pH value of 7.4 negatively charged blood proteins of the blood plasma are filtered out.

Also, size selectivity is present for the filtering process of the renal corpuscles. The individual layers of the blood-urinary barrier are only permeable to molecules up to a radius of eight nanometers. This size selectivity, together with the charge selectivity, is also referred to as the permselectivity of the blood-urinary barrier. Due to the permselectivity of the anatomical structure, the barrier hardly filters out any constituents that are important for the body. For example, albumin is the most important plasma protein. For this reason, it should be filtered out only to a small extent. The protein weighs about 69 kDa and has a negative total charge.

The radius of these molecules is about 3.5 nanometers. Therefore, it can only pass through the blood-urinary barrier to a limited extent and remains in the body instead of being filtered out. For the filtering process, the difference between the pressure in the capillaries and the pressure in the Bowman capsules is all crucial. This pressure difference results from the colloid-osmotic and hydrostatic pressure. As the vascular bundle of kidney corpuscles passes through, the hydrostatic pressure remains at a certain level.

Because of the total cross section of the parallel capillaries, there is little resistance. The ultrafiltrate is pressed off in this way. The plasma proteins stay behind instead. Thus, the concentration of proteins in the passage of the capillaries increases piece by piece. The protein concentration also increases colloid osmotic pressure. The effective filter pressure drops as a result and reaches zero as soon as there is a balance of filtration.


The most prominent disease associated with the blood-urinary barrier is glomerulonephritis. Glomerulus capillaries are affected by inflammation in this condition. As a result, the pores of the filter structure enlarge and the negative charge in all layers of the blood-urinary barrier is lost. Hence any macromolecules can pass the barrier.

The permselectivity of the anatomical structure is so lost. Neither the radius of the molecules nor the charge properties are still valid as filter criteria. For this reason, a hematuria sets in. This means that the patients notice blood in the urine. In addition, albuminuria may occur. Albumin is excreted in unnaturally large amounts through the urine. As a result, as a result, the nephrotic syndrome sets in. The protein in the blood is lowered in the context of this syndrome. Blood lipid levels increase and peripheral edema occurs.

A nephritic syndrome can also occur as a result of the symptoms described. In addition to pain on the flank, there is increased tissue tension. The renal corpuscles can be permanently damaged in inflammatory processes and cause permanent renal insufficiency. Glomerulonephritis can develop in the context of various primary diseases.

Tumor diseases are to be considered as well as autoimmune diseases or syphilis and HIV. The onset of glomerulonephritis may also be associated with the use of various medications. In addition to gold, for example, penicillamine can trigger the inflammatory reactions of the renal corpuscles.

Typical & common urethral disease

  • Incontinence (urinary incontinence)
  • Urethritis (urethritis)
  • Urethral cancer (rarer)
  • urethral stricture
  • Frequent urination

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