Blood-urinary Barrier: Structure, Function & Diseases

The nephrologist understands the blood-urine barrier to be a filtration barrier consisting of renal corpuscles and Bowman’s capsule. Because of the permselectivity of the barrier, blood proteins are not filtered out by the kidneys. In inflammatory processes in the renal corpuscles, the blood-urine barrier may be disrupted.

What is the blood-urine barrier?

The blood-urine barrier is a three-layer filtration barrier. As a filter membrane, it mechanically separates particles from a suspension. In the vascular tangle of the kidneys, the primary urine is filtered out as an ultrafiltrate of blood. This filtering process takes place in the renal corpuscles, which are enclosed by the so-called Bowman capsule. The blood-urine barrier decides which molecules are filtered out. For this purpose, the anatomical system contains highly specialized structures. Around 120 milliliters are filtered in the blood-urine barrier per minute. Most of the filtered primary urine is reabsorbed in the tubules of the kidneys. About 1.5 liters of urine are thus formed per day. The most important property of the blood-urine barrier is permselectivity. It is this permselectivity that ensures that the kidneys only filter out harmful substances, while important proteins such as albumin are retained in the blood.

Anatomy and structure

The three layers of the blood-urinary barrier consist of the endothelial cells of the capillaries, the vascular tangle of the basement membrane, and the Bowman capsule. The first layer contains two selectivity-filtering systems. Large-molecule and negatively charged proteoglycans and glycosamine glycans reside in the endothelial cells of the capillaries. In the intercellular spaces of the epithelial cells there are also pores whose diameter corresponds to 50 to 100 nm. The mechanical filter barrier of the blood-urine barrier is formed by the vascular tangle of the basement membrane. The tightly woven meshwork of this barrier is negatively charged and permeable only to molecules above 200 kDa. The cytoplasmic projections of the Bowman capsule delimit the intercellular spaces to 25 nm. A proteinergic slit diaphragm in the intercellular spaces reduces the pores to five nm. Thanks to the slit diaphragm, only molecules exceeding a weight of 70 kDa can pass through this part of the blood-urine barrier.

Function and tasks

The blood-urine barrier is impermeable to blood cells, anionic molecules, and macromolecules. This impermeability results from the pore size and the anionic charge. This is also referred to as charge selectivity. The negative charges thus prevent negatively charged blood proteins of the blood plasma from being filtered out at a ph value of 7.4. Size selectivity is also present for the filtering process of the renal corpuscles. The individual layers of the blood-urine barrier are permeable only 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-urine barrier. Due to the permselectivity of the anatomical structure, the barrier thus hardly filters out components that are important for the body. Albumin, for example, is one of the most important plasma proteins. For this reason, it should only be filtered out to a small extent. The protein has a weight of about 69 kDa and has negative overall charge. The radius of these molecules is about 3.5 nanometers. Therefore, it can only pass the blood-urine barrier to a small 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 everything. This pressure difference results from the colloidosmotic and hydrostatic pressures. As the vascular tangle of renal corpuscles is traversed, 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 squeezed out in this way. The plasma proteins remain instead. Thus, the concentration of proteins increases bit by bit as they pass through the capillaries. As the protein concentration increases, so does the colloid osmotic pressure. The effective filtration pressure decreases as a consequence and reaches zero once filtration equilibrium is reached.

Diseases

The best-known disease associated with the blood-urine barrier is glomerulonephritis. In this phenomenon, the glomerulus capillaries are affected by inflammation.As a consequence, the pores of the filter structure enlarge and the negative charge in all layers of the blood-urine barrier is lost. Henceforth, any macromolecules can pass through the barrier. The permselectivity of the anatomical structure is thus lost. Neither the radius of the molecules nor the charge properties are still valid as filter criteria. For this reason, hematuria sets in. This means that patients notice blood in their urine. In addition, albuminuria may occur. In this case, albumin is excreted in the urine in unnaturally large quantities. As a rule, nephrotic syndrome develops as a result. The protein in the blood is reduced in the context of this syndrome. Blood lipid levels increase and peripheral edema occurs. Nephritic syndrome may also occur as a result of the symptoms described. In addition to pain in the flank, increased tissue tension is present. The renal corpuscles can be permanently damaged in inflammatory processes and cause permanent renal insufficiency. Glomerulonephritis can occur in the context of various primary diseases. Tumor diseases should be considered as well as autoimmune diseases or syphilis and HIV. The onset of glomerulonephritis can also be associated with the use of various medications. In addition to gold, penicillamine, for example, can trigger the inflammatory reactions of the renal corpuscles.

Typical and common urethral diseases.