Each kidney consists of about one million functional units, called nephrons. The number of nephrons is individual determined and depending on many factors.  For the kidney to function properly, it is necessary to keep at least 30% of the nephrons in operation to work well. One nephron is composed of the kidney corpuscle and the tubule. The kidney corpuscle consists of a complex network of capillary loops called a glomerulus and an encompassing Bowman's capsule.

The glomerulus as a filtration unit

The kidney corpuscle (glomerulus+ Bowman’s capsule) has two poles: a vascular pole and a tubular pole. The arterioles from the kidney circulation enter and leave the glomerulus at the vascular pole. The glomerular filtrate leaves the Bowman's capsule into the proximal tubule at the tubular pole.

Preliminary filtration of urine takes place in the glomeruli of the kidney – in its capillary network (blood vessel). The wall of this capillary network has a three-layer structure and consists of the following layers:

  1. Endothelial Cells that pad the glomeruli capillaries from the inside.
  2. Glomerular Basement Membrane (GBM), a trilaminar structure composed principally of type IV collagen with laminin, heparan sulfate, and enactin as well as smaller components of other proteins and proteoglycans.
  3. Epithelium, a layer of cells called podocytes. They have long extensions called pedicels, after which they are named (podo-, -cyte). They cover the capillaries from the outside. The pedicels rest on the basement membrane. Between them there are diaphragm filtration gaps containing various proteins.

The glomerular filtration assembly:

The glomerular filtration assembly is composed of three main cellular barriers that are critical for the ultrafiltration process, the fenestrated endothelium, glomerular basement membrane and highly specialised podocytes.

GBM (Glomerular Basement Membrane) is not only a stabilizing element of the structure of capillaries and glomeruli, but also the only continuous, homogeneous layer of the filter barrier. Its thickness changes with age, in children it is from about 110 nm, in adults 310-350 nm; in males significantly thicker than in women.

► The general task of the GBM is to selectively pass only certain components of the blood toxins and metabolic products, while retaining the desirable components in blood circulation.

In the light microscope the normal basement membrane appears as a continuous line. An electron microscope shows that the structure of that wall is composed of a collagen network made up of alpha 3, alpha 4 and alpha 5 chains. These chains come together, leading to the formation of a well-organied solid network. When any of these chains are abnormal, the entire collagen IV network is abnormal leading to a progressive alteration of GBM.