The glomerular capillary wall

Observations of the glomerular capillary wall of rats and rabbits in normal and experimental conditions, including hemoglobin excretion, clarify the fine structure of the different components involved in the process of glomerular filtration. During excretion of large amounts of hemoglobin, the protein fills the inner and outer layers of the basement membrane, indicating a fairly loose gel structure. The lesser density of the central layer of the basement membrane and of the surface coat on the foot processes indicate rather compact structures without much space for hemoglobin to accumulate, although it must pass through both layers. The absence of hemoglobin between most foot processes when other parts of Bowman's space are filled with it, suggest that normally in vivo the surface coats between adjacent foot processes are usually in contact for the entire height of the processes. The glomerular filtrate must pass through the two surface coats filling the slit approximately 240 Å wide between the foot processes. Several lines of evidence support the following conclusions. Glomerular endothelial fenestrations offer little or no barrier to filtration. The central layer of the basement membrane is a dense structure normally stopping molecules larger than about 100 Å. The slit membrane and the polysaccharide surface coat beyond it appear to act as a finer filter, but are permeable to hemoglobin (64 Å in largest diameter) and smaller molecules.