Reviewed by Rishi Desai, MD, MPH
Filtration of blood into Bowman’s space, ie. glomerular filtration, is the first step in urine formation and produces an ultrafiltrate without proteins or blood cells. The filtration rate across glomerular capillaries is much higher than systemic capillaries because of the porous endothelium and podocyte epithelial layer, though the basement membrane and size of the filtration slits between podocyte food processes do not allow for filtration of plasma proteins or cells. The net negative charge of the barrier, thanks to negatively charged glycoproteins, also repels negatively charged solutes - especially plasma proteins which generally are negatively charged at physiologic pH.