The glomerulus, which looks like a wad of yarn, fills the indentation. Fluid is filtered from the glomerulus into the Bowman capsule. The filtered fluid then flows into the proximal convoluted tubule, which carries it away from the Bowman capsuleA Bowman capsule has an outer layer, called the parietal layer, and an inner layer, called the visceral layer (figure 26.5b). The parietal layer is constructed of simple squamous epithelial cells. The epithelial cells become cube-shaped at the beginning of the proximal convoluted tubule. The visceral layer is constructed of specialized cells called podocytes, which wrap around the glomerular capillaries.
Because the kidneys' main function is to filter the blood, the glomerulus has several unique characteristics that make these capillaries especially permeable. Numerous, windowlike openings, called fenestrae (fe-nes′trē), are in the endothelial cells of the glomerular capillaries. Gaps, called filtration slits, are between the cell processes of the podocytes that make up the visceral layer of the Bowman capsule (figure 26.5c). A basement membrane lies sandwiched between the endothelial cells of the glomerular capillaries and the podocytes of the Bowman capsule. Together, the capillary endothelium, the basement membrane, and the podocytes of the Bowman capsule form the kidney's filtration membrane (figure 26.5d), which performs the first major step in urine formation. Urine formation begins when fluid from the glomerular capillaries is filtered across the filtration membrane into the lumen, or space, inside the Bowman capsule.