Final answer:
The thousands of small holes in the glomerulus that allow for filtration of blood are called fenestrations. These pores are essential in the renal filtration process, helping to form urine while retaining larger elements in the bloodstream.
Step-by-step explanation:
The thousands of small holes that perforate the three layers of the glomerulus are called fenestrations. These fenestrations are present in the endothelial cells of the capillaries and are essential for the filtration process that occurs in the kidneys. Because of these tiny openings, the glomerulus can effectively filter the blood, retaining large elements such as blood cells and proteins while allowing smaller molecules to pass through and form what will eventually be urine. Podocytes, with their extensions called pedicels, interdigitate around these capillaries to form additional filtration slits that further regulate the filtration process. This complex structure ensures that the filtrate composition is very similar to plasma, minus the cells and large proteins.