Kidney glomeruli. Coloured scanning electron micrograph (SEM) of a resin cast of glomeruli capillaries (red) and the larger blood vessels (orange) supplying them with blood. Glomeruli are found in the kidneys. The core of a glomerulus is a tightly coiled balls of capillaries, the smallest blood vessels. The permeable capillary walls allow substances carried in the blood to be exchanged with the surrounding tissues. Each kidney has hundreds of glomeruli that filter toxic waste from the blood. The resulting waste fluid drains to the bladder as urine. The cast was made by injecting resin into the blood vessels. The surrounding tissues were then chemically digested.