This is a picture of a glomerulus of a mouse kidney, taken with a scanning electron microscope (magnification ~ 10,000X). The glomerulus is the filtration unit of the kidney. It consists of a knot of capillaries through which blood flows; large molecules like proteins are retained in the blood, while smaller molecules like salt, water, and sugar pass into the urine. The capillaries are covered with cells called podocytes that have amazingly interdigitated “foot processes”. (In this image, one podocyte and all of its processes have been pseudo-colored in blue.) The water and small molecules from the blood pass through the slits between these foot processes. In chronic kidney diseases, these slits are affected and the filter system is compromised.