Renal blood vessels. Light micrograph of a section through a healthy human kidney showing its blood vessels (red). This is the cortex, the outer layer of the kidney. Part of the renal artery (thick lines, upper right) branches into several smaller arterioles (centre left). These carry blood to the glomeruli (small circles, four seen here). Each glomerulus is a network of blood capillaries that is responsible for the filtration of waste products from the bloodstream. This waste later forms the urine. Magnification unknown.