Light micrograph of a blood vessel (centre) in the cortex of a kidney. Red blood cells (erythrocytes, round) can be seen in the lumen of the blood vessel. Surround it are layers of muscle cells (blue) that function to regulate blood flow by making the vessel wider or narrower. The kidneys filter waste from the blood and excrete it as urine. Magnification: x400 when printed at 15cm wide,