Transplanted kidney. Power Doppler ultrasound scan of a transplanted kidney with partial blood flow (orange) in a 35-year-old man. The kidney is seen as the oval shape at centre left, outlined by a dark band (cortex, outer layer). Transplantation replaces a failed, or failing, kidney with a donor kidney. The body's two kidneys filter nitrogenous waste (mostly urea) from the blood, excreting it in urine. Doppler ultrasound scanning detects moving fluids, such as blood, by the frequency change they cause when reflecting high-frequency sound waves. Power Doppler scans use colour to show the moving fluid, but do not show the direction of the flow.