Kidney dialysis. Colour Doppler ultrasound scan of arterial blood flow (red) as it leaves a 65-year- old woman to undergo dialysis. The pulsing of the blood flow is shown on the graph at lower centre. The peak blood flow velocity is 2.5 metres/second. The blood will be cleaned of nitrogenous waste in a dialysis machine. This will be a support for a failed, or failing, kidney. It is the body's two kidneys that normally carry out this function. Colour Doppler ultrasound scanning colour-codes moving fluids, such as blood, by detecting changes that they cause in the frequency of reflected high-frequency sound waves. Flow towards (red) and away (blue) from the detector can be shown.