Light micrograph of a section of polyurethane containing hollow cellulose fibres (dark circles), the surface of a filter element of a kidney (haemodialysis) machine. Haemodialysis is a filtering operation performed on the blood of persons whose kidneys have failed. Blood removed from an artery is passed through the dialyser on one side of the filter membrane, while a electrolyte solution compatible with blood circulates on the other side. Water & waste products pass through the filter, whose pores prevent the passage of blood cells & proteins. Magnification: x62 at 6x4.5cm size, x36 at 35mm size. Differential Interference Contrast 62:1