Bladder lining. Coloured scanning electron micrograph (SEM) of the internal surface folds of a bladder in its contracted state. The bladder is the organ that receives and stores urine from the kidneys. The walls of the bladder (a hollow organ) have to expand and contract to cope with varying volumes of urine. Urine is a waste product that is produced by the kidneys. The kidneys filter the blood to remove metabolic waste such as urea. The resulting urine drains from the kidneys through the ureters into the bladder. The urine is then removed from the body by urination through the urethra. Magnification unknown.