 The pericardial duct contains an ultrafiltrate of the blood known as the pericardial fluid, and the rate of filtration is partly controlled by the muscle- and nerve-rich branchial hearts.The renal appendages move nitrogenous and other waste products from the blood to the renal sacs, but do not add volume.The renal fluid has a higher concentration of ammonia than the urine or the blood, thus the renal sacs are kept acidic to help draw the ammonia from the renal appendages.