 These long, ciliated ducts filter the blood into a pair of kidney sacs, while actively reabsorbing glucose and amino acids into the bloodstream.The renal sacs actively adjust the ionic concentrations of the urine, and actively add nitrogenous compounds and other metabolic waste products to the urine.Once filtration and reabsorption are complete, the urine is emptied into "O. vulgaris"' mantle cavity via a pair of renal papillae, one from each renal sac.