crrt - replacement fluid?

With regard to continual renal replacement therapy, from my understanding, replacement fluid is used to cause convection, ie. to flush the blood through the filter giving it extra 'umph' so that more ultrafiltrate is created. How can adding replacement fluid after the filter be of any use in this respect? Is adding it after just so that you can add drugs without them being filtered, but not getting the benefits of convection? Any help anyone?