 Stroke volume of the systemic heart changes inversely with the difference between the input blood pressure through the vena cava and the output back pressure through the aorta.The hemolymph, pericardial fluid and urine of cephalopods, including the common octopus, are all isosmotic with each other, as well as with the surrounding sea water.It has been suggested that cephalopods do not osmoregulate, which would indicate that they are conformers.