118 131 an alternate method of creating backpressure on the vein of the organ 2 is shown in fig1 this removes the requirement to have the entry port 8 into reservoir a 9 at a higher level than the organ 39 s vein 5 by a distance u2018 h u2019 6 perfusion fluid flows out from the vein 5 and into the entry port 157 of a back pressure regulator system the perfusion fluid 1 is directed to a ball 158 and ball seat 159 in one embodiment the required backpressure for the organ 2 is adjusted solely by the weight of the ball 158 against the seat 159 when the required pressure is obtained the ball is lifted from the seat and perfusion fluid 1 is allowed to flow to the exit port of the back pressure regulator 160 which can be at the same level as the reservoir a entry port 8 the pressure to lift the ball 158 is calculated by taking the weight of the ball 158 while considering the buoyancy of the ball 158 in the perfusion fluid 1 and dividing the balls net weight weight minus buoyant force by the area of the seat opening 159 once the required backpressure is obtained and the ball is lifted only slightly greater pressures will be seen with considerably larger flow rates in the preferred embodiment a spring 161 can be added to help close the valve without relying on ball weight alone this is the preferred embodiment due to the fact that the spring 161 is less susceptible to inertial forces seen more often in portable systems with this approach the flow can move horizontally rather than upward through the ball seat as shown in fig1 virtually any.