r37f33 BACKGROU~D OF THE I~VENTION _ 1. Field of the Invention . This invention relates to a blood reservoir for use in an extracorporeal blood circulating system. 2. Description of the Prior Art The transfer of mass or heat from one fluid to another, where the fluids are separated by a membrane, is the objective of many biological and industrial processes. For example, an artificial lung, an artificial kidney and a blood heat-exchanger all use this principle. In an artificial lung, blood is separated from gaseous oxygen or another oxygen-rich fluid by a thin permeable membrane, the oxygen passes through the membrane into the blood, and carbon dioxide is released from the blood in the reverse direction. In an artificial kidney, the oxygen rich fluid is replaced by a dialysing fluid, and a wettable, permeable membrane particularly adapted for dialysis is used~ Since the development of the first membrane ~0~'7~33 blood oxygenators and dialysers, membranes have been made progressively thinner and more effective. The resistance set up by blood flowing in channels or tubes has, however, been found to severly limit the rate of mass transfer across the membrane. Thus, the efficiency of these exchanges depends both on the permeability of the membrane which separates the two fluids, and on the ease with which the species being transferred (e.g. oxygen or carbon dioxide in the case of an oxygenator or artificial lung) is diffused within the fluids. Since oxygen is relatively insoluble in blood, it diffuses very slowly, and the efficiency of simple artificial lungs is very low. However, if the blood is well mixed within the blood channels, oxygen transfer can be greatly increased. In a prior patent application, I have described a method and apparatus for enhancing the mixing of blood and enhancing the rate of mass transfer across the membrane com- prising forming the blood channels into furrowed surfaces, and pulsating the blood across these surfaces, so that eddies or vortices are generated within the hollows, thereby mixing the blood thoroughly and enhancing gas transfer. More particularly, the apparatus for effecting heat or mass transfer between two fluids through a membrane des- cribed in my said prior application comprises a conduit for flow of one fluid, said conduit being at least partly defined by said membrane and the configuration of said conduit in a plane orthogonal to the general direction of flow varying periodically along the general direction of flow either in- herently or in response to fluid pressure therein in such a manner that when said fluid is pulsed along the line of the general direction of flow, a component of motion is induced ~0~7'33 therein which is mutually orthogonal to the surface o the membrane and the direction of flow. In preferred embodiments described therein, the conduit configuration varies periodically along the general direction of flow (either inherently or in response to fluid pressure) in order to give rise to separation and reattachment of the flow at a multiplicity of zones within the conduit, whereby secondary flow is induced within said zones. The zones, which can vary considerably in configuration, are generally spaced one from another by constrictions to flow through the conduit. The specification of my said prior application dis- closes that when fluid is pulsed across a constriction the flow is detached therefrom and reattaches at a neighboring con- striction, with eddy formation in the zone between the con- strictions. The eddies, which reduce the boundary layer effect, generally take the form of vortices, the axes of which are transverse to the general direction of flow. I have pos- tulated that the vortices are formed primarily during deceler- ation of fluid which has been subjected to pulsation, thevortices tending to be ejected during acceleration, The conduit surface, typically the surface of the mernbrane, may vary along the general direction of flow to provide the zones either intermittently or, preferably, con- tinuously, as with an undulating membrane surface. The zones may take a variety of configurations, In particular, they may extend across the surface of the conduit transverse to the direction of flow to provide furrows or may be present as local depressions such as dimples in a conduit surface, such depres- sions preferably having curved bases and conveniently beinghemispherical. - ~69793 SUMMARY OF THE INVENTION It will be appreciated that when the fluid in the conduit is blood, flow must be non-turbulent if trauma is to be avoided, The present invention provides means by which an improved flow across a conduit can be induced in blood, substantially without turbulence. I have now devised a method and apparatus by means of which excellent transfer rates to or from blood may be achieved with the basic conduit structure disclosed in said prior application but employing pulsatile flow of low ampli- tude fluctuation compared with the mean flow rate, as produced by a roller pump. In accordance with one aspect of the present invention, there is provided a mass or heat-transfer device with vortex flow features of my prior application but which, surprisingly, operates under the low amplitude fluctuation pulsatile flow - which is produced by roller pumps. Thus, I have discovered that vortices can be generated by fluid passing over a surface with cavities in it, and under certain continual flow condi- tions which are produced by a roller pump as opposed to revers- ing flow, and distinct from truly steady flow as achieved by gravity flow using a constant head tank. In both reversing flow and continual, gently fluctu- ating pulsatile flow, accelerating flow tends to displace fluid within the hollows, and vortex formation occurs readily during decelerating flow. Reversing flow permits vortex formation - even if the mean velocity is low, because the peak velocities are higher, and the accelerating and decelerating phases of the pulse assist fluid mixing. However, in gently fluctuating pulsatile flow, when the velocity is low, the vortices form only by friction, with negligible exchange between fluid in - 793 the hollow and fluid in the main stream. Surprisingly, at intermediate higher velocities, even with the relatively small disturbances produced by the roller pump, the vortices are much stronger, and considerable exchange of fluid occurs between the vortex and the main stream. At much higher velocitie$, the flow becomes turbulent and thus potentially damaging to sensitive liquid compositions such as blood. It is postulated that the vortices that occur in inter- mediate gently pulsating continual flow are effective in mixing blood because they are formed convectively (that is, by either continuous or rapidly alternating inflow and outflow processes). r~hey form only when the flow speed is high enough to overcome viscous effects which tend to keep the boundary layer attached to the wavy wall of the conduit. Vortex formation is observed in gently pulsating flow when the Reynolds number Up reaches about 200 (U being velocity of the fluid in the channel, v kinematic viscosity, and p hollow pitch - i.e. distance between ridges. For water in 1 mm depth semi-cylindrical hollows, the mean velocity has to exceed only about 3 cm/sec., but is preferably at least 10 cm/sec to form convective vortices. No substantial turbulence is observed at mean velocities below about 50 cm/sec., but the mean velocity preferably should not exceed about 30 cm/sec., in order to assure avoiding substantial turbulence. This 30 cm/sec. mean velocity upper limit corresponds to a Reynolds number, as defined above, of about 3000. r~he ratio of pitch to depth of hollow is at least about 2:1, is generally below about 8:1, and is preferably in the range of between about 2:1 and, more prefer- ably about 4:1. I have recently observed that vortices are also formed convectively in the foregoing apparatus using steady flow, as ~0697~3 from a constant head tank. Howe~er, much higher mean veloci- ties, of the order of 30 cm/sec. are required to achieve this effect, In addition the Reynolds number based on channel width should not exceed 2000 if turbulence is to be avoided, In an oxygenator, it is necessary to have a large sur- face area and low resistance, which means, as a practical matter, average blood velocities too low for vortex formation with gently pulsating flow given the available blood supply from the patient - unless special means are provided to over- come this problem. In a preferred embodiment of the present invention, I provide a system for adapting blood treating devices such as artificial lungs embodying a particular con- duit configuration for achieving eddy formations to the use of gently pulsating flow, to thereby afford such additional advantages as the use of simpler, more readily available pump- ing equipment than would be required for creating reversing or violently pulsatile flow. This system comprises a blood reservoir, means for conducting blood from the patient to the reservoir, a roller pump for conducting blood from the reser- voir through an oxygenator conduit as a pulsatile flow, of low amplitude fluctuation compared with the mean flow rate, and at a mean flow rate which will achieve a Reynolds number between about 200 and about 3000, means for returning a portion of the oxygenated blood to the patient at a selected perfusion rate independent of the rateof flow of the blood through the oxygenator, and means for recycling a portion of the oxygenated blood to the reservoir, Preferably this system includes means, conveniently associated with the blood reservoir, for intro- ducing into the system stored blood, blood components or otherfluids, and for venting trapped air from the system. -- 7 -- ~069793 In another aspect of my invention I provide a blood reservoir, particularly suited for use in the foregoing system, this reservoir also acting as a mixing chamber for the blood and comprising a rigid lower housing, a flexible, bag-like upper portion sealed to the periphery of the lower housing, first and second inlet means, preferably in the lower housing, for intro- ducing blood from the animal to be treated, and, respectively, from the oxygenator, into the reservoir, means for mixing the blood in the reservoir, and outlet means for drawing the mixed blood from the reservoir, for conveyance to the oxygenator. Preferably, as illustrated in the drawings, the inlet means are tangential to the housing, so that the introduction of the blood therethrough serves to create a vortex in the chamber whereby efficient mixing with substantially no trauma to the blood is effected, the preferred inlet means thus serving as the mixing means as well. According to a further broad aspect of the present invention there is provided a blood reservoir for use with an extracorporeal blood circulating system. The reservoir comprises a rigid lower portion having a substantially circular cross- section. The reservoir also has a flexible upper portion and first inlet means for introducing blood thereinto from an animal. A second inlet means is provided for introducing blood into the reservoir from apparatus for effecting heat or mass transfer between the blood and another fluid. Outlet means is provided for discharging blood from the reservoir. Means is provided for mixing the blood within the reservoir and this mixing means is provided, at least in part, by at least one of the inlet means constituting tangential inlet means. BRIEF DESCRIPTIO~ OF THE DRAWI~GS In the drawings: Figure 1 is a perspective view of the apparatus suit- able for the oxygenation of blood with parts broken away -- 8 -- ,06637~3 Figure 2 is an enlarged fragmentary portion of Figure 1, in section, Figure 3 is a fragmentary longitudinal section of a tubular mem~rane suitable for use as an alternative conduit to that illustrated in Figure 2, Figure 4 is a schematic view of a complete circulat- ing system suitable for use in accordance with the present invention with the apparatus shown in Figure 1, Figure 5 is a front view, partly in section, through a blood reservoir suitable for use in the circu~-ting system illustrated in Figure 3, Figure 6 is a section along lines 8-8 of Figure 5, Figure 7 is a plan section along lines 9-9 of Figure 5, and Figure 8 is a graph of clearance time vs. Reynolds number using disturbed flow. DESCRIPTION OF_THE PREFERRED EMBODIMENTS The present invention permits the design of high- efficiency artificial lungs and kidneys and also heat exchangers for a wide variety of applications, ranging from heat exchangers for regulating the temperature of blood, to automobile radiators, and de-salination equipment, which can be used with gently pulsating fluid flows, using standard roller pumps, without introducing fluid turbulence. Turbu- lence produces damage to blood formed elements, and incurs high energy losses in industrial mass or heat exchangers. The invention flows from the observation of gently pulsating water flow, at measured velocities, across a furrowed channel, using dye injected into the hollow of a furrow to observe the flow patterns therein. ~al697~3 Water flow rates through the channel were measured with a graduated cylinder and stopwatch, and the time taken for the dye to clear from a given semi-circular hollow was measured, again with a stopwatch. In an early experiment, when mean velocity within the channel was below about 10 cm/sec, dye clearance times were relatively long, but for higher velocities within the channel, the dye cleared rapidly. This transition could be observed by closely following the flow patterns (made visible with dye) and corresponds to a value of Reynolds number of about 200, where, as indicated previously, Reynolds number = velocity in channel x pitch of hollow . kinematic viscosity These results are shown in Figure 8, which is a graph of clearance time vs. Reynolds number. To discover when the transition from convectively- formed vortices to turbulence would occur in gently pulsating flow, I used hot-film anemometry to measure instantaneous velocities within the hollows. I found that transition to turbulence occurred at velocities of about 50 to 150 cm/sec. corresponding to a Reynolds number based on hollow pitch of about 1000-3000. Thus, gently pulsating flow rates corresponding to a Reynolds number as defined above between about 200 and about 3000 provide the desired vortex flow and consequent efficient, essentially non-turbulent, mixing when a fluid is caused to flow across a furrowed path. Preferably, the Reynolds number is in the range of between about 200 and about 2000, and, more preferably, is from in excess of 200 to about 1000. -- 10 -- 1~6~793 In later experiments in which a roller pump was used, it was observed that it produced periodic fluctuations in velocity, at a frequency double the shaft rate of rotation, since it had two rollers. The frequency of these velocity fluctuations was directly proportional to flow rate and was 71/min at a flow of 1000 ml/min. The ratio of peak to mean flow rates varied from 1.22 at a mean flow of 1860 ml/min to 1.46 at a mean flow of 154 ml/min. Although the waveforms of the flow produced by the roller pump were similar in shape, with 2/3 of the cycle at a roughly constant velocity, then with a sharp drop in velocity for the remaining 1/3 of the cycle, the shapes did vary with pump revolution rate. Thus changes in channel spacing altered not only the frequency of velocity fluctuations at the same mean velocity in the channel, but their amplitude also. Despite this, dye clearance times correlated well with mean velocity in the channel and was substantially independent of channel spacings. With mean velocities greater than 3 cm/sec, the vortices within the hollows were always formed convectively, so dye clearance time ~O can be expected to depend on the time for a fluid particle to traverse the chord and curved - surface of a hollow (L). This time is of the order of u~ where u is mean fluid velocity in the channel. This implies that -L (which is non-dimensional) is constant, provided viscous effects are negligible. The constant -L~ = 62 was obtained empirically for the roller-pump perfusion experiments. This equation satis- factorily correlates the data for the four different hollows and the three channel spacings used in the experiments. It indicates that viscous effects are less important than convective effects, in the range of interest. '7~3 Illustrative embodiments of the present invention will now be described in connection with the accompanying drawings. Referring now to Figures 1 and 2, the apparatus comprises a stack (11) of plates (12) each of which consists of two membranes (13) of silicone rubber or the like separated by and supported on a rigid support plate (14) comprising a series of ridges (15). Furrows (16) are formed in the membrane (13), the bottoms of which furrows are spaced from the support plate (14), the space between furrows (16) and support plate (14) defining oxygen flow channels (17). The ratio of pitch to depth of the furrows in the illustrated embodiment is ~r: 1. Each plate is vertically spaced from its neighbors by - pairs of spacing strips (18) sealed to the membrane near opposing edges and parallel thereto, thereby forming a mem- brane envelope between adjacent plates for blood flow at right angles to the flow of oxygen. The disposition of adjacent plates (12) is such that the ridges (15) in support plates (14) vertically correspond. Typically, the ridges are about 0.4 mm. apart. Adjacent membranes are, if desired, sealed at their edges to provide further security against leakage of blood through any gap inadvertently present between the spacing strips (18) and membrane (13). The stack (11) is provided with an oxygen distribution chamber (19) which communicates with the channels (17). The stack also communi- cates through blood inlet and outlet ports (21, 22) with a suitable pump (not shown) such as a conventional roller pump, having sufficient capacity for providing the desired rate of flow through the stack. The type of membrane to be used in apparatus accord- - 12 - 1~697913 ing to the present invention will, of course, depend upon the application, For heat exchange, the membrane is generally metal, for oxygenation of blood or removal of carbon dioxide therefrom the membrane may be, for example, silicone elasto- mer, or microporous polyester, polyurethane, or polytetra- fluoroethylene, and for haemodialysis or reverse osmosis, the membrane may be of a wettable porous material such as "cellophane" or "Cuprophane" (registered trademarks). It will be appreciated that all materials which contact the blood, e.g. those from which the conduit and membrane are fabricated, should be blood compatible. Referring now to Figure 3, there is illustrated an alternate transfer membrane (23), of circular cross-section, forming a conduit (24) for flow of fluid therethrough. Con- duit (24) has constrictions (25~ along the length thereof which space zones, in the form of circumferential furrows (26), one from another. When fluid in the conduit is flowed along the length thereof in the direction shown by the arrow A and at the steady flow rates of the present invention, vortices are set up in the zones as shown. When the fluid is blood, and dialysing fluid or a gas comprising oxygen is passed over the outer surface of the membrane, dialysis or oxygenation can be effected. Figure 4 is a schematic illustration of a suitable blood circulating system employing an oxygenator in accordance with the present invention. Blood is drawn from animal 27 and transported through conduit 29 to a reservoir 31. Addi- tional whole blood, blood components, or other desired addi- tives may conveniently be added to reservoir 31 through a suitable port (not illustrated). Blood is pumped, at a ~รป~9 f 93 constant but disturbed flow rate, from reservoir 31 through - conduit 33 and oxygenator 37 by a first pump 35, suitably of the conventional roller type A portion of the oxygenated blood exiting oxy- genator 37 is returned to the animal 27 through conduit 39 by the action of a second pump 41. Pump 41 may be of the same general construction as pump 35. The capacities of the two pumps are so adjusted that the flow rate through conduit 39 is a safe rate for perfusing blood into animal 27 - usually the same rate at which the blood is being withdrawn from the animal and blood is pumped through the oxygenator 37 at a steady rate suitable to obtain the desired Reynolds number. The remainder of the oxygenated blood is returned via conduit 42 to reservoir 31, where it is mixed with the oxygen-poor blood from the animal. In other words, pump 41 runs at the desired perfusion rate for the animal, whereas pump 35 runs at a rate sufficient to keep the Reynolds number in oxygenator 37 in excess of about 200. A preferred blood reservoir suitable for use in the above-described system is illustrated in Figures 5-7. Reservoir 43 comprises a rigid lower portion 45 and a flexible upper portion 47, both portions suitably being made of blood compatible synthetic polymer. Two tangential inlets 49, 51 are provided in the lower, rigid portion 45 for introducing into the reservoir blood from the animal and the oxygenator, respectively. An outlet 53, preferably at the bottom of rigid portion 45, is adapted to be connected with tubing to the pump inlet. The tangential flow at - 14 - 106~7~3 the inlets facilitates mixing of blood in the reservoir, while minimizing trauma thereto. The location of the outlet at the bottom of the reservoir enhances this effect. The upper portion 47 oE the reservoir is adapted to expand and contract to accommodate different volumes of blood, since, as is known, contact with air is harmful to the blood. Accordingly, the blood level in the reservoir is conveniently above the top of the rigid portion 45. Alternatively, the flexible portion 47 may be adapted to collapse into the upper portion of the cavity defined by rigid portion 45 in the event the blood level is below the top of portion 45. Preferably, suitable support means are provided for reservoir 43. As shown in Figure 5, these means comprise a metal frame 55 fixed to rigid portion 45 as at groove 57. Suitably, additional hanger means 59 may join frame 55 to flexible portion 47, for example, through rings 61. A port 63 is preferably provided in flexible portion 47 for introducing stored blood and other fluids therethrough. Port 63 may also serve as a vent for venting air from the system, as during priming. As will now be apparent to those skilled in the art, many variations and modifications may be made without departing from the spirit and scope of the invention. This application is a division of Canadian Patent Application Ser. ~o. 227,568 filed May 22, 1975. - 15 -