as already mentioned above , the exchange structure according to the invention is , to a certain extent , like a fiber structure in the sense that it lends itself to being made in the form of an elongate filamentary body of indefinite length of which fig1 , 5 and 6 show any portion whilst fig2 and 7 show a section taken on any diametrical plane ( the longitudinal position in which this section is taken varies only the relative orientation of the elements considered ). an exchange structure according to the invention , generally indicated 1 , comprises essentially two elements , both of indefinite length and therefore capable of being cut to any desired length or wound in any manner entirely similar to those utilized for fibers , that is to say : a core 2 , 2 &# 39 ;, 2 &# 34 ;, 2 &# 34 ;&# 39 ;, which defines the general longitudinal development of the exchange structure 1 , and a sheath or covering 3 of generally tubular form ( although this configuration is not essential in itself which surrounds the core , 2 , 2 &# 39 ;, 2 &# 34 ;, 2 &# 34 ;&# 39 ;, more or less loosely , for example being fitted over the core itself . in this connection it should be stated that even though explicit reference will be made in the following description to an embodiment in which the core and the sheath are physically separate from one another , being constituted by two separate bodies , the invention also lends itself to being made with arrangements in which the core and the sheath are at least marginally connected together , for example because they are made simultaneously or almost simultaneously , for example by an extrusion , co - extrusion or like operation . whatever specific constructional arrangement is adopted , the sheath 3 defines an exchange surface between at least one first medium , which can flow within the space between the internal surfaces of the sheath 3 and the outer surface of the core 2 , 2 &# 39 ;, 2 &# 34 ;, 2 &# 34 ;&# 39 ;, and at least one second medium which , in use , flows over the outer surface of the sheath 3 in a mechanism generally similar to that which regulates the functioning of hollow fiber or tune exchange structures . in the embodiment of fig1 and 2 , the core 2 is formed by two filamentary or fiber elements ( here exemplified in the form of solid fibers ) preferably wound , at least locally , in a generally helical configuration . a substantially similar arrangement is adopted for the core 2 &# 39 ; of the embodiment of fig3 and 4 in which there are three filamentary elements 4 ( here also shown in the form of solid fibers ), preferably coupled in a generally stranded arrangement , also with a generally helical configuration in this case . in the embodiment of fig5 and 6 , the core is constituted by a profiled body 2 &# 34 ; or 2 &# 34 ;&# 39 ; with a lobed cross - section , for example with four substantially identical lobes equiangularly spaced at 90 ° about the longitudinal axis of the core . the difference between the embodiment of fig5 and the embodiment of fig6 lies in the fact that , whilst the core 2 &# 34 ; of fig5 has a generally rectilinear form , the core 2 &# 34 ;&# 39 ; of fig6 is , so to say , twisted along its axis such that the lobes 5 are in a generally helical configuration . with regard to the embodiment illustrated in fig1 and 6 it should be said that , in these drawings , the helical configuration of the core 2 , 2 &# 39 ;, 2 &# 34 ;, 2 &# 34 ;&# 39 ; has been exaggerated ) in order to illustrate the principal clearly ( compared with the real situations currently considered preferable . a characteristic common to all the embodiments illustrated is the fact that the sheath 3 surrounds the core 2 , 2 &# 34 ;, 2 &# 34 ;&# 39 ; in such a way as to leave spaces 6 between the core and the inner face of the sheath 3 . these spaces define longitudinal flow channels within the exchange structure 1 for one of the media which is involved in the exchange process ( of mass or heat ) in use of the structure . for example , in the case of a blood oxygenator device , in a haemofilter or a haemodialiser , the spaces 6 define the flow channels for the oxygen , the blood filtrate or the dialysis solution , whilst the blood which is oxygenated or purified flows over the exterior of the sheath 3 . in embodiments such as those of fig1 to 4 , the spaces 6 have a cross - section which is generally v - shaped with rounded sides ( defined by the outer surfaces of the fibers 4 ). to these is added , in the arrangements of fig3 and 4 , a central channel or space 6 &# 39 ; of approximately triangular shape . not being exposed to the sheath 3 and therefore not being usable in the exchange mechanism , this central space 6 &# 39 ; can be closed or utilized for other purposes ( for example for the passage of guide wire ). it goes without saying that the arrangement described in fig1 to 4 can readily be extrapolated to the production of further variants of the invention ( not illustrated here ) in which the core is formed by a strand or braid or four or more filamentary or fiber elements 4 . the provision of a plurality of fibers 4 in a generally helical arrangement enables the outer surface of the sheath 3 also to have generally helically - ribbed shape if fitted tightly , or even slightly loosely around the core 2 or 2 &# 39 ;. this particular conformation is advantageous in all those situation in which it is useful to have a degree of turbulence in the medium which flows over the outer surface of the exchange structure in order to achieve effective exchange : for example , in the cases cited above , blood which is to be oxygenated or purified . similar , substantial turbulence , or at least non - stagnation , of the medium which flows within the exchange structure 1 is promoted by the generally rough form of the channels 6 . the above also applies essentially to the embodiments shown in fig5 to 7 , especially for the embodiment of fig6 in which the core 2 &# 34 ;&# 39 ; has a generally helical form . the variants of fig5 to 7 have the further advantage that the profiles of the connector surfaces , indicted 6a , interconnecting the lobes 5 can be modified selectively . the surfaces 6a , together with the inner surfaces of the portions of the sheath 3 facing them define the cross - sectional profiles of the spaces or channels 6 . thus , for example , whilst the drawing of fig7 shows surfaces 6a of generally arcuate shape , these surfaces could have an l or v shape or a polygonal or more complex shape . all this within the recognized preference for shapes in those regions of the lobes 5 intended to come into contact with the sheath which are generally rounded in order to support it : corners which are too sharp would in fact induce wear and possibly rupture of the sheath 3 itself . it will likewise be appreciated that , in all the embodiments illustrated , it is possible to make the channels 6 of such a shape that , in those regions in which the lobes 5 lie close to the sheath 3 , corner regions with extremely small radii are formed . all this is for the purpose of causing , when liquids are present or flowing in the channels 6 , capillary phenomena to occur which prevent the formation of gas - liquid fragmentation with the exchanger structure , according to the teachings provided in ep - a - o 521 430 . as regards the choice of constituent materials the structure according to the invention is extremely flexible and can be varied according to specific fields of application . for example , for the production of mass - exchange structures ( oxygenators , dialysers , structures for reverse osmosis , etc . ), for the sheath 3 it is possible to use all materials conventionally utilized for the production of exchange membranes or fibers , for example , polymers , polyethylene , polypropylene , polyurethane , polysulphone , silicone , cellulose materials , etc ., up to the use of metals in the case of heat - exchange structures , for example metals having a high thermal conductivity ( copper , aluminum , steel , etc .) there is also the possibility , as far as the wall thickness of the sheath 3 is concerned , of using extremely small values , for example , of the order of a few microns , which is optimum for the majority of the applications indicated above , and of making use of unified technologies ( for example extrusion , spinning , co - extrusion , etc .) which do not impose particular overriding limitations as regards the dimensions of the structure . these can therefore be shown according to the specific requirements of use , for example as far as the overall section of the channels 6 is concerned . by way of example , structures of the type illustrated in the appended drawings can be made with diameters of the order of 150 - 400 microns , from which it will easily be understood how , in the drawings , and in particular in the sections of fig2 , and 7 , the thickness of the sheath 3 , which is a thin - walled body , is shown with dimensions which are greatly enlarged with respect to the real relative dimensions for reasons of clarity of illustration . the above is also true in essence as regards the production of the cores 2 , 2 &# 39 ;, 2 &# 34 ;, 2 &# 34 ;&# 39 ;. here , too , the choice of materials in just as wide , it being possible to range from very diverse types of polymer to metals ( above all when it is desired to give the structure a certain longitudinal rigidity or strength , for example for the production of devices for catheterization ). all this can be achieved by the manufacturer with the use of known manufacturing techniques , such as , typically , extrusion , co - extrusion of the sheath on the core , or fitting of heat - shrinkable sheaths and / or ( in the case of structures such as those illustrated in fig1 and 3 ) spinning and winding techniques . as already mentioned , the sheath and the core may then consist equally well of two separate bodies or parts of a single body . this is regardless of whether they are made from the same material or are formed from different materials . naturally , the principle of the invention remaining the same , the details of construction and the embodiments can be varied widely with respect to those described and illustrated , without thereby departing from the ambit of the present invention .