9 with a very small diameter see fig3 a which may increase see fig3 b 3 d as it moves towards the inside of the duct portion 4 where once a certain dimension has been reached it remains constant in embodiments the tubes according to the present disclosure may be produced by extrusion using three different extruders a first extruder may be responsible for the extrusion of tubular wall 3 while a second and a third extruder may be respectively responsible for the extrusion of rigid core of the rib and of outer part of the rib in particular by changing the speed of an extruder screw of the second and the third extruders it is possible to dose the quantity of both the rigid core material and the outer material of the rib while maintaining unchanged the rib external diameter in other words the combination of the co extrusion variability together with a variable pitch mandrel allows a progressive decrease of pitch during extrusion without stops coupled with decrease of rigid core screw velocity and increase of soft material screw velocity the tubes of the present disclosure may have the same properties as the hoses of the prior art including rigidity and flexibility of the different portions of the tube itself however the tubes of the present disclosure do not require the use of specific fittings for their assembly in the respiratory circuit and therefore do not require gluing operations in fact the.