Abstract: A tubular set portion for circulating blood between a patient and an extracorporeal blood treatment device. The set portion has: an arterial tube for conveying blood from a patient toward the blood treatment device; a venous tube for conveying blood from the blood treatment device back towards the patient; and a pair of spaced, transverse tubes that each connect between the arterial tube and the venous tube, each of the tubes being capable of clamp sealing. The arterial and venous tubes are clamp sealable between the spaced, transverse tubes. Flow reversal on the patient's side of this device is possible by appropriate clamping of two opposed tube sections of the rectangular array which may be formed by the joined tube sections, while flow remains unchanged on the side of the tubular set portion that is connected to the extracorporeal blood treatment device.