“…Attention also needs to be paid to the possible accumulation during dialysis of headache-producing substances or, on the other hand, to a wash-out and subsequent decrease of headache-protective substances which allow headache to develop unopposed. Substances of this type which are of current interest in the investigation of vascular headache are histamine, 10 serotonin, 11 bradykinin, 12 and the formed elements of the blood such as basophils, 13,14 platelets, 15,16 and white cells. The possibility that these substances are altered during the course of dialysis, either in the patient's body or in the extracorporeal circulation, may be readily studied in the dialysis patient by sampling of the easily available arterial and venous bloods in both the internal and external circulation.…”