Vascular access for haemodialysis. Position of the tip of the vascular catheter and its relationship in the input opening (superior vena cava or inferior vena cava). Correlationship with the recirculationVascular access for haemodialysis. Position of the tip of the vascular catheter and its relationship in the input opening (superior vena cava or inferior vena cava). Correlationship with the recirculation