ECMO circuits usually have a side port just downstream of the pump that is used for continuous renal replacement therapy (CRRT). The side port allows blood to be easily drawn off from the circuit, and returned after going to the dialysis filter. This is incredibly convenient as most patients that are initiated on ECMO often have concomitant renal failure requiring either full continuous hemodialysis or at least ultrafiltration for volume overload. The flows in the ECMO circuit can reach 5 L/min and the blood through the CRRT circuit is only 100-200 m/min so it represents a very small portion of the total blood flow. While Perfusionists are often reluctant to use the circuit for hemodialysis, the availability of the side ports eliminates the problem of additional patient venous access for the dialysis catheter.
The location of the side port and relationship to the pump can be seen in the photo below. The side port has a Luer-Lok connector and projects orthogonally from the tubing of the ECMO circuit.