Patients with kidney failure can be treated in different ways, including haemodialysis. Which of the following statements describes a feature of haemodialysis?

1. Blood is passed through an external machine containing a synthetic semi-permeable membrane.
2. Heparin is added to the blood to prevent clotting while it is outside the body.
3. The peritoneal membrane lining the abdominal cavity of the patient acts as the filtration barrier.