Apparatus and methods are described, including apparatus (20) for implanting in a heart of a human subject. The apparatus includes an interatrial anchor (22) shaped to define an opening (26) having a diameter of 4-8 mm, and a bag (24) in fluid communication with the opening of the anchor. The apparatus is shaped to fit within a right atrium of the heart of the subject, and has a capacity of between 4 and 20 cm3. Other applications are also described.
