Apparatus and methods are described including an impeller configured, in a radially-expanded configuration thereof, to pump a fluid by rotating. A radially expandable cage is disposed around the impeller, such that, in radially-expanded configurations of the impeller and the cage, the impeller is separated from an inner surface of the cage. The impeller is disposed within the cage such that, in response to the cage becoming radially contracted and axially elongated, the impeller radially contracts and axially elongates. Other applications are also described.