Apparatus is provided including first and second chambers, adapted to be in fluid communication with first and second volumes of oxygenated blood of a subject, respectively, the first and second chambers having first and second surfaces, respectively. A third surface is adapted to apply an elastically-derived force, at least a first and second portions of the third surface in mechanical communication with, respectively, the first and second surfaces, during at least a portion of a cardiac cycle. A pressure-sensitive valve is coupled between the first and second chambers, the valve adapted: (a) to be in an open position during at least a portion of systole, such that the first chamber is in fluid communication with the second chamber; and (b) to be in a substantially closed position during diastole, such that the first chamber is substantially not in fluid communication with the second chamber. Other embodiments are also described.