The invention describes a heat exchanger that continuously and passively regulates the transfer of heat across each part of the heat exchanger's partitions. Such partitions composed of typically two tubes, one inside the other's bore, that at various points, contact each other or separate from each other, thereby increasing or decreasing heat transfer across their walls at those points. Such separations and contacts being spontaneously effected by changes in the shape of at least one of the tubes which is driven by elements connected to that tube and which change their shape in response to temperature changes, such as shape memory alloys (SMA's) or bi-metal strips.