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June 14, 2008

Beat The Heat:Testis Cools The Easiest Way

single electron refrigerator cools objects near absolute zeroScientists are developing new ways to cool matters. In one instance, Jukka Pekola of the Helsinki University of Technology and his team has devised a way to cool things down to 10 mKelvin. He uses single electron cooling to achieve this feat. A transistor like device ferries an electron (a real hot electron) from the object to be cooled to a superconductor. This process called tunneling is a peculiar one, involving quantum mechanics. A particle which does not have sufficient energy to jump over an energy barrier, can still pass this barrier by this process called tunneling. It is facilitated by the application of a voltage across the 'channel/barrier'. As the picture on the left portrays, the orange colored bar is the one to be cooled. Attached to it are 4 superconducting leads. Two of these acting as temperature sensors, the other two are meant for cooling. The 'T' like thing in the left is the 'gate' of the device; acting capacitively with the metal object to be cooled. A high frequency current oscillates between the 'object' and the 'gate', through capacitive dielectric. This oscillation helps in taking away the heat from the metal to the superconductor. It also replaces the traditional voltage-gating needed in the 'herding' of the electrons mentioned before. To boost efficiency, scientists have implemented 'Coulomb-blockade' as the electrons try to exit. As a result of this, only the hottest electron can exit (sounds like Darwin's survival of the fittest).

Phonon transistors and heat transferIn another instance, A. Majumdar and co-workers at the University of California at Berkeley have built the first nanoscale solid state thermal rectifier (diode). It is not tough to guide electrons, since an electrical or magnetic field will do the job nicely. Guiding heat, on the contrary, is a bit tough; heat being a phonon by nature can not be acted upon by electromagnetic interactions. So scientists rely on a very common phenomenon: resonance, to transfer heat. The basic principle is somewhat like this. When you tune two chords of two different guitars and strum just one, the other guitar too will sound in unison. This occurs due to resonance, as energy transfer is maximum at resonance. Every natural object has its own natural resonating frequency. By changing this resonant frequency by altering the physical properties of the object/medium (for example, by introducing inhomogeneities in the paths of carbon nanotubes, a one dimension structure) one could attach directional properties to phonons. A schematic picture of a thermal transistor is shown above.

But the testes give a damn about quantum tunneling or resonating phonons. This luxury loving structure is born inside the abdomen, by the side of the developing kidney, in the form of 'genital ridge' also called gonadal ridge. It needs to descend to the scrotum, its ultimate abode, to stay away from the hot core body temperature of 37 degree Celsius. The heat is detrimental to spermatogenesis or gametogenesis, (one of its two important functions; the other being the endocrine function of production of androgens). As it descends, it draws its blood supply, lymphatics and nerve supply along, from the abdomen down to the scrotum. Since the (arterial) blood is a good conductor of heat, the testes are supposed to have the same core body temperature. But still the testes cools (in fact, it even warms up when necessary!). countercurrent exchange of heat and testosterone in testisThis occurs because, testicular veins and a rich venous plexus called the pampiniform venous plexus run alongside i.e. parallel to it. Since the venous blood flows in opposite direction to that of the artery, it can be said that they (veins and arteries) are antiparallel. The heat the arterial blood carries, is efficiently transferred to the veins, which lie in close proximity, even before the arterial blood reaches the testis. The testes are no longer exposed to the core body temperature then. This is called counter-current heat exchange mechanism. A similar mechanism restricts the hormone testosterone to the testis itself, rather than diluting out in the systemic circulation.

Another mechanism of temperature regulation is contributed by the cremaster muscle covering the testes. When it is hot, the muscle relaxes, the testis hangs lower from the abdomen. Thus it can escape the direct conduction of heat from the abdomen. The pampiniform plexus then plays the role of an effective radiator. Conversely, in cold, the muscle contracts, pulling the organ closer to the abdomen. The testis then warms up. Using both mechanisms of coutercurrent and cremasteric regulation, the testicles maintain an optimum temperature of about 2 degree centigrade below core body temperature, the most suited temperature for gametogenesis. Isn't it cool?

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