|MadSci Network: Anatomy|
First of all, thanks a lot for your very insightful question – as is the case with all good questions, you have seen that one explanation for a well-known fact (the existence of an external "sack" to protect the heat-sensitive sperm cells) leads to a conflict with a plain fact that everybody can see is true: There are regions in the world where bringing the testicles out of the (too hot) body is not probably of any use, since the outside is even warmer than the inside.
Second, let me apologise for the fact that it took me longer than the promised 8 days to churn out an answer. Oh, the workload...!
And third, here is your answer:
To begin with, the explanation for the existence of external (well, fairly external) testicles is true. From children in whom the testicles fail to descend into the „sack“ (the scrotum) during the final weeks of pregnancy we know that they are not fertile later in life – unless this state is corrected by an operation relatively early in life. The condition is known by its Latin name, "maldescensus" or "nondescensus testis." Even if one testicle makes it down into the scrotum but the other one does not, the nondescended testis should still be operated, because at its uncomfortable temperature (32 °C would be optimal), it is prone to develop testicular cancer. The degree to which the testicles are kept apart from the higher body temperature is, by the way, finely regulated. There is a muscle leading into the scrotum, the musculus cremaster, which activates in cooler temperatures, dragging the testicles closer to the body and thus heating them. If you don’t mind asking, a member of the opposite sex will readily confirm that after a bath in a cold lake, more or less everything "down there" looks quite shrunken and indrawn.
But you are asking about heat. Now this leads to a much broader problem. If we humans encounter temperatures higher than our own temperature, how do we manage to keep ourselves at a „cool“ 37 °C – or 32 °C, in the case of the testicles. In other words: Where is the refrigerator?
Basically, there are three ways to loose one’s excess heat. The three mechanisms are used by all animals that regulate their body temperature (mammals, marsupials and birds). One is radiation. You may have seen pictures of infrared cameras showing humans in a radiant glow of heat radiation. Problem is, radiation only works if the ambient temperature is lower than that of the body – thus, no help for our poor sperm cells. The second way is convection – you can see that the veins on your forearms grow large and bulging when you are in a hot environment. This way, they transport heated blood to the skin where the heat is transferred to the air. I do not know from where you are writing, but you may know this Italian soccer referee with a bald head – whenever he referees a match on a cold day, you see the air over his head going blurry from the heat convecting from the skin on his head (like over a fire or a hot street tarmac). But again, this method is no help to the sperm cells, because this mechanism too relies on the environment being colder than the body.
The only way we can get rid of our heat in an environment hotter than ourselves is by evaporation – plain, good old sweating. Refrigerators work by that principle (the evaporating fluid being the infamous chlorofluorocarbons or CFCs), and it is also the reason why it is so nicely cool around water fountains in summer – the evaporating water cools the air. We also evaporate water (up to 2 litres /hour, if you are really used to it), and the skin of our testicles does the same – only more of it, so the sperm cells inside remain at a comfortable 32 °C. And so, whether you want to cool down a human body to 37 °C or a human testicle to 32 °C on a very hot day (i. e., more than 37 °C) – it will be done by sweating.
I hope that this is a satisfactory answer. Write back if it isn’t.
Jens Peter Bork References – basically any good physiology and anatomy book will tell you in depth about heat regulation – although it might not say anything about this particular region. One potentially helpful website I found is http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatreg.html.
Have fun exploring!
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