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The testes (at least in human males) are vascular organs, the vascularization extending even o the interstitial spaces of the seminiferous tubules. The function of the scrotal sac, is to isolate the testes from the abdominal cavity, and provide a lower temperature ($1-2^\circ \text C$ lower than normal body temperature of $37^\circ \text C$).

But since the blood is the main temperature-buffer maintaining the normal body temperature and the scrotal (testicular) tissues are considerably vascularized, why doesn't the blood flow raise the scrotal temperature to normal body temperature?

If the reason is the location of scrotum external to the abdomen, then all limbs and limb-extremities are located in the similar way (and in those cases, heat dissipation would be faster than in case of scrotum due to less surrounding insulation by thighs and clothes), and hence the temperature of all parts except the interior of the head and abdomen must be at considerably lower temperature? Is this the case?

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well... fingers and toes do become cold and pale in winters.. :P –  WYSIWYG Nov 12 '13 at 7:22
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up vote 3 down vote accepted

The fingers and toes (for example) ARE at lower temperatures than the interior of the torso. It's why it's so easy to get frostbite on the extremities. As for temperature regulation of the testes, you have to also consider that humans evolved without clothes...i.e. The testes just "hang out" and get lots of airflow, as opposed to modern times, when they are typically under (usually) more than one layer of clothing.

Also, for reference, I took some quick IR temp readings on myself:

Room temp: 19.9C

Fingertip: 27.0C

Palm: 31.8C

Axilla: 34.6C

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You didn't include your scrotal temperature, the area of interest here. –  Ram Manohar M Nov 12 '13 at 19:16
    
I thought it would be odd to use the kitchen thermometer to measure the temperature of my "junk". Next best thing I could come up with was the axilla. –  Andrew Bonnell Nov 13 '13 at 14:31
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It probably does reduce heat dissipation in all cases, the reduction dependent on the nature of the clothing. Scrotal temperature is actually regulated by a couple of methods however: blood flow can be adjusted by changing the flow resistance of the vascular beds, and scrotum itself contains muscles that adjust how closely the testes sit relative to the body. Take a cold shower, and you will notice the scrotum will pull up snugly against the body. A hot shower and it will hang loosely. Of course, if temperature is not perfectly regulated, it's not the end of the world either. –  Andrew Bonnell Nov 17 '13 at 14:50
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The lower temperature optimizes sperm production conditions, but a degree here or there will not render one sterile. It may slightly reduce sperm count, but you'd probably be hard pressed to measure a difference. The outside of the body will probably always be about 1C lower than the core body temp, so you aren't looking at a huge alteration in the testicular temperature even if all thermoregulation were stopped by the clothing. –  Andrew Bonnell Nov 17 '13 at 14:53
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ncbi.nlm.nih.gov/pubmed/7496419 Apparently special undergarments can reduce male fertility significantly. I'm not sure about how rigorous that particular study is, but I suppose it's something to consider. I'm not going to attempt to replicate their work, at any rate :D –  Andrew Bonnell Nov 17 '13 at 14:58
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