Wikipedia:Reference desk/Archives/Science/2014 March 3

= March 3 =

Blood donation risk factors
Why do different countries, even with similar statistics for HIV risk etc have different eligibility criteria for donating blood? For example, in the US those who have had tattoos in a licensed state, are not barred from donating blood but in the uk, they are barred for 4 months despite licensing for tattoo parlours existing across the uk. Similarly, men who have sex with men have a lifetime ban in the US, whilst only a 12 month ban in the UK. Other risk factors such as those who pay for sex are not even considered a risk factor in the us, whilst there is a 12 month ban in the uk. Why are these restrictions so different? — Preceding unsigned comment added by 2.126.193.17 (talk) 11:09, 3 March 2014 (UTC)
 * Because different people in different places are making different decisions. -- Jayron  32  13:14, 3 March 2014 (UTC)


 * As a regular donor I can tell you it's because of the Window_period for various infectious diseases. The ban is an attempt to screen donors in that period where the disease is not detectable in a blood/serum test. This relies on people telling the truth about their sexual activities. Both systems (US and UK - and even South Africa) have risks to the recipient. When my father went for his prostate op my brother and myself donated and "banked" our blood for him. The blood is still tested but dad knows we would not lie about the screening questions. 196.214.78.114 (talk) 13:49, 3 March 2014 (UTC)


 * Also as a regular donor, I can tell you that in the USA there IS a 12-month deferral period for anyone who pays or receives payment for sex. 24.5.122.13 (talk) 03:49, 4 March 2014 (UTC)


 * It's four months for a tattoo in the UK now - see Who Can't Give Blood?. They keep changing these rules as research about the diseases improves. It wasn't long ago that your blood wasn't acceptable in the UK if you were a man who had EVER had sex with another man, but things move on. I understand that many countries won't accept your blood if you lived in the UK during the 1980s, because of fears of Variant CJD. Obviously, if such a rule was applied in the UK, there wouldn't be many people who could donate. Alansplodge (talk) 13:43, 4 March 2014 (UTC)


 * You've already had some good answers here particularly relating to different people making different decisions based on a variety of factors (such as differing views on the evidence). It's worth noting that the restrictions are not without controversy. Notably the Gay male blood donor controversy, particularly as the indefinite restriction in some countries is argued to be inconsitent with other deferrals for high risk groups (such as people who have been paid for sex or paid for sex or had a very large number of sex partners at some stage) which are for shorter periods in some countries. Of course as some above have indicated there is a risk of people not telling the truth so it is possible that statisticly there is a higher risk for some reason (e.g. a person by be willing to admit they engaged in MSM at some point in their life, but not recently which doesn't happen with some other grouping). Although I've also seen it suggested that by making the criteria too strong such that people feel it's unreasonably discriminatory, people might be more likely to lie. Nil Einne (talk) 15:16, 5 March 2014 (UTC)

Human body parts that we have more than 2 of?
What is the body part that we have the fewest of but we have more than 2 of?

Besides fingers and toes.

And not general body parts like "facial orifices" or "bones in the forearm". They should be discrete body parts like kidneys or femurs or hairs on the head. — Preceding unsigned comment added by 24.228.94.244 (talk) 15:40, 3 March 2014 (UTC)


 * Wisdom teeth? --109.189.65.217 (talk) 17:53, 3 March 2014 (UTC)


 * The pineal gland is historically a third eye, and was active as a light sensor in early fish. The liver usually has three lobes, but that is not vital to its function.  There are all sorts of nerves, muscles and bones that come in small sets, like cranial nerves.  The inner ear has three canals for balance in the three dimensions. μηδείς (talk) 18:18, 3 March 2014 (UTC)


 * Hmmm, that's good... except you have two ears, making it 6. :) Getting to 3 is difficult, but I'll go with ... the tricuspid valve.  Except, that is, sometimes it doesn't have 3. :)  For 4 there are more choices, such as canine teeth. Wnt (talk) 19:59, 3 March 2014 (UTC)


 * That valve is a part of the heart. The OP would need to clarify just what he means by "body part". Does a portion of a larger organ count? ←Baseball Bugs What's up, Doc? carrots→ 21:35, 3 March 2014 (UTC)


 * Most people have two nipples. Some have three.  A case could be made that the average person has two-point-something nipples.  Also apparently applies to kidneys. (Other than that, an internet search for "people have three" finds little, other than "three brains" which probably doesn't qualify.) 88.112.50.121 (talk) 21:32, 3 March 2014 (UTC)


 * The human body, like most animal life, has bilateral symmetry. So most of what we have comes in pairs.  -- Jayron  32  23:20, 3 March 2014 (UTC)


 * But rather incomplete bilateral symmetry, as it doesn't apply to our liver, gall bladder, pancreas, heart, most of our digestive system, etc. StuRat (talk) 23:50, 3 March 2014 (UTC)


 * The small intestine is divided into 3 parts, the duodenum, jejunum, & ileum. StuRat (talk) 23:44, 3 March 2014 (UTC)


 * Veins and arteries. All connected, but many tributaries. Fewest and most. InedibleHulk (talk) 23:47, 3 March 2014 (UTC)


 * If you toss in capillaries, that makes 3 types of blood vessels. StuRat (talk) 01:16, 4 March 2014 (UTC)


 * Cheeks.165.212.189.187 (talk) 15:50, 4 March 2014 (UTC)
 * This one is right - you have 4. :) The eyelids are more of a stretch. :) Wnt (talk) 06:04, 5 March 2014 (UTC)
 * I thought you would have said that about the sphincters! I think eyelids is it165.212.189.187 (talk) 14:09, 5 March 2014 (UTC)


 * Eyelids165.212.189.187 (talk) 15:51, 4 March 2014 (UTC)


 * Limbs165.212.189.187 (talk) 16:04, 4 March 2014 (UTC)


 * Sphincters.165.212.189.187 (talk) 16:06, 4 March 2014 (UTC)


 * Testicles...all manner of reproductive bits. SteveBaker (talk) 17:59, 4 March 2014 (UTC)


 * Giving that answer without explaining what you mean takes a lot of balls. :-) StuRat (talk) 19:37, 4 March 2014 (UTC)


 * Yes, but at least we now know that User:SteveBaker comes from Cape May, lucky bastard. μηδείς (talk) 19:50, 5 March 2014 (UTC)

Oxygen
What's the lowest level of oxygen humans can tolerate? Is 2% enough to guarantee death? I heard some people can tolerate like 1%, which I find bull. Money is tight (talk) 23:21, 3 March 2014 (UTC)
 * It's not the percentage of oxygen, it is the partial pressure of the oxygen that matters. That is, lowering the percent oxygen by raising the overall pressure (such as adding an inert gas to the breathing gas) shouldn't have a significant effect.  The actual amount of oxygen you need to survive is more complex than just a single number, it can be found most easily using the Oxygen–haemoglobin dissociation curve which depends on many factors, including blood pH, partial pressure of O2 in the air, partial pressure of CO2, etc.  -- Jayron  32  23:26, 3 March 2014 (UTC)


 * To be completely clear, if you doubled the atmospheric pressure, you'd only need half the usual PERCENTAGE of oxygen - and if you halved the atmospheric pressure, you'd have to double the percentage. So this question only has meaning if the atmospheric pressure is specified.  We know that people can survive (albeit with difficulty - and for limited amounts of time) at the top of Mt Everest - which has a third the atmospheric pressure (and hence a third of the oxygen) than at sea level.  At sea level, we have 21% oxygen - so clearly 7% (at sea level) ought to be survivable for at least some amount of time.  However, as Jayron says, it's a complicated business - people who live at high altitudes can survive with much less than people who normally live at sea level, so we should expect wide variations.  Also, your needs are different with exercise rather than when relaxed.  There is a big difference between what you could survive for an hour at - and what would keep you alive for 5 minutes or an entire day. SteveBaker (talk) 17:57, 4 March 2014 (UTC)


 * How many people could live for 5 minutes without O2? Tolerance of course varies on if you get to hyperventilate first, hold your breath, whether brain damage is "tolerating" or not, and whether you were exposed to vacuum which would make it impossible to hold onto any oxygen. (your lungs might explode if you tried) Even a deep exhalation still keeps some air in the lungs. Sagittarian Milky Way (talk) 17:25, 5 March 2014 (UTC)


 * 22 minutes is possible. Count Iblis (talk) 13:49, 6 March 2014 (UTC)