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43 votes
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Why do human females have periods?

Short answer Shedding or reabsorbing the endometrial lining is energetically advantageous to the female.The advantage of shedding over re-absorption may be that sperm-born pathogens are removed from ...
AliceD's user avatar
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42 votes
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Why do the fastest runners tend to be black?

It's an interesting question and one that has been asked before. NPR did a story in 2013 on this topic, but their question was a bit more focused than just "why are so many black people good runners?" ...
Forest's user avatar
  • 1,879
19 votes
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How deep under water can humans open their naked eyes without damaging them?

Crushing damage from pressure occurs due to pressure differences. Imagine you have a rigid container. If you have equal gas pressure inside and outside, the pressure acts on just the walls and the ...
Bryan Krause's user avatar
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15 votes
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How does aspirin "thin" blood?

Aspirine irreversibly inhibits the enzyme cyclooxygenase. This enzyme facilitates the reaction from arachidonic acid to prostaglandin G2/H2. The further reaction leads to the generation of Thromboxane ...
Chris's user avatar
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13 votes
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Can you bleed from your bones?

Yes, bones too bleed if they get damaged (break or bruise). This is because bones are highly vascularised organs and breaking of bones will indeed rupture the blood vessels causing bleeding. Main ...
JM97's user avatar
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13 votes

How deep under water can humans open their naked eyes without damaging them?

Goggles do not, cannot, protect eyes from pressure. The pressure on both sides of goggles should be nominally the same. In fact, divers tend to use a mask, which communicates with the nose, to be able ...
Neil_UK's user avatar
  • 231
11 votes

Why do the fastest runners tend to be black?

Take a look at Will Grover's answer here, I will cite the genetic diveristy explantion here, I think that sounds the most logical: Genetic Diversity This is my favorite answer, and the one that ...
KingBoomie's user avatar
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11 votes
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How is vitamin B12 stored in the body although it is water-soluble?

Vitamin B12 (cobalamin) is a cofactor required by the enzymes methionine synthase (MTR) and methylmalonyl-CoA mutase (MUT). These are the only known enzymes that use B12 in humans, and are abundant in ...
Roland's user avatar
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11 votes
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How can we move our lips even though they don't have any bones?

Because of the Orbicularis oris muscle, it's a complex of muscles in the lips that encircles the mouth, It forms the greater part of the substance of the lips, lying between the skin and the mucus ...
Bilal's user avatar
  • 761
11 votes

Pathogens vs Microbes and the Immune System

Not all microorganisms are pathogens Firstly, not all microorganisms are pathogens - some can be harmless and some can be even essential for the host organism functioning (e.g., many microorganisms ...
Roger V.'s user avatar
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9 votes
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Why don't we feel hungry after waking up?

I will try to keep the answer as simple as possible, but feel free to point out if I make this too simple. Well, you are not alone with this observation. This is quite a common, but rather complex, ...
another 'Homo sapien''s user avatar
9 votes

How deep under water can humans open their naked eyes without damaging them?

Currently the record for the deepest dive wearing scuba equipment is 332.35m. The descent took only 15 minutes while the ascent lasted 13 hours 35 minutes. Liquids do not compress, so the sinuses, ...
bandybabboon's user avatar
  • 10.4k
8 votes

Why do the fastest runners tend to be black?

I also found this hypothesis from a scientific paper which suggests that asian and white people have a lower center of gravity than black people. See the general explanation: Blacks tend to have ...
Mud Warrior's user avatar
8 votes
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Why are we able to differentiate between colored objects without the presence of light?

You seem to have many misconceptions about how we see and how that relates to light. I'll address them one sentence at a time: Why are we able to differentiate between colored objects without the ...
Oosaka's user avatar
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8 votes
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Are 'homeothermic' and 'endothermic' synonymous?

[D]o homeothermic and poikilothermic have the same meaning as endothermic and ectothermic respectively[?] No. Once you are referring to the source of the heat, while the other time you are referring ...
Remi.b's user avatar
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8 votes
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Immune System - B-cell receptors

This is an excellent question. You are on the right track with the random match idea. B-cells (as a group) develop receptors that bind every possible antigen and then get rid of the receptors that ...
De Novo's user avatar
  • 8,811
8 votes

Why does exhaled air still contain oxygen?

You seem to be confused about diffusion versus bulk flow of fluid (air/gas). In the lungs, there is diffusion between blood and the gas in the lungs. There is sufficient surface area that these ...
Bryan Krause's user avatar
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8 votes
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Does LSD "stay" in your fat storage and then "returns" back?

I've heard this rumor for years, both from pro- and anti-drug people, and it is not true. According to this review of LSD's pharmacology, its half-life in the body is about 5 hours, with it being ...
MattDMo's user avatar
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8 votes
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How does increased resistance to flow decrease blood pressure?

There is not enough information in the question to solve it. The answer key from the original question makes a logical error: Viscosity is directly proportional to resistance. This is true. An ...
Bryan Krause's user avatar
  • 46.1k
7 votes
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Do humans produce rennin?

Scanning various reviews it seems that everyone who mentions the possibility of a human chymosin refers to a single paper. So for example this 2014 review has a single reference to a human chymosin: ...
Alan Boyd's user avatar
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7 votes
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Wim Hof Method claims Alkaline Blood is Good. Contradictions? Counterproof?

The Wim Hof method unfolds its health effects by hormetic stressors, respiratory alkalosis is one of them (a minor one). The WHM does not follow the pH theory, even though Wim himself gave ambiguous ...
Hannes's user avatar
  • 86
7 votes

Why is arterial pO2 normal in carbon monoxide poisoning?

First, let's think about what pO2 means: pO2 is not the same as "oxygen concentration." pO2 is the partial pressure of oxygen, you can think of it as proportional to the "number of oxygen molecules ...
Bryan Krause's user avatar
  • 46.1k
6 votes
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Why antipyretics do not bring the body-temperature below normal?

Antipyretics doesn't work by reducing the body temperature but blocking pathways that make it higher. For paracetamol/acetaminophen the mechanisms implied are not currently completely clear (as ...
DavideN's user avatar
  • 180
6 votes

Do human populations differ in core body temperature?

Yes there are different physiological temperatures in different ethnicities. To know which temperature range differences are inherited and which are learnt by living in different climates requires ...
bandybabboon's user avatar
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6 votes

What happens to the mold spores we inhale?

Mold spores are capable of landing and becoming dormant. Unless conditions are severe enough, mold spores will not just simply die off; they will essentially wait for conditions to improve and once ...
TheDemsk's user avatar
6 votes
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Why is glycerol kinase absent from adipocytes but present in the liver?

The short answer to this question is given in the Wikipedia article on glycerol kinase: Adipocytes lack glycerol kinase so they cannot metabolize the glycerol produced during triacyl glycerol ...
David's user avatar
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