43
votes
Accepted
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 ...
- 52.3k
42
votes
Accepted
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?" ...
- 1,879
25
votes
Accepted
Why is water flavorless?
Consumer water is not flavorless. Sources of flavor include (1) the chemical and microbial content, which is most influenced by geology and ecology; (2) chemicals added or removed during water ...
- 52.3k
19
votes
What is the maximum altitude where humans can survive?
Short answer
Between 62,000 and 63,500 feet (18,900 and 19,350 meters) blood begins to boil at body temperature. This altitude, referred to as the Armstrong limit, is generally considered to be the ...
- 52.3k
19
votes
Accepted
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 ...
- 44k
15
votes
Accepted
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 ...
- 51.5k
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 ...
- 231
12
votes
Accepted
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 ...
- 4,786
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 ...
- 2,380
11
votes
Accepted
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 ...
- 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 ...
- 3,832
10
votes
Accepted
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 ...
- 5,675
9
votes
Why do we get sleeply after we eat?
After we have eaten... the maximum blood supply is transferred towards the digestive system so that digestion is done, and therefore the brain to does not get adequate blood supply. Am I right about ...
- 24.7k
9
votes
Accepted
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, ...
- 14.1k
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, ...
- 10.3k
8
votes
Accepted
Could a trans-female person ever become pregnant?
Medical science has reached the point where it is able to consider performing uterine transplants in genetically female (cis-female) patients born without a uterus.
In the MedlinePlus report, First ...
- 4,985
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 ...
- 867
8
votes
Accepted
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 ...
- 3,245
8
votes
Accepted
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 ...
- 68k
8
votes
Accepted
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 ...
- 8,751
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 ...
- 44k
8
votes
Accepted
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 ...
- 15.2k
8
votes
Accepted
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 ...
- 44k
7
votes
Accepted
Why do humans masturbate?
One word. DOPAMINE
See this article on Reward System and this one on Dopamine Those basically answer your question as to addiction, urge, and feelings of relaxation.
There is absolutely nothing ...
- 4,985
7
votes
Accepted
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:
...
- 22.7k
7
votes
Accepted
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 ...
- 86
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