This tag is for questions about the general anatomical features of human beings as opposed to the anatomy of non-human animals.

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115
votes
4answers
15k views

Why do I only breathe out of one nostril?

I was just sitting with my hand next to my nose and I realized that air was only coming out of the right nostril. Why is that? I would think I would use both, it seems much more efficient. Have I ...
24
votes
1answer
3k views

Is humans' height ideal?

More or less we humans are around 1.70 meters tall. My question is, would it be possible to scale us? that is, is it in principle possible having humans 1 cm tall, or 10 meters tall or is there some ...
20
votes
3answers
23k views

If the brain has no pain receptors, how come you can get a headache?

I've read many years ago in books, that the brain has no nerves on it, and if someone was touching your brain, you couldn't feel a thing. Just two days before now, I had a very bad migraine, due to a ...
20
votes
1answer
15k views

How does cerebrospinal fluid circulate in the central nervous system?

Cerebrospinal fluid (CSF) is produced in the choroid plexus of the lateral ventricles and in the 4th ventricle of the brain. CSF then circulates through the ventricles of the brain and the ...
19
votes
1answer
2k views

Why do stars disappear when I look at them?

I've noticed that when I'm out at night stars seem to vanish when I focus on them. I assume the reason is physiological and not astronomic. Why does this happen? I would expect that things would be ...
18
votes
4answers
1k views

Why do we need red blood cells?

From what I know, the main function of red blood cells is hemoglobin transport. So, why do we need cells packed with hemoglobin: why can't it travel freely in the bloodstream? My own thoughts were: ...
16
votes
2answers
362 views

Why would diffusion be faster across a non-specialised tissue?

The standard protocol for a person experiencing chest pains is to chew a 300mg aspirin tablet, the argument being that chewing rather than swallowing the tablet results in the aspirin entering the ...
16
votes
5answers
7k views

Does red light preserve your night vision?

A number of companies have started marketing LED lamps that can be switched to a red mode. The claim is that red light is less harmful to one's night vision. Given that our eyes are less sensitive to ...
14
votes
4answers
1k views

Could we transmit smells electronically?

We can encode sound and images in radio waves and send them, but presumably there's some physiological reason that we can't easily make a picture or video of a smell. Could we realistically break ...
11
votes
2answers
2k views

Will humans and animals be harmed by frequencies outside their hearing range?

The human ear (for example) is sensitive to frequencies from 20Hz to 20kHz. Any sound beyond that limit would not be heard by us naturally. Will it harm us (effect our ability to hear) when we hear ...
10
votes
2answers
2k views

How is eye color in humans inherited?

In high school we studied the inheritance of eye color, as it was explained to us in the most simple way: blue eye color is a recessive, monogenic, autosomal trait. Now I know that it is a bit more ...
10
votes
1answer
309 views

Foveal ganglion cell density (Tay-Sachs Disease)

I’m currently reading on Tay-Sachs disease and have stumbled upon something regarding the typical “cherry red” macula symptom. On the one hand it is mentioned that the macula is almost devoid of ...
9
votes
1answer
2k views

Why can hair grow without limit while eyebrow cannot?

I cut my eyebrow and it grows until reach a certain length. Hair can grow without limit. Why can hair grow without limit while eyebrow cannot?
9
votes
2answers
2k views

How do the lungs act as a sieve to trap blood clots?

Does anyone know? I'm curious to find out as my human anatomy and physiology book doesn't go into much detail on how the lungs function as such
9
votes
1answer
614 views

Why are there nail growth differences between humans and other mammals?

When a cat is growing, his nails are growing with him to some extent. A grown cat has a fixed nail length that is not extending. By contrast, human finger nails just keep on growing, so we have to ...
9
votes
3answers
3k views

Why does human facial and head hair continue to grow?

Many people can grow extremely long head hair and facial hair. Are there evolutionary theories as to why this is the case? It seems like having long hair could be a disadvantage, and extremely long ...
9
votes
2answers
1k views

What is the evolutionary reason behind the fragility of teeth?

Almost all organs in the human body have a rather large threshold within which the organ or tissue is capable of repairing itself using materials supplied by the body, whether it's made from organic ...
8
votes
1answer
120 views

Is it the sphincter that flexes when a human pushes a bowel movement?

Of course the sphincter muscle is at the exit point. To use a toothpaste tube analogy, if I want to squeeze out some toothpaste, it does me little to no good to jostle the nozzle; I need to squeeze ...
8
votes
1answer
327 views

Coincidence that navel is near the womb?

The attachment point of the umbilical cord in a baby girl is very close to where her own baby would develop. Is there something in the evolutionary history of mammals that explains this, or some ...
8
votes
1answer
234 views

How fast is the blood flow in various vessels?

How fast is the blood flow in different blood vessels, such as the aorta and capillaries?
8
votes
1answer
415 views

Why are we using upper teeth and lower lip on labiodental sounds?

I came to wonder this when studying language (as well as other same theme question posted just few ago). For example the word "fantastic" we use upper teeth and lower lip to produce F sound, instead ...
7
votes
3answers
267 views

Weighing 2 vertical halves of the human body

I had this thought while bathing: Assume a perfectly developed physical human body and anatomy in terms of size, dimensions and proportions. In theory, if I cut the body vertically into two exact ...
7
votes
2answers
76 views

What is the benefit of near-emptying the bloodstream in the mammalian kidney?

In the normal function of the kidney, the bloodstream is near emptied of fluids as it moves through the glomerulus, in which glomerular filtrate is formed. As it proceeds through the Nephron, a ...
7
votes
1answer
340 views

What happens to bones that do not heal?

I would like to know the steps the body takes to heal a broken bone and what happens if it cannot heal properly. I am not looking for advice or explanations on resting and not doing activity, I am ...
6
votes
2answers
2k views

Can any other animal choke on food?

I read somewhere (I think it was Bill Bryson's book on the origins of the English language) that of all animals, we are the only ones that can choke on food (having something to do with how our larynx ...
6
votes
1answer
877 views

Chest muscle and female breast size

Do female chest size have anything to do with muscle? I know many female athlete and their average breast size is much bigger than normal female that does not tend to play sports. Do this have any ...
6
votes
1answer
82 views

Is it biologically possible for an adult's eye color to change?

Can it be that the adult eye can change color? Specifically my question is about a unilateral color change, such that the color of one eye remains constant, while the other changes color over time. ...
6
votes
1answer
741 views

How and why did mouth and nasal cavity evolve separate?

My initial objection is that nose filters air, mouth is for eating but is used for breathing also, plus they both are used to create sounds. What is the cause and reason in this case, why do we need ...
6
votes
1answer
429 views

Why don't we have hair on palms?

We have hair all over the body except palms. What is the biological reason behind this? Thanks
6
votes
1answer
1k views

What is the average Leg-to-Foot Length/Width Ratio?

I'm making a program that estimates a probable size (widest width and longest length) for a human's foot given the length and width of the leg to which it is attached. However, try as I might, I can't ...
6
votes
1answer
68 views

Healing rate of cut while suffering from a cold

I recently cut my finger fairly deeply with a box cutter and had to have it stitched. During the healing process I contracted a strong cold/mild flu (a sore throat and runny nose with a high fever, ...
5
votes
1answer
236 views

Would a spinal injury decrease human height?

A woman I know has been losing height at a rate of few cm a year following a severe spinal injury. How is this possible? What kind of injury could cause such progressive loss of height? Could this be ...
5
votes
2answers
9k views

What could cause hairs to gray at the tips but not the roots?

I have noticed that some of my sporadic gray hairs are gray at the tip side but oddly, not near the roots. Some are even only gray in the middle. I find all of this very counter intuitive, and I ...
5
votes
1answer
99 views

Reproduction and defecation

In mammals and birds the external reproductive organs and waste removal organs see to be the same. Is this always the case with other animals such as insects etc...? Are there any good reasons for it? ...
5
votes
1answer
162 views

Why does the sacrum fuse together?

Everywhere I look describes how the 5 sacral vertebrae start fusing together during adolescence and are totally fused by the time you're 30 or so. But nobody seems to explain why this happens. It ...
5
votes
2answers
378 views

How would constantly growing nails have aided early human?

Nails grow rapidly and constantly, such that without constant artificial trimming they would reach lengths difficult to manage. How did this benefit early humans, say 200kya? Were they used like ...
4
votes
2answers
175 views

Where do the 31 spinal nerves start/meet?

Which part of the brain is the first place (from top to bottom) where all 31 pairs of spinal nerves (on each side) meet? Or if they all start at one place - where is that? Nominally, the beginning of ...
4
votes
1answer
696 views

Difference between intervertebral and vertebral foramina

In the human spine, what is the difference between intervertebral and vertebral foramina? Is it just a matter of perspective and do they refer to the same thing or are they different?
4
votes
2answers
8k views

Why do toenails grow much slower than fingernails?

As the title suggests, I've noticed that my fingernail grow much faster than my toenails. For instance, I usually trim my fingernails every two week or so, but for toenails the number would be about 6 ...
4
votes
2answers
307 views

Extraretinal photoreception in mammals? [duplicate]

A Finnish firm Valkee sells light-ear-plugs against thing such as jetlag. I asked a researcher in Aalto university how do they really work and he responded ...
4
votes
1answer
220 views

what are the mechanics of sweating?

The causes of sweating aside, i.e. I'm not interested in which and how nerve signals are transmitted to sweat glands. But I would like to read a detailed account on what a sweat gland consists of, ...
4
votes
1answer
119 views

Accurate Human Skull Model for Educational Purpose

I want to buy a model of Human skull for education purpose. However, I really want a very accurate model and at a decent price. I got the info that many of the available skulls in the market are not ...
4
votes
1answer
108 views

Why do we yawn?

I've read a new study which suggests that yawning may help you keep a cool head. Also, the findings might hold some hope for sufferers of insomnia, migraines, and even epilepsy. Is there any ...
4
votes
1answer
2k views

3D models of adult male brain in the Blender software? Any open-sourced version for research?

I am trying to find 3D model of brains. I am particularly interested in adult-male -brains in Blender-model. I could find such 3D models of brains for sale here but because I am researching I would ...
4
votes
0answers
76 views

How do duplicate brain regions (ex: left/right amygdaloid body) operate together?

I frequently hear talk about parts of the brain like "Amygdala" or "Hypothalamus", so I looked them up in an app called "essential anatomy". What I see is that there's mirror symmetry, and most of ...
3
votes
4answers
205 views

Bad Eyesight and Evolution

Why do many humans have bad eyesight, such as near-sightedness, which hampers performance in a wide variety of tasks? Shouldn't there be evolutionary pressure towards better eyesight?
3
votes
1answer
6k views

What is the standard deviation of adult human heights within sexes?

The wikipedia article on human height reports mean height for many different countries by sex but it does not report standard deviations. What is the standard deviation of adult human heights within ...
3
votes
1answer
57 views

Do human visual functions degenerate due to genetic factors or by external factors?

Does eyesight primarily deteriorate 'naturally' due to genetics (genomic/epigenomic factors), or due to external factors such as normal wear and tear, or disease? By normal wear and tear I include ...
3
votes
1answer
100 views

How to maximize lung oxygen intake?

What method of breathing provides a sudden surge of oxygen into the lungs filling it as full as possible in volume and density. Deep breaths? Shallow breaths? Breaths taken while bent over? Deep ...
3
votes
1answer
405 views

Do adipose cells divide in adults?

I have a dim recollection of having heard that when humans gain weight, adipose cells just get larger, rather than dividing. True?