246

Apparently you're not the first person to notice this; in 1895, a German nose specialist called Richard Kayser found that we have tissue called erectile tissue in our noses (yes, it is very similar to the tissue found in a penis). This tissue swells in one nostril and shrinks in the other, creating an open airway via only one nostril. What's more, he found ...


141

This is a natural phenomenon called the nasal cycle. It is discussed in this paper by Telles et al. (1994), among many others. The nostrils are used on an alternating cycle of about 2-3 hours, controlled by the autonomic nervous system. If you notice alternating congestion, that also seems to be coupled to the nasal cycle (Hasegawa and Kern 1977, 1978). ...


51

It is true that nearly all arteries carry oxygenated blood and nearly all veins carry de-oxygenated blood, but that is not what defines them. Arteries are blood vessels that carry blood away from the heart, and veins carry blood towards the heart. If you look at the situation in that light, the naming makes sense: the pulmonary artery is carrying de-...


39

As others have said, this phenomena is called the nasal cycle, a process controlled by the autonomic nervous system that alternants congestion between your nostrils. Mentalfloss of all places has an article about this that explains: ...it makes our sense of smell more complete. Different scent molecules degrade at different rates, and our scent ...


38

Humans can, to some extent, be scaled: While most humans are two meters tall; the human body essentially works throughout the entire range from around a meter to around two and a half meters, although you tend to run into problems at the extremes. However, outside of that range you run into the tyranny of the square-cube law: As you scale a creature (or, ...


37

I found many plausible claims that fingerprints increase friction. However, the following article claims, at least under their experimental conditions, that fingerprints actually decrease friction with smooth surfaces by reducing contact area. Fingerprints are unlikely to increase the friction of primate fingerpads. It is generally assumed that fingerprints ...


35

Brain, indeed, cannot feel pain, as it lacks pain receptors (nociceptors). However, what you feel when you have a headache is not your brain hurting -- there are plenty of other areas in your head and neck that do have nociceptors which can perceive pain, and they literally cause the headaches. In especially, many types of headaches are generally thought to ...


35

While others have addressed the big picture aspects of your question, I think it would be useful to look at the specifics. Have a look at the heart (or more accurately, the hearts) of the earthworm: They're nothing more than veins with some pumping muscles wrapped around them. It seems almost a stretch to call them hearts, they are shaped so different ...


34

To balance the debate, from a neutralist evolutionary perspective... There does NOT have to be a direct selective pressure for a trait's contribution to an organism's expressed phenotype. Three alternative, neutral, explanations: "Hitch-hiker" traits: It (the trait) could be a by-product of a more necessary component whose function is directly ...


24

The uniqueness of irises and fingerprints are, as you said, limited to the number of possible permutations of irises and fingerprints. A similar problem exists in computer science, and is known as a hash collision. Given sufficient samples, there will always be a collision for a hash of finite size. However, the sample space is sufficiently large for iris ...


22

Your best bet is the Terminologia Anatomica, which is the international standard for anatomical terminology. The 1998 edition is freely available. It lists only a few named tendons though, which is consistent with my experience as an anatomist: very few tendons are named separately from the muscles to which they are connected. Central tendon of the ...


20

This is a very good question. Red light is routinely used by scientific laboratories to do low light dissections of retinas, and of course it is used in other low light contexts such as printing plate development. In both of the above contexts, you have a clear subject: the retina being dissected or the printing plate being developed. In the case of the ...


20

Pregnancy test, just like any test, has some non-zero false positive rate. So yes, of course a test could be positive when a man pees on it. Pregnancy tests work by detecting the presence of a hormone called Beta-HCG. This hormone is produced by both genders but is produced in much higher dose during early pregnancy. Teenagers also produce a relatively high ...


19

Hemoglobin molecules used to manufacture these products are not contained by a red cell membrane, and when released into the vasculature, these molecules rapidly scavenge nitric oxide.This can result in systemic vasoconstriction, decreased blood flow, increased release of proinflammatory mediators and potent vasoconstrictors, and a loss of platelet ...


18

Overview. This is a very interesting question. The ideas behind this have been around for a while and the methods are covered in great detail elsewhere. The overall answer to the hypothetical question is rather surprising. There may be actually potential benefits to perpetual liquid ventilation for people with certain diseases. Trials so far have not found ...


17

Anatomical terms must be able to fit a wide variety of organisms, from insects to fish, dogs, horses, chimpanzees to humans. That's why the terms are sometimes confusing to people who are thinking only of bipedal humans. In anatomy, the dorsum is the upper side of animals that typically run fly, swim or crawl in a horizontal position. In vertebrates the ...


16

You are correct in that the neurons themselves do not sense pain. However, the brain contains layers of coverings, blood vessels, the scalp and some muscles. All of these other structures have pain receptors. The coverings of the brain are called meninges and consist of the dura, arachnoid and pia. The dura in particular has a lot of pain receptors and may ...


15

There are two types of sweat glands: (1) eccrine sweat glands and (2) apocrine sweat glands. Eccrine sweat glands are present from birth in humans and secrete sweat that is mostly water and functions in evaporative cooling. Apocrine sweat glands are found in the armpits and groin regions and become active in humans at puberty (although the distribution ...


15

It's probably around 7 cm for men and 6 cm for women. The Evolution of Adult Height in Europe, which is a source for some of the statistics reported on that Wikipedia page, gives averages, standard deviations, and sample sizes for both sexes across 10 european countries (and across a number of different age cohorts). Taking their numbers for men and women ...


15

The wikipedia article links to two papers. The first article has data for 120 pain-free and 51 affected patients with data on the number of coccygeal segments in each. We can back-calculate a reasonable approximation of segment distributions from their percentages: 1 Segment: 7% 2 Segments: 51% 3 Segments: 38% 4 segments: 4% So 2 or 3 segments makes up ...


14

First, for reference, see here for a discussion about the difference in directional terms between bipeds and quadrupeds as well as a fairly complete explanation of word meanings/etymology. The etymological meanings of the various anatomical directional terms should help explain their usage in body organs. For example: Ventral -> "belly" side Dorsal -&...


13

Developmentally, the urinary and genital systems (typically you will hear them referred to as "urogenital system") are derived from the same embryonic tissue, the intermediate mesoderm. The embryonic kidneys are drained by the mesonephric duct in both females and males. This embryonic tissue also gives rise to the ovaries and testes. The mesonephric duct ...


12

The concept of transgenders has nothing to do with sexual organs at birth. Transgenders is a concept that has to do with sociology, not biology. See the wiki link. intersex (previously called Hermaphroditism) has to do with biology. The wiki link makes a great job at defining intersex. In short: A transgender person is born anatomically either male or ...


12

Short answer The relative large surface area of the white sclera in humans has been linked to an enhanced ability to detect eye gaze. Background The white of the eye is caused by the sclera. Human eyes indeed have the highest relative amount of visibly exposed white sclera. The amount of visible sclera provides information about the orientation of the ...


12

Are males taller than females? Best data I could find come from the Statistical Abstract of the United States (1999) > Section 3. Here is a table reporting the percentage of the male and female population which height is lower than a given threshold Note that this data collection was done among students in US universities and is therefore not ...


12

This kind of question was raised in a book called "Darwin's Black Box" by Michael Behe, who is a biochemistry professor in the U.S. - he calls this 'irreducible complexity' (IC). For example, the blood clotting cascade system where you have a large number of components that are all apparently essential for the process. Now I have to say I find the idea that ...


12

The FMA lists 705 tendons, but note that it includes separate terms for left and right instances. As @kmm says, many of these simply shadow the list of skeletal muscles (and is likely incomplete). You can browse the list on OLS, or if you want to extract a table you can query this SPARQL endpoint, just type in the query here: SELECT DISTINCT ?x ?v0 WHERE { ...


11

How humans evolved to have head / beard hair that continues to grow longer than other animals is a topic that many anthropologists & biologists are still not sure about & there is no general consensus as to "why" yet. The three main views that I am aware of however are : 1) Evolution of the "Aquatic Ape." (Ingram, 2000: Morgan 1997; 1982) ...


11

Questions of the type, "why does organism X have feature A?" invite teleological (just so) explanations which are difficult to substantiate. For example, the number of teats on a cow are difficult to explain in terms of providing milk to humans! We should look to evolutionary history to explain human traits, not "just so" stories. The simple answer is that ...


11

Here is my overly succinct answer. I doubt we will ever know this for sure. But, it basically comes down to ancestral bilateral symmetry in development; this defaults to two organs and is broadly symmetrical except where the organs are central. See this article for a more thorough answer regarding bilateral symmetry. Further exceptions to symmetry occur ...


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