Hot answers tagged

154

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 ...


81

This is 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). ...


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, ...


30

Hair actually does have a limit to its growth. All hair cycles between periods where it grows, and when a new stand appears, which pushes the older, long hair out. Hairs that are generally shorter, like eyebrows or arm hairs, have a shorter growth period before a new hair pushes the old one out. The hair on your head actually grows much faster than many ...


26

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 ...


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

When there is little light, the color-detecting cone cells are not sensitive enough, and all vision is done by rod cells. Cone cells are concentrated in the center of the eye, whereas rod cells are very rare in the center (image source): When you focus on the star, the light is projected close to the center of the retina, where it will hit few rod cells. ...


21

There are several issues here: 1) Any mucous membrane is a specialized tissue for absorption. Mucous membranes are indeed not so good for passive diffusion, that makes them absolutely perfect tools for active absorption of certain substances, almost independently from the membrane type. To provide some examples: many drugs like cocaine are inhaled and ...


21

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 ...


20

There are several points here. Arachnoid granulations are not the only "sinks" for CSF. Even though it is true that most of the CSF is eliminated from ventricular system and subarachnoid space through these granulations, there are also suggestions that there are also other potential mechanism of shunting CSF into the venous system: Cranial nerves leaving ...


15

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 ...


14

There are only three kinds of optical receptors in the eye, but more than 900 kinds of olfactory receptors. Thus you can encode pictures with the three primary colors, but there is no small set of primary scents. To transmit a smell via "primary scents", you'd have to create an artificial nose that monitors the response of each of the olfactory receptors, ...


14

It would also be logical to note that a chewed tablet increases its surface-area-to-volume ratio for absorption relative to an intact tablet, for any absorptive tissue.


14

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 ...


13

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 ...


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

Cat claws are growing all the time, like horse hooves, or human nails. However, cats and horses usually use their claws/hooves, so they get shortened through mechanical action. An indoor cat may need their claws trimmed if it doesn't use them enough (that's why cats will want to scratch everywhere), or if has supernumerary toes that don't normally touch the ...


12

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 ...


12

The only sensible answer to these questions is "sometimes". The reason: even a simple monotone must be described by both frequency and amplitude. Any frequency can cause harm with enough amplitude, and any frequency can be harmless when the amplitude is low enough. If you want to actually quantify the damage, you'll need something like an equal-loudness ...


12

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 ...


11

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 ...


10

The female breast is composed primarily of fat and connective tissue plus lobules and ducts for milk production and delivery. The pectoralis muscles form a thin layer beneath the breast, as seen in the figure in the link. An athletic female can enlarge the size of the pectoralis muscle and the laterals muscles, just as male athletes can enlarge their "pecs" ...


10

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 ...


9

Just as an intro... The heart pumps deoxygenated blood from the right ventricle, through the pulmonary arteries (pic) which then eventually split into small capillary networks that surround the alveoli. The alveoli are formed by the trachea eventually branching off. So when you breathe in, the alveoli become filled with higher levels of oxygen. The blood ...


9

The two halves likely do weigh approximately the same. Since the density of most tissues that are not filled with air is basically equal, it doesn't matter that, e.g., heart, spleen, and stomach are on the left and liver is (mostly) on the right. In the left/right axis, it evens out. As for the location of the center of mass of a human: it's located about ...


9

Probably because it's easier to retain what the body wants than get rid of what the body doesn't want. What does your body want to keep from your Urine? Pretty much water and selective ions (Cl-, K+, Na+, Ca+2, etc.). Maybe a few other things, depending on how healthy you are. Now, what does your body want to get rid of in your Urine? Well, anything it ...


9

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) ...


9

It's the renal artery because it will contain everything from the liver as you describe plus all waste accumulated on the way from the liver to the kidney. Here it gets filtered, so there cannot be a place after the kidney that is less "pure".


9

Fracture healing occurrs in several steps: haemorrhage: blood and surrounding cells fill the space created by the fracture. fibrous callus: chondrocytes colonize the fracture space, with neovascularization. bony callus: osteocytes colonize the fracture space and rearrange in woven bone remodelling: in long bones, woven bone is remodelled into lamellar bone ...



Only top voted, non community-wiki answers of a minimum length are eligible