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21

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 ...


20

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 ...


18

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. ...


17

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 ...


16

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 ...


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 ...


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

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, ...


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 ...


8

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 ...


8

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 ...


7

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 ...


6

I would say it is a coincidence because: the navel is in the same place in males, so if it was there for some evolutionary advantage the charachter would be sexually dymorphic, which clearly is not the case. I cannot think of any specific function of the navel that is related to pregnancy or delivery (or of a specific function of the navel at all for that ...


6

The veterinarian in our group offers this: For humans, who choke much more frequently than other mammals, it is likely to be a cognitive problem. We talk and eat at the same time and so give ample opportunity to allow food passed the epiglottis and choking. Animals do choke - dogs can, cats can. Not all animals can vomit and this is particularly a problem ...


6

Humans, like all vertebrates, belong in subregnum bilateria, a broad class of animals whose characteristic trait is having a bilaterally symmetric body plan at least in some of their life stages. The common ancestor of all bilaterians was presumably something like a small marine worm. For a primitive animal living in water, an obvious advantage of ...


6

The pressure that you apply when you push during a bowel movement derives from an increase in the pressure of the abdomino-pelvic cavity. You generate this pressure by closing the glottis (the opening to the lungs) and contracting the anteriolateral abdominal muscles (i.e., the external oblique, internal oblique, and transversus abdominus). This reduces ...


6

Here is a comparison of the range of wavelength sensitivities for both rod cells (labelled R) to the 3 subtypes of cones cells (labelled S, M and L) from Wikipedia. If one is exposed to red light (above ~650 nm), it would activate the L-type cones mainly (possibly some M-type activation), but no rod activation. Rods are the low light receptor cells in our ...


6

Neither the nostrils nor the mouth originally evolved for breathing. Fish have (two pairs of) nostrils which they use to smell and mouths which they use to eat, but they breathe through their gills. Some lobe-finned fishes (the ancestors to tetrapods) evolved a connection between the posterior nostrils and the oral cavity called choanae. A fossil called ...


5

In expansion to biocs' excellent answer, I would like to highlight some practical limitations of this. Suppose we did manage to create a huge database of exact chemical mixtures which produce all smells recognisable by humans. You would still meet some complications: The output device (analogous to headphones or screen) would either need to be able to ...


5

In the middle cerebral artery blood flow at rest is 73.7 cm/s. After a period of 3 minutes of hyperventilation, blood flow in the same artery decreases to 37.6 cm/s. This is published work. Vena cava measures: Peak velocities during ventricular systole ranged from 30 to 45 cm/sec in the inferior, and from 10 to 35 cm/sec in the superior, vena cava, also ...


5

There is no direct link between the capillaries in the sinuses and the ear. The Eustachian tubes drain the middle ear (between the eardrum and the inner ear) into the nasopharynx, the part of the throat that is just behind the nose. The para-nasal sinuses drain into the nasal passages themselves at different points. The sinuses and middle ear constantly ...


5

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 ...


4

It is mainly determined by the OCA2 gene, but it also likely involves several other genes, including TYR, TYRP1, HERC2, and several others. To complicate things further, it is not a "mutant/wild type" trait, but multiple normal alleles can be found in the population, and it is believed that the sum of these single nucleotide polymorphisms (SNPs) determine ...


4

Another reason why red lights are now sponsorized for night illumination is because they are supposed to be safer in terms of interference on the circadian cycle. This is not related to better vision, but better health. The mammalian eye senses the light by the conventional rode and cone cells. However, a third light-sensing cell type has been recently ...


4

The cloaca, which is the common opening of the urinary, excretory, and reproductive systems, is present in birds as well as in non-avian reptiles (and thus presumably dinosaurs), amphibians, and monotremes (e.g., duck-billed platypus). To answer your first question, yes, this condition does seem to be universal for those groups mentioned above. To answer ...


4

The general belief is that these agents are irritants which do not have a very specific effect. That is to say that there is no specific cell or biological function that they set off, but that they irritate the cells, so the cells respond by putting up general defenses to insulate themselves from the noxious chemicals. Mucous and tears wash away the ...


4

My foray into the literature suggests that mature adipocytes do not divide. See, for example: Lefterova, MI et al. (2009) New developments in adipogenesis. Trends in Endocrinology and Metabolism 20:107-114 In the context of looking at the possibility of stimulating the conversion of white adipose tissue to brown adipose tissue, the authors outline the ...


4

The answer is probably No. "Long" bones - like the tibia, fibula, femur, humerus, etc. grow at the ends during childhood via a special formation called the Epiphyseal plate (also known as "growth plates"). The plates are composed of a special cartilage that grows, and slowly calcifies as a person reaches adulthood. When the growth plates completely stop ...


4

Mechanical force can compress neurons and cause action potentials as you probably experienced in the form of hitting the funny bone. Strong enough acceleration of the brain tissue may be causing massive excitation of neurons as indicated by animal EEG study. It suggests that the loss of consciousness is due to generalized epileptic seizure. However, it is ...



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