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101

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 ...


41

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). ...


32

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, ...


23

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

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. ...


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 ...


20

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 ...


13

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.


13

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 ...


12

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, ...


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

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 ...


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

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 ...


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

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" ...


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 ...


8

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 ...


7

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 ...


7

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 ...


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

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

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

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 ...


6

This is a well worded question. There are more reasons than the following but the most apparent is hemoglobin production and its transport. The erythrocyte is a cell whose structure is optimal for its function-the production and transport of hemoglobin. It lacks a nucleus and organelles so that it can devote almost 100% of its energy to hemoglobin ...


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 ...



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