146 votes

Why do I only breathe out of one nostril?

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, ...
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96 votes
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Why do we have two kidneys but one liver?

Your organs can be grouped into two categories. The digestive tract organs, which are singular. These form from the endoderm. The liver and pancreases are all direct outgrowths from the digestive ...
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  • 13.2k
52 votes
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Why are the pulmonary vein and artery not like the rest of circulatory system?

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 ...
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  • 14.8k
39 votes
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Is humans' height ideal?

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 ...
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39 votes

Why do I only breathe out of one nostril?

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 ...
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  • 521
37 votes

What is the function of epidermal ridges on human fingers (that produce fingerprints)?

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 ...
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  • 17.5k
36 votes
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How did the cardiovascular system evolve?

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: ...
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  • 1,377
34 votes

What is the function of epidermal ridges on human fingers (that produce fingerprints)?

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 ...
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30 votes

Why do blood vessels in the eye not obstruct vision?

Avoid the fovea Figure 2 from the same paper shows the distribution relative to the fovea: As you can see, it's pretty much devoid of this superficial vasculature, so anything you are directly ...
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  • 36.5k
24 votes
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The Uniqueness of DNA Paradox

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 ...
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  • 9,314
23 votes

Where can I find the list of all tendons in the human body?

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 ...
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  • 12.1k
21 votes
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What are the side-effects of long-term liquid breathing?

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 ...
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  • 11.3k
20 votes
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Why do we need red blood cells?

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 ...
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  • 4,450
20 votes
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Could a man be proved positive for a pregnancy test?

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 ...
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  • 67.6k
17 votes
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Why is the opposite of plantar flexion called "dorsiflexion"?

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 ...
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15 votes
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Human spine: Is the coccyx really fused?

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 ...
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  • 12.1k
14 votes
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Using anatomical terms for human organs and parts of plants

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 ...
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13 votes

Why are the organs for reproduction and urinary excretion combined?

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 ...
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  • 12.1k
12 votes
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Why do we have two of some organs, but not all?

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 ...
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  • 11.3k
12 votes
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Are transgender people born physically different from others?

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 ...
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  • 67.6k
12 votes
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What is the function of the human eye white?

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 ...
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  • 51.1k
12 votes
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Are males taller than females in humans?

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 ...
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  • 67.6k
12 votes

How did the cardiovascular system evolve?

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 ...
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  • 880
12 votes

What makes humans omnivores, and not herbivores?

As the answers given already point out, humans are generally considered omnivores that are able to use food sources available to them opportunistically. Apart from that, there actually is not much to ...
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  • 2,878
12 votes

Where can I find the list of all tendons in the human body?

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). ...
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  • 291
12 votes

Why do blood vessels in the eye not obstruct vision?

You CAN see your blood vessels! Following-up from Bryan Krause's answer: Bryan mentioned that the blood vessels are typically not visible partially because they are unchanging (i.e., static). In ...
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11 votes

What is the evolutionary reason behind the fragility of teeth?

The evolutionary biologist in me would argue that, on average, teeth are exactly as strong as they need to be. In other words, the fitness cost of producing and maintaining teeth is balanced with the ...
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  • 1,194
11 votes
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Chest muscle and female breast size

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 ...
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11 votes
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Do the right-handed people tend to use the right side teeth of their jaw to chew food more often than the left-handed people?

Individuals have not only hand dominance but also a dominant foot, eye and ear. There has also been a belief that this sidedness applies to chewing as well. However, the short answer is that no one is ...
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