100 votes
Accepted

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 ...
  • 13.7k
53 votes
Accepted

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 ...
  • 15.1k
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 ...
  • 17.6k
36 votes
Accepted

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: ...
  • 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 ...
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 ...
  • 39.2k
24 votes
Accepted

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 ...
  • 9,414
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 ...
  • 12.2k
21 votes
Accepted

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 ...
  • 11.3k
20 votes
Accepted

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 ...
  • 67.8k
17 votes
Accepted

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 ...
15 votes
Accepted

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 ...
  • 12.2k
14 votes
Accepted

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 ...
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 ...
  • 12.2k
13 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 ...
12 votes
Accepted

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 ...
  • 11.3k
12 votes
Accepted

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 ...
  • 67.8k
12 votes
Accepted

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 ...
  • 51.8k
12 votes
Accepted

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 ...
  • 67.8k
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 ...
  • 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 ...
  • 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). ...
  • 291
12 votes

Why does 360 degree rotation not seemed to ever have evolved?

Actually there are propeller like features called flagellum (3) evolved on many organisms like plants, animal sperms (4), bacteria (1) and flagellates (2). There are known only one macroscopic form of ...
  • 121
11 votes
Accepted

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 ...
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 ...
  • 1,194
11 votes
Accepted

How can we move our lips even though they don't have any bones?

Because of the Orbicularis oris muscle, it's a complex of muscles in the lips that encircles the mouth, It forms the greater part of the substance of the lips, lying between the skin and the mucus ...
  • 761
10 votes

Can any other animal choke on food?

I don't agree with the accepted answer which seem to assume that food is chewed. I don't think that can be assumed if generally talking about animals and choking. So in addition to Larry's statement ...
  • 994
10 votes
Accepted

Why can't people talk while inhaling?

Via deep scientific analysis (i.e. trying it myself 5 seconds ago), I have determined that you can in fact speak while breathing in, it just sounds funny. Think of the vocal chords as being like the ...
  • 3,781
10 votes
Accepted

Why do humans have only two nipples?

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 ...
  • 1,320
10 votes
Accepted

Can defibrillation be done with a car battery?

In general, no. Car batteries are designed to provide a large amount of amperage, to turn the starter with a high amount of torque via an electric motor. Generally this is done at 12 or 24 volts. ...

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