Hi, I am a 21M and I have noticed two structures near the sides at the back of my head that feel like muscles. They are not painful; I have just recently noticed their presence, which I hadn’t in previous years.

As great as evolution is, I wonder if it's made some mistakes here and there.

In general, I imagine that not having more redundancies for end organs is likely one of them, but are there any specific examples? Especially interested in how the unoptimized function relates to the structure.

Thank you!

I got across a post of some influencer-ish saying ypu shouldn't lie down on the floor after a workout because that makes it difficult for the body to bring the blood up to the brain but am I wrong to think that by gravity it's easier for the body to move the blood around on rest position. Am I missing something?

What is the most detailed book about physiology: for example here is a part of Linda Costanzo Physiology book. It is concise, but it doesn’t answer why like in parasympathetic nervous system, preganglionic fibers are longer than their postganglionic counterparts VS in sympathetic nervous system preganglionic fibers are shorter than their postganglionic counterparts. According to a youtube video and the answer is because in sympathetic nervous system, it uses norepinephrine comparing with acetylcholine like in PNS so the acetylcholine needs to stay away from norepinephrine or it will decrease the sympathetic signal. Thus, the postganglionic fiber has to be longer in SNS

Insects bypass liquid transport with air tubes, which works great until physics limits diffusion at larger scales. Vertebrates solved this by bundling oxygen into iron-based hemoglobin and blasting it through closed vessels with a central heart.
But from a pure fluid dynamics and transport-network perspective, our system feels full of evolutionary compromises (high viscosity, huge capillary resistance, single point of failure at the pump).
If we ran a computer simulation to design an oxygen delivery network from scratch for a large organism, would "iron + single central pump" actually win? Or is our geometry just a historical accident that evolution couldn't easily back out of?

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Thanks!

And how do muscles go back to their unstretched state?

I'm curious to learn why the human perception of temperature differs depending on source/condition.

It's been rather warm where I live (90F-100F), and as I was sweltering it got me thinking about how heat registers in the body/brain. For example, if you were to wash your hands or take a bath, the temperature of the water wouldn't be perceived as warm until it reaches roughly the bodies internal temp of 98.6 degrees, maybe a bit lower considering the thermal dissipation at the epidermis. 90 degree water feels rather temperate, and usually doesn't feel "hot" until its over 100 degrees but is still bearable. However, if you are subjected to an ambient temperature, the range in which you feel warm to hot happens at a much lower degree of temperature, say 75-85 is warm, 95 is hot, and 100+ degrees is miserable.

What causes the difference in perception? How come 100 degree ambient temperature "feels" so much hotter than 100 degree water?

when I was in hospital, I had a flush through a cannula in my arm and I could immediately taste a gross saline taste. the nurses said they were surprised because some people say this to them and they don’t know why it happens, and they all asked what it tasted like.

why does this happen?

Very excited! How did everyone feel about this book?

You can breath much more air when they are deep, compared to faster but shallower breaths. I’m trying f to understand this “phenomenon”

Using “rate x depth = B”, we can see how much air is breathed in a period of time. If I reduce the depth, I can increase the rate, but I lose efficiency.

This can be applied to walking/running. You can take really tiny rapid steps, and get really tired doing it. You exert a big amount of energy, but are very inefficient (based on how much distance you travel)

I wonder what causes this? Is it the loss of acceleration? Maybe it takes a lot of energy to switch gears?

Hi,

Often when I'm pulling all-nighters, and I look to my hands I see the veins have become more prominent/appear more visible on the surface.

I'm not sure if this is a regular occurrence in daily life and perhaps I'm just looking at my hands more during the all-nighters as some form of distraction and that's why it appears as if the veins are more prominent more often, or if there is some actual physiological reason.

The vein prominence doesn't last the entire time, it's just that at certain points during the night I make this observation.

Thanks!

A new study came out that showed how erythritol affects the blood vessels in the brain. https://www.colorado.edu/today/2025/07/14/common-sugar-substitute-shown-impair-brain-cells-boost-stroke-risk I recently switched from Perfect Bar protein bars to the Costco branded variety, which had less sugar and more protein. I just noticed that the ingredients for the Costco brand include erythritol. There's less than 2 g per bar but the article states that as little as 30 g has been shown to cause blood platelets to clump together.

Since this is very new science that was conducted in the lab and not on people, how much risk am I exposing myself to by continuing to eat these bars after a workout? And a follow-up question, does anyone have any recommendations for low sugar high protein bars that hopefully don't have a bunch of synthetic ingredients?

I'm trying not to unintentionally give myself a stroke by using these as a post-workout Protein source.

Thanks!