In physics, there are simple laws and complex consequences. In math, a good formalism (like a “graph”) may well appear wherever you concentrate your attention. In economics, incentive structures fractally explain behavior at a range of scales, from intrapersonal to international.

Physiology seemed like a grab-bag of random processes which no one really understands. If you understand a physiological process—congratulations, that idea probably doesn’t transfer much to other domains. You just know how humans—and maybe closely related animals—do the thing. At least, that’s how I felt. (These sentiments tend to feel sillier when spelled out.)

I haven’t totally changed my mind. But I do know that I love understanding my own body better.

This content is actually easy stuff

At least at this level.

You just have lots to memorize, which is easy with Anki. Take the circulatory system. Is there anything crazy happening here?

There might be some unfamiliar arrangements of letters, but the concepts are easy. Combine that with modern llms and it’s a walk in the park. I studied this book over about two weeks of effort, diving into random bits I didn’t understand and liberally adding Anki cards.

“Ah, so that’s how it works!”

No physical tract from the mouth to the bladder

For some reason, I thought there was a “urinary tract” since childhood

The mouth sends food to the stomach. After you drink water, you often need to pee. So why wouldn’t the mouth be sending water directly to the bladder?

Of course, upon closer examination this theory falls apart.

1. Where would the water branch out from the digestive tract? The tract doesn’t bifurcate. And if it did, how would water be filtered out of the food-slop (the “chyme”) after leaving the stomach?
2. How would the body absorb water? On the way? Why would you just instantly urinate out freshly consumed water, unless you were truly that close to maximum capacity?
3. How would the water be (mechanically) filtered out to the other pathway? It’s not like you have a second mouth just for drinking fluids.

The real answer is basically that the small intestine absorbs water for use. Separately, in the kidneys, there are a bunch of thin tangly spirals called “glomeruluses”… er, “glomerulii”? I don’t know. Whatever, the singular is “glomerulus”!

The semipermeable membrane of the glomerulus and the high pressure together push the blood’s small particles (including water) into this tube thing (called the “Bowman’s capsule” and then the “renal tubule”). In the tube, a bunch of water gets reabsorbed. The unabsorbed filtrate gets shipped down to the bladder. When you haven’t drank water in a while or are otherwise low, hormones tell the tube things to reabsorb as much water as possible. That’s why your pee is darker when you’re thirstier.

So! There isn’t a direct physical path from the mouth to the bladder, but the truth is rather more interesting—wouldn’t you say?

Cardio and weightlifting literally exercise different kinds of muscle

And that’s why they’re so distinct.

There isn’t a continuous spectrum of muscle specializations, just two kinds of muscles and activities which engage those kinds to varying degrees. Also, the explosive power of weightlifting comes from the battery-like nature of white muscle fibers. The white fibers (weightlifting) store their energy like a battery while the endurance red fibers are “plugged in” to energy received from the circulatory system.

Anus vs rectum

To be honest, I didn’t know these were different. I just knew they were both involved in pooping near the end of the process. The anus is like the final doorway out of the digestive tract, whereas the rectum is like a waiting room for that doorway.

Can all eggs get fertilized at once?

For the longest time, I had a cursed question: What if (somehow) a bunch of sperm made it into an ovary and then fertilized tons of eggs? This doesn’t actually happen—fertilization generally takes place in a fallopian tube. But what if this did happen? Would the woman’s body attempt to develop an ungodly number of babies at once?

No. No, that can’t happen. There are several reasons which I now understand:

1. A woman is born with millions of ovarian follicles, which contain proto-eggs (formally called oocytes). Oocytes aren’t mature and so can’t be fertilized.
2. Each month, a single¹ oocyte matures into an egg, leaves its follicle behind, and enters the nearby fallopian tube.
3. Even if the immature oocytes could be fertilized, the zygotes are only provided nutrients when their former follicles secrete hormones into the uterus to tell the body to airdrop some support. Presumably that process wouldn’t work correctly if the zygote is still in the ovary.

You swallow food like you squeeze toothpaste

In peristaltic motion, your muscles inch the food down your esophagus. In fact, once you know what to look out for, you can feel it! Try swallowing. Pay attention to the sensations of your throat muscles.

Nostrils have no direct scent receptors

Odorants are actually just detected at the olfactory epithelium, whereas I’d thought there were detectors directly in my nostrils. Hence, you can’t smell as well when you have a stuffy nose.

Confusions I still have

Why is there only one fovea?

The fovea is a pit in your retina which contains a high concentration of cones, giving you sharp vision in the center of your field of view. Why not have multiple high-resolution spots?

As the above image may make obvious, the lens focuses light onto a point. That point lands on the fovea. So I guess you’d need several lenses to concentrate light on several different foveae, which probably isn’t worth the hassle? I’m still confused as to the final details.

Gemini’s explanation

Why one fovea in humans (and most vertebrates):

• Lens Limitation: You’re correct that the lens plays a crucial role. Our single lens focuses light onto a specific area of the retina. Having multiple foveae would indeed require a fundamentally different optical system, perhaps multiple lenses or a more complex lens structure, to focus light onto different points sharply. This would add significant complexity and potential drawbacks.
• Brain Processing: Even if we had multiple foveae, our brains would need to process the high-resolution information coming from each one. This would require significantly more neural processing power and a more complex visual cortex. Our current brain structure is optimized for a single point of high-acuity vision that we can move around by moving our eyes.
• Eye Movements: We’ve evolved a highly efficient system of rapid eye movements (saccades) that allow us to quickly shift our fovea to different points of interest. This gives us a wide field of view with high acuity wherever we direct our gaze, without the need for multiple foveae.
• Trade-off between Acuity and Field of View: A larger fovea or multiple foveae would necessarily mean a smaller area of peripheral vision. While high acuity is useful, a wide field of view is also crucial for detecting movement and potential threats in our surroundings. Our single fovea system strikes a good balance between these two needs.
• Developmental Complexity: Developing multiple foveae and the corresponding neural pathways would be significantly more complex during embryonic development. Evolution often favors simpler solutions that are “good enough.”

I’m not sure whether animals with compound eyes (like dragonflies) have multiple foveae, or if that’s just not a sensible question. They at least have multiple lenses.

Not sure what’s up with the inner ear

The semicircular canals track changes in your head’s orientation. The otoliths track which way is down. But why not just combine them? Why did they evolve to be separate? Can’t you infer changes in gravity’s direction from signals from the semicircular canals?

Vestibular system

As movements consist of rotations and translations, the vestibular system comprises two components: the semicircular canals, which indicate rotational movements; and the otoliths, which indicate linear accelerations.

That kinda answers my question. But I’m kinda still confused and am not going to deep-dive right now.

Is the lymph system just dumping random garbage back into the bloodstream?

Surely not, but my gut thinks so.

I know there’s some filtration at the lymph nodes, but my initial impression was that the filtration is just the lymphatic immune cells (i.e. the lymphocytes) killing off intruders. Since the lymph system does general garbage collection and keeps interstitial fluid at acceptable levels, I’d expect there to be non-invasive waste products.

Maybe that’s fine! After all, it’s the kidneys’ job to filter out waste. The kidneys need that waste to be in the bloodstream.

Gemini’s elaboration

Cellular debris collected by the lymphatic system is primarily broken down by macrophages within the lymph nodes. The components are then either recycled for use by the body or further processed for disposal, ultimately being eliminated through the liver or kidneys.

Osmotic pressure seems weird

Osmosis, Wikipedia

Osmosis is the spontaneous net movement or diffusion of solvent molecules through a selectively permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region of higher solute concentration), in the direction that tends to equalize the solute concentrations on the two sides.

Basically, if you have more of the solute (the particles in the image), water goes to that side. But why? For a given volume of water, the pressure on top of that volume is roughly $\rho gh$ where $\rho$ is the density of water (about 1,000 kg / m³). That is, the higher the water level, the more pressure pushes down (and therefore back through the membrane). So how is the osmotic pressure defying or avoiding this mechanism?

Imagine I’m standing in my bedroom. The door is open to my kitchen. Then I toss a ton of glitter (the solute) into the air (the fluid or solvent). Osmotic pressure feels like it’s saying, yeah, now air comes rushing into your bedroom. But why? The room pressures were already equilibrated! Why does throwing a bunch of tiny particles change it? Surely it’s because the solute particles are ions or react with water or something, right?

Here’s some random cool stuff

An anime about your body: Cells at Work

This anime is fun. Not as fun as Doctor Stone, but fun. Its representations of cells are sometimes hard to forget.

For example. Neutrophils are a first-responder immune cell and a type of white blood cell. They catch invaders, envelop them, and then dissolve them—a process called phagocytosis. Cells at Work portrays neutrophils as knife-wielding maniacs.

Conclusion

Physiology is an area where my reaction was “gee, that’s hard and complicated”—now it’s just “yay, another subject that’s fun to read random Wikipedia pages about.”

It’s nice to look down at my hand and be able to pop through multiple understandings of my body. I start with the circulatory system and I might imagine how oxygenated hemoglobin turns bright red, tracing the flow back down to my median cubital vein (where blood is drawn). My mind’s eye flickers as I imagine the peripheral nervous system and its reflex arcs, wondering at the preconditions which trigger spinal short-circuits (and at what preprocessing is done to customize the reflex to the situation). Another flicker and I see the muscles and joints and their lubricants and connectives; gazing through flesh at my ulna and remembering the repetitive stress injury which once tormented me…

So many ways of looking at one unified object—my body—my home.

☙❧

Sequence: Becoming Stronger

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