#/blood is imitation seawater/ is the part that’s confusing
Picture this: you are a Thing That Lives In The Ocean. Some kind of small multicellular animal a long time ago, before proper circulatory systems existed. “Wow,” you think, metaphorically, “it sure is difficult to diffuse chemicals across my whole body. Kinda puts a hard limit on the size and distance of what specialised organs I can have. Good thing I have all this water around me that’s the same salinity as my cells (they have to be that way so I don’t explode or shrivel up) so I can diffuse and filter chemicals with that.”
“Wait a minute,” you say a couple of generations later, because you’re not actually a small animal but an evolutionary process personified and simplified to the point of dangerous inaccuracy for the purposes of a Tumblr post, “instead of losing all these important chemicals to the water around me, how about I put it in tubes? I can keep MY water separate from the rest of the world’s water! Anything I want to keep goes in my water! Anything I don’t, I dump back into the outside water! I’m a genius! An unthinking natural trial-and-error process that’s a GENIUS!”
“Wow,” you think a great many generations later, “being able to have such control over such high concentrations of important chemicals is so great. Look how big I’m getting. I even have a special pump to move my seawater around, and these cool filter systems to keep the chemicals in it right, and that control and chemical concentration has let me grow so many energy-intensive, highly specialised organs! Being big is so hard. I need special cells just to carry my oxygen around now, to make sure my enormous, constantly-operating body has enough of it.”
At this point you are embodying a fish, and eventually, fish start straying into water with different pressures and salinity levels. (I mean, they do that since befor ehty’er fish, but… look, I’m trying to keep things simple here.) “What the FUCK,” you think. “My inside water is at a different salinity and pressure to the outside water?? How am I supposed to deal with that? I can’t have freshwater inside my seawater tubes! My cells have a set salinity and they would explode! I need to start beefing up my regulatory and filter systems so that my inside seawater STAYS SEAWATER OF THE CORRECT SALINITY even if the outside water is different! Fortunately, adding salt to my seawater is a lot easier than removing it, and I want to be saltier than this weird outside water.” At this point you beef up your liver and urinary systems to compensate for different salinities. (Note: the majority of fish, freshwater and saltwater, have a fairly narrow band of salinities they can live in. Every fish doesn’t get to deal with every level of salinity; they are evolved to regulate within specific bands.)
You also, at some point, go out on land. This is new and weird because you have to carry all of your water inside. “It’s a good thing I turned myself into a giant bag of seawater,” you think. “If I wasn’t carrying my seawater inside, how would I transport all these important chemicals between my organs and the environment?” As you specialise to live entirely outside of the water, you realise (once again) that it’s a lot easier to add salt to water than to remove it in great quantities. Drinking seawater in large amounts becomes toxic; your body isn’t specialised for removing that amount of salt. Instead, you drink freshwater, and add salts to that. The majority of your organs are, at this point, specialised for moving your seawater around, protecting it, adding stuff to it, or taking stuff out. You have turned yourself into an intelligent bag for carrying and regulating a small amount of imitation seawater, and its salinity (and your commitment to maintaining that salinity) is based entirely on the seawater that some early animals started to build tubes around a long time ago.
And that’s what a human is!
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