#/blood is imitation seawater/ is the part thatâs confusingÂ
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. âWow,â you think, metaphorically, âit 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 thatâs the same salinity as my cells (they have to be that way so I donât explode or shrivel up) so I can diffuse and filter chemicals with that.â
âWait a minute,â you say a couple of generations later, because youâre 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, âinstead 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 worldâs water! Anything I want to keep goes in my water! Anything I donât, I dump back into the outside water! Iâm a genius! An unthinking natural trial-and-error process thatâs a GENIUS!â
âWow,â you think a great many generations later, âbeing able to have such control over such high concentrations of important chemicals is so great. Look how big Iâm 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.â
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 ehtyâer fish, but⌠look, Iâm trying to keep things simple here.) âWhat the FUCK,â you think. âMy inside water is at a different salinity and pressure to the outside water?? How am I supposed to deal with that? I canât 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.â 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 doesnât 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. âItâs a good thing I turned myself into a giant bag of seawater,â you think. âIf I wasnât carrying my seawater inside, how would I transport all these important chemicals between my organs and the environment?â As you specialise to live entirely outside of the water, you realise (once again) that itâs a lot easier to add salt to water than to remove it in great quantities. Drinking seawater in large amounts becomes toxic; your body isnât 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 thatâs what a human is!