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I came up with a pretty creative idea:
An alien life form having an organ near to the brain that translates information out of the blood stream into feelings.

My questions:

  • The medium of communication: What molecules or salts do not react with hemoglobin and other molecules in the blood?
  • Distinction of this medium: Somehow the organ has to distinguish the sent molecules out of the whole body to accurately tell where e. g. the pain is coming from.

Of course you can also describe how unlikely this evolution line must be.

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    $\begingroup$ Do you know what a hormone is? How is this different from the human hormonal signalling system? For example, in humans, the human pituitary and pineal endocrinal glands are located in the brain. Also, in humans the endocrine system interacts and is partially controlled by the nervous system. $\endgroup$ – AlexP Jul 2 '18 at 5:54
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    $\begingroup$ Have you considered an organism whose nerve tissue is embedded in the walls of the cardiovascular system? $\endgroup$ – pojo-guy Jul 2 '18 at 12:11
  • $\begingroup$ This would be kind of trippy, but what if the blood was conductive enough that you could send bioelectric impulses through the bloodstream? (note that in this case, you still have to deal with seperating out signals to their respective endpoints) $\endgroup$ – trevorKirkby Jul 2 '18 at 18:31
  • $\begingroup$ The Krogan in Mass Effect had something to this effect. No idea if the authors ever supplied any technobabble about how this worked that you could draw from. $\endgroup$ – Adonalsium Jul 2 '18 at 19:07
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The main problem with this set up is the transmission rate.

If your hand is touching a hot surface, by the time the blood has carried the signal to your brain and then from the brain to the muscles to retract it, the hand will be somewhere around medium-rare.

Also, once in the stream you cannot pick where the signal goes: unless you have a specific molecul per action and actuator (i.e. distinguish between relax biceps and relax sphincter), you are just generating noise.

This can work only for "slow" reactions, more or less what happens with our hormones or adrenaline.

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    $\begingroup$ This may still be feasible for animals whose survival doesn't depend on the reaction time of a pain response, such as perhaps a plant-like animal which wouldn't be moving much anyway. $\endgroup$ – Neil Jul 2 '18 at 7:58
  • $\begingroup$ @Neil, plants do not have an organ exclusively devoted to analize signals called brain $\endgroup$ – L.Dutch - Reinstate Monica Jul 2 '18 at 10:12
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    $\begingroup$ Nor do there exist organisms which pass signals to the brain via blood stream. If you think we're talking about nonfiction here, then I think you're on the wrong site. $\endgroup$ – Neil Jul 2 '18 at 11:22
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    $\begingroup$ @Neil - L.Dutch is right. This is nonfiction aka endocrinology. Hormones like progesterone and small molecules like oxytocin and catecholemines circulate in the blood and pass messages to the brain. Feelings and emotions can be enhanced or suppressed by these molecules. $\endgroup$ – Willk Jul 2 '18 at 13:14
  • $\begingroup$ @Neil, I am just following the OP's statement that there is a brain involved. And my brain uses adrenaline released into blood stream to stay alert in case of danger, just saying... $\endgroup$ – L.Dutch - Reinstate Monica Jul 2 '18 at 13:53
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Every individual organ and limb would have to use a different set of chemical signals. That's far too complicated and imprecise for higher life-forms - even relatively simple creatures such as jellyfish have a nervous system (although it's rather decentralized).

However, what if you discard that part and instead merely have the "nerves" form as part of the blood vessels? The creature's circulatory and nervous systems would be so closely integrated that it may be impossible to tell them apart on initial examination.

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    $\begingroup$ I don't know if this possibility would work for the OP, but conceivably this would make sense. The nerves traveling on the arterial wall would also perhaps be far more sensitive to blood poisoning or moods directly deriving from the state of the blood. $\endgroup$ – Neil Jul 2 '18 at 8:02
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Other than the answer above by LDutch, what makes this system inefficient is that the moment you get a cut or a gash, information of the damage literally flows out of your body. A physical trauma that causes swelling will contain the information in the damaged area.

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AlexP has the answer: unless you want the system to work for specific immediate actions hormones work fine. Adrenalin for example is a wonderful hormone that targets a lot of organs specifically. It makes fatcells and the Liver dump more sugar into the bloodstream for the muscles, increases erectocytes (that clot blood when wounded), periferal nerves partially close off so more blood goes to organs that need it (muscles etc), it increases the heartrate and breath, increased attention, lower bloodflow to intestines... The list goes on.

This shows how a single hormone can target different organs simultaneously and entice them to do effects it wants. The advantage over a nervous system is that hormones can last for an extended time after its released and in many cases even be used by the cells in question for the processes, something thats harder to achieve with an electrical signal.

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