"Circulatory system", you say?

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White parrot
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"Circulatory system", you say?

Post by White parrot » Fri Nov 16, 2018 12:42 am

Quasar posted a roadmap and I have nothing better to do, so I decided to take an item, recapitulate what I know (or think I know) about it and throw some reflections on implementation.
This time, it's about circulatory system.

Blood in meatspace

What and why?

Let's begin with a basic definition.
To stay healthy and functioning, cells need influx of various materials (nutrients, oxygen if aerobic...), expulsion of wastes and to stay at an appropriate temperature (we'll come to that later). Single cells manage as they can by communicating with the outside environment; small colonies of cell do likewise and can somewhat trade substances through cellular pores, which is barely better than diffusion.
However, as the size and complexity of a multicellular organism augment, some cells usually become too remote from the outside environment or the specialized systems (digestive, excretory etc.) in contact with it (let's call them "interface systems" so I don't have to type that again) to directly interface with them, and cell-to-cell transport is just too inefficient to maintain communication.
Hence circulatory system: the public transport system of the organism, used to relocate whatever needs to be relocated across the body.
Whether it is beneficial or not, the circulatory system will inevitably also transport heat, and this feature/annoyance drives some important adaptations.

The reason I pointed out the obvious is to make it easier to grasp the situation in which a circulatory system isn't needed: you don't need to move things between random cells and interface systems if your interface systems are already in contact with all of your cells.
You may be thinking of flat organisms, but it is also feasible to have branching interface systems with omnipresent tendrils. If I remember correctly, jellyfish and flukes have a digestive system based in this idea, while insects have spiracles that allow their muscles to directly breathe air. Arguably, instead of having a circulatory system, such beings could be said to have a few, each devoted to a specific type of needed-to-be-moved substances.
Note that having a circulatory system isn't a binary choice. Indeed, the insects I just mentioned do have a more familiar circulatory system alongside their spiracle system; it's just that transporting oxygen isn't in the purview of their "blood".
It seems that having a singular circulatory system has the advantage of versatility, which makes it easier to scale and probably explain why beings with no or many circulatory systems are confined to small size.

Sure, but what does it look like?

For obvious reasons, I'll focus on animal circulatory systems.

At its most basic, a circulatory system is an empty cavity, the haemocoel (literally "blood cavity") filled with blood (well, "hemolymph", which combines the role of blood and lymph. I'll explain what the deal is with lymph later, but I hope you'll forgive me if I just use the word "blood" in a colloquial sense.). Blood sloshes about in the haemocoel according to the movements of the animal's muscles, but it can't really be said to circulate in a specific pattern; it is, however, an improvement over an absence of circulatory system.
Such a simple circulatory system is said to be "open".

(Note that moving molecules aren't necessarily free-floating in the blood, which can contain transport molecules and blood cells. As usual, such features are more or less efficient in different conditions and can thus be remarkably diverse, but I don't intend to focus on them in this post, where I operate at a more anatomical level.)

A heart is a system of pumps wholly dedicated to setting blood into motion, without having to rely on the animal's other activities. Better yet, a heart covering the whole length of the haemocoel (either by its muscly self or by being extended by crude blood vessels) can take blood from one end of the cavity and giving it back at the other, forcing the liquid to move in a single direction inside the whole circulatory system. As you can guess, keeping a heart (or, for that matter, a few) beating is energetically costly but brings substantial gains in efficiency. It is not unusual to adjust the frequency of the beating to the situation (as interpreted though emotions), so as to only "bring the heat" when necessary.

A further step to improve the system is to use less blood by only irrigating the required body parts instead of relying on a bloated haemocoel, through the use of blood vessels. I'm not sure whether it comes from the haemocoel slimming down and being partitioned or if blood vessels are extensions of the heart(s) that came to make the haemocoel irrelevant (perhaps with intermediate stages in which some body parts where directly linked by blood vessels and other still used the haemocoel), but in any case an innovation gave birth to a "closed" circulatory system, in which blood is safely contained within blood vessels.
As it happens however, no circulatory system is entirely "closed", because we are still talking about making pressurized liquid flows through pipes made of meat here. Blood vessels evolved to be rather watertight, but plasma (the liquid part of the blood) and tiny molecules filter through: properly speaking, blood is the haemolymph-like substance that stays inside and lymph is the filtered blood that left the system. The very existence of lymph require the existence of a parallel circulatory system, the lympathic one, whose main purpose is to collect lymph as it leaks and to bring it back to a site where it is reinjected into the bloodstream. Evolution being what it is, the lymphatic system was then repurposed as a priority lane for immune cells and acquired its own identity, but at its core, it was still born of blood vessels failing at keeping all their blood inside.

So here we are, with blood looping through blood vessels between, say, lungs and other organs. A single two-chambered heart could be enough to move the blood around, but this would involve having a lot of blood pressure at the single exit point, which would be risky; conversely, an organism could have a lot of less powerful "relay hearts" across the circulatory systems, but that's a lot of pumps to maintain. The four-chambered heart we're familiar with is the result of a compromise: we have functionally two two-chambered heart, but the circulatory system twists around to use a single muscle mass for both. Not sure how we came that way (two independent hearts fusing, or a blood vessel sneaking alongside a heart and growing there?) but in any case here we are.

A few notes on wounds

Obviously, damaged blood vessels fail to properly desserve their assigned body part. It is well-known blood coagulates (... and I don't feel like giving more details :P ) to seal the breach; in the meantime blood pressure rises to maintain the amount of delivered blood despite the losses. This all works out if the wound is small enough to seal quickly, but in major cases the organism enter a vicious circle of sending more blood to the wound to compensate for the blood loss, resulting in major loss of blood before the system can be closed; and of course, a general lack of blood in the circulatory system is damaging to its functionality.
In the case of internal wounds, another danger is simply that the lost blood has to go somewhere and often ends up where it shouldn't: crushing brain cells, drowning lungs, and more generally making a nuisance of itself.
Meanwhile, a wound opened to the outside world is a major infection risk since it opens a path that bypasses the skin barrier to let pathogens enter directly into the body's internal delivery system!

Vasoconstriction and blood-bending

A benefit of having a circulatory system comprising a large number of blood vessels each desserving a specific part of the body is that, by having ring muscles able to pinch these tubes, you can control in a targeted way the amount of blood any given organ receives. This is called vasoconstriction (and vasodilation for the converse) and brings a lot of surprising benefits.

You may think "Why? Don't all organs need blood anyway?" -this is true, but not at the same time. During a digestive nap, the digestive system works fully but arms and legs don't do much; and remember, maintaining blood pressure is energetically costly. Why bother maintaining some muscles at their full potential when they aren't in use anyway?! Vasoconstriction allows a clever workaround: through judicious pinching, the organism can have the heart beat less, but only reverberate the pressure drop in the organs that aren't fully used. Result: energy saved with no negative consequence. Similarly, in a life or death situation, an organism with vasoconstriction can take the risk of taking pressure from organs which provides long-term benefits to provide more resources for immediately relevant organs. This is a risky but potentially life-saving move, which is only permitted by this feature.

Another use of vasoconstriction is temperature control (remember I mentioned heat at the beginning?). Animals exchange heat with the outside world through their skin, but the important players are the blood vessels under the skin, because blood brings heat or cold to the whole organism. Skin doesn't require that much blood flow to work, so the organism is free to increase or decrease blood flow to increase or decrease in turn exchange with the outside world. This is why, if you have a clear skin, you may grow red when hot: your skin blood vessels are dilating to bring in more blood in (indirect) contact with the comparatively colder outer world. Contrariwise, people grow paler when they are cold and the organism takes back the blood from their skin.
(I don't know if vasoconstriction is involved, but speaking of temperature... "Reptiles" are said to have a hole between the ventricles of their heart, meaning oxygenated and deoxygenated blood constantly mix up, which is bad design if the goal is to keep the body breathing.
The goal, as it happens, isn't to keep the body breathing. As ectotherms, reptiles don't have metabolic needs as high as us mammals and can endure receiving a bit less oxygen; what they do need is a better control of their internal temperature, and lungs are by design a massive contact zone with the (sometime) cold outside world. What appears at first to be an unredeemable flaw is actually an adaptation relying on the same principle as cold-motivated vasoconstriction: keep blood as far away from the rest of the world as possible.)

An extreme case of vasoconstriction is one of the mechanisms involved in autotomy -the ability for some animals to jettison parts of their own body. Other adaptations are needed for the break itself to happen, but vasoconstriction is there to shut down blood vessels leading to the severed body part and avoid catastrophic (and embarrassingly self-inflicted) blood loss.

Blood for the blood god: circulatory system in Species

Including a detailed circulatory system rather than an abstraction is going to raise the complexity of creatures, especially if Q. wants to open the possibility of the most exotic variants. Once again, I'm unsure of the starting complexity of Primum sp., which can hardly be described as "barely multicellular" (I blame the media for their unrealistic body image issues) and thus may be too big for the most primitive systems. I personally like having vasoconstriction at disposal because it allows to wave away some level of abstraction by decreeing the organism simply uses it to autoregulate some aspect.

Since the very purpose of a circulatory system is to move molecules between organs, it implies the presence of different organs in creatures -such as (individual?) muscles, lungs and digestive tract. While obvious, this raises a small complication regarding the diagram meant to represent the metabolic pathways as they are used: different paths will take place in different organs, meaning reactions will be slowed down by the need to move the reactants around. Do you somehow represent this spatial component on the diagram? Do you ignore it, at the risk of confusing players who see pathways stopping temporarily for unrepresented reason? Do you consider transport time to be negligible? Do you proceed with an "acceptable breach from reality" and only move around end results such as energy?
In any case, a choice will have to be made.

In the context of combat system overall, Quasar once posted these notes, which, by treating of the consequences of wound, have implications on how the circulatory system functions in the first place. I think I'll take inspiration from that.

Let's name the value we're dealing with the blood flow -which may replace the use of stamina. Each organ has a specific calculated blood flow, which represent its ability to send or retrieve molecules from the overall circulatory system, which is abstracted as a sort of hammerspace, in a given unit of time. Blood flow should factor in calculation of temperature variation, either in skin (if an organ by itself) or simply in body part (ex: a limb through which much blood flow lose or gain more heat than the same arm without much); not sure yet how (or if) to implement vasoconstriction to deliberately manage heat.
Blood flow is implicitly a unit of volume by time. The volume part is provided by a genetic value specific to the organ, vascularization: how much blood vessels are integrated into it. The time part is pressure: in this context, how much blood is pumped in a given capillary in a certain amount of time. A creature can only have a maximum amount of pressure determined by its single heartbeat, which consumes energy to be maintained; this overall value is then spread across the individual organs. Depending on how much Quasar wants to go into details, an implicit system of vasoconstriction could allow individual organs to only take from the common store the amount of pressure they need to work and not more. Maximum heartbeat is, at its core, a genetic value, but it could vary depending on creature emotions, or simply depending on how much pressure they wish to use overall (which should be similar but more precise: heartbeat rises when we're afraid because of the pattern of preparing to use muscles in dangerous situations. Using the amount of pressure needed rather than than emotion is simply cutting the middle man.).

A greater blood flow (or at least the potential of it) is obviously beneficial since molecules are consumed or generated in the normal working of organs, and high values may be necessary to proceed at peak efficiency. On the other hand, relying on having a high blood flow makes the organ (and the creature as a whole) highly vulnerable to wounds.
I'd imagine an organ to have two health gauges, one for itself and one for its associated blood vessels; both are damaged at the same time, but they can heal differently and their health losses have different consequences. I'm only talking here about the consequences of damaging the circulatory system part: the more damage an organ's blood vessels suffers, the more it loses molecules while attempting to send or retrieve them (yes, including the molecules required for emergency healing :twisted: ), and the more pressure it makes the heartbeat lose (which either will have to take more energy to maintain overall pressure, or drop and let some organs suffer from it). Since this will result in situations where there simply isn't enough pressure to go around, we may need a priority value for each organ to determine in which order they'll be sacrificed through vasoconstriction for the common good so that others can still receive pressure. Then again this is kinda crude,there has to be a more elegant way...

It could be interesting to determine what are the molecular requirement from the creature brain(s) to function, and what happens when it begins to starve or be damaged... :twisted:

Depending on its location, a damaged blood vessel could have two additional effects: an internal wound may inflict structural damage on its organ relative to the local blood pressure, while on external wound could make infection more likely.

There is, at last, the matters of blood use by creature themselves. Bloodsuckers, for one: it feels like some creatures should be able to "attach" to a given external wound and siphon into their digestive tract whatever molecules are lost from blood loss. Such creatures may even have access to a move, "active sucking", that uses energy to artificially bring more pressure to the wound from the victim's heartbeat (... I may need to work on my terminology) to increase blood loss.
And there is the matter of blood trails: it seems intuitive that creatures should be able to smell or see blood, but at the moment I don't feel clear enough on the creatures' sensorial and intellectual abilities to speculate on how to do this.


There are some points I'm not clear on, but this here should launch discussions... :P
At this point, we shouldn't be surprised by anything nature does. She's like a meth addict whose drug-fueled rampages unfold in slow motion and span millions of years.
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Re: "Circulatory system", you say?

Post by Therminator » Fri Nov 16, 2018 7:28 am

Let me start by saying that a short comment does no justice to the large amount of time and effort you have put into this, and I think the content is great.

However, allow me some points of criticism:

- circulation systems would be invisible anyway (blood loss aside), so it would be mostly a black box, and be of little interest to players
- adding complexity for the sake of complexity is just putting strain on the simulation I think. Does implementing it solve problems that the simulation has? I think there's a difference between adding detail because you want more detail, and adding systems because the current behaviour doesn't make intuitive sense.
- the simulation is simply too coarse-grained to have dynamic processes of this size and detail, to literally simulate something like liquid flow.

I think it's better in this case to add general genes for overall phenotypes, rather than coding for the base parts and hoping for emergence.
Like the existing genes for thermoregulation for example: they asume hidden molecular mechanisms, but they don't tell us what it is. So it could be in part by something like vasoconstriction, but I think it's better that it's just implied that this is a complex of one or more "hidden" systems that coevolve as one genetic variable.
For the same reason eyes don't need to be coded as retinas with a cross-section and amount of cones per mm², they can just exist as sliders of "how good this eye sees". The reasons for why exactly this is are hidden and less relevant.

So I think it's more practical to take the simpler route, and do it via genes that determine a time (implying coagulation) of blood loss after wounding damage, a rate (implying pressure), and of course all the metabolic rates and efficiencies (that each in turn also imply molecular mechanisms of transport). All this without explicitly modeling organs and vessels.

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Re: "Circulatory system", you say?

Post by Therminator » Fri Nov 16, 2018 7:35 am

I just thought of an obvious tradeoff where the attacking creature can have sharp or blunt attack weapons, and how to connect that to this topic.
So the net blood loss and actual tissue damage are in part genetic (determined directly for blood loss rate, and indirectly through thickness of skin + directly for genetic blood pressure) but also in part determined by the weaponry of the foe. Sharp claws would give little direct damage but faster bleeding (cutting arteries), blunt hits damage a lot but don't cause (much) bleeding.

Boom, tradeoff that will cause prey specialization and predator coevolution. A lot of armor? Either very blunt weapons to hit the armor with brute force, or sharp claws that bleed them out. Little armor? Matters less how you damage them, they'll probably try to outrun you (because low armor = higher speed).

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Re: "Circulatory system", you say?

Post by White parrot » Sun Dec 09, 2018 9:45 pm

Therminator wrote:
Fri Nov 16, 2018 7:28 am
- circulation systems would be invisible anyway (blood loss aside), so it would be mostly a black box, and be of little interest to players
- adding complexity for the sake of complexity is just putting strain on the simulation I think. Does implementing it solve problems that the simulation has? I think there's a difference between adding detail because you want more detail, and adding systems because the current behaviour doesn't make intuitive sense.
- the simulation is simply too coarse-grained to have dynamic processes of this size and detail, to literally simulate something like liquid flow.

I think it's better in this case to add general genes for overall phenotypes, rather than coding for the base parts and hoping for emergence.
Like the existing genes for thermoregulation for example: they asume hidden molecular mechanisms, but they don't tell us what it is. So it could be in part by something like vasoconstriction, but I think it's better that it's just implied that this is a complex of one or more "hidden" systems that coevolve as one genetic variable.
For the same reason eyes don't need to be coded as retinas with a cross-section and amount of cones per mm², they can just exist as sliders of "how good this eye sees". The reasons for why exactly this is are hidden and less relevant.

So I think it's more practical to take the simpler route, and do it via genes that determine a time (implying coagulation) of blood loss after wounding damage, a rate (implying pressure), and of course all the metabolic rates and efficiencies (that each in turn also imply molecular mechanisms of transport). All this without explicitly modeling organs and vessels.
I completely see your point. The roadmap did include circulatory system at the junction of combat system and dietary chemistry and Species tends to favour emergent design so I felt like I had to give it a go, but I do have doubts about the benefits of a complex description...

EDIT: how does Dwarf Fortress do it? Quasar once mentioned he wanted to take inspiration from DF for the combat aspect, and given the reputation for complexity of the game, any system more developed than DF's is probably too much developed. :mrgreen:
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Re: "Circulatory system", you say?

Post by Quasar » Tue Dec 11, 2018 12:14 am

I'm going to need time to absorb the idea's in this thread so I can't really give them their due right now, but I can share some of the design work I've done on this subject.

The 'Circulatory System' is partially about resource management (sort of like Dietary Chemistry for Oxygen), but mostly about wounds and tying specific functionality to specific body parts. The central conceit is this: rather than a central Health Bar, creatures have a Functionality Bar for each body part, which affects the creatures stats (or causes them to fall unconsious and ultimately die in the case of the brain). This Functionality Bar will take damage if the part does, but it will also drop whenever the creature fails to pay an oxygen upkeep cost to that part.

That cost is taken from the lungs/gills and paid automatically. Under normal circumstances, it will be paid immediately.If there's not enough oxygen to go around, it'll go to the brain first to keep the creature alive, then the torso for digestion, then the extremities.

Where circulation comes in is that creatures bleed (and bleed more from body parts with high oxy requirements). If blood pressure drops, that means less oxygen delivery, which means less functionality for body parts.

This ties in with combat because of the elemental weapon/armour types I'm plotting:

Code: Select all

Soft armour      beats Blunt attacks,        <- No bleeding
Blunt attacks    beats Plate armour,
Plate armour     beats Slashing attacks,     <- Heavy bleeding
Slashing attacks beats Dodging,
Dodging          beats Piercing attacks,     <- Light Bleeding
Piercing attacks beats Soft armour, 
Although I may end up relying on a simpler, easier to communicate system:

Code: Select all

Armour           beats Sharp attacks        <- Bleeding
Sharp attacks    beats Dodging
Dodging          beats Blunt attacks
Blunt attacks    beats Armour
But that's all Combat Overhaul stuff. A prerequisite of any of this to get predators functioning (ie. hunters should evolve on most maps and sustainably coexist with herbivores).

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Re: "Circulatory system", you say?

Post by Natural_20 » Wed Dec 12, 2018 7:11 pm

White parrot wrote:
Sun Dec 09, 2018 9:45 pm
EDIT: how does Dwarf Fortress do it? Quasar once mentioned he wanted to take inspiration from DF for the combat aspect, and given the reputation for complexity of the game, any system more developed than DF's is probably too much developed. :mrgreen:
Dwarf Fortress defines different types of tissue, starting with a base material and then adding tags for nervous and vascular density. Then it uses a body plan to layer them on in the right order. Hits actually affect a skin layer, then fat, then muscle, etc, until they can't penetrate deeper. DF's system is probably too detailed in some respects and not enough in others; it doesn't really care about things like biochemistry, but it's also designed with smaller numbers of units than Species has to deal with.
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Re: "Circulatory system", you say?

Post by White parrot » Thu Dec 13, 2018 5:06 pm

Quasar wrote:
Tue Dec 11, 2018 12:14 am
I'm going to need time to absorb the idea's in this thread so I can't really give them their due right now, but I can share some of the design work I've done on this subject.
[...]
A prerequisite of any of this to get predators functioning (ie. hunters should evolve on most maps and sustainably coexist with herbivores).
True, but I'll still write a bit more about it! :P

There's a specific point I have problem with in this preliminary design:
Quasar wrote:
Tue Dec 11, 2018 12:14 am
The 'Circulatory System' is partially about resource management (sort of like Dietary Chemistry for Oxygen), but mostly about wounds and tying specific functionality to specific body parts. The central conceit is this: rather than a central Health Bar, creatures have a Functionality Bar for each body part, which affects the creatures stats (or causes them to fall unconsious and ultimately die in the case of the brain). This Functionality Bar will take damage if the part does, but it will also drop whenever the creature fails to pay an oxygen upkeep cost to that part.

That cost is taken from the lungs/gills and paid automatically. Under normal circumstances, it will be paid immediately.If there's not enough oxygen to go around, it'll go to the brain first to keep the creature alive, then the torso for digestion, then the extremities.
The thing about oxygen is that IIRC the documentation about Dietary chemistry, not every creature is meant to used it! While extremely potent, aeroby is ultimately facultative, thus it feels wrong to use it as the main purpose of circulation.
As I see it, this leaves us with either circulation being used to move around all sort of molecules (including oxygen but not only), or skipping even oxygen to circulate pure energy. Or some midpoint: moving around only enzymes, lipids and energy. In any case, defining what is being moved is crucial to the notion of circulatory system.

(Note: IRL Adenosine TriPhosphate, or ATP, is the usual ultimate material vehicle for energy, which it liberates by having one phosphate cracked apart (becoming Adenosine DiPhosphate -ADP- in the process). The thing is, ATP being easily "cracked" is precisely what makes it delicate to store or transport, as it would tend to spontaneously react before reaching its destination; this is why ATP is produced and consumed very locally.
Consequently, circulation moving ATP around would be a deliberate breach of realism in the name of programming efficiency, and should be accepted or rejected as such.)

Quasar wrote:
Tue Dec 11, 2018 12:14 am
Where circulation comes in is that creatures bleed (and bleed more from body parts with high oxy requirements). If blood pressure drops, that means less oxygen delivery, which means less functionality for body parts.
While bleeding should certainly impacts Functionality, it seems to me that healing bleeding wounds (scarring?) should be disjointed from an organ's Functionality healing: it feels like it is possible for circulation to heal fully before an organ regains its full functionality, and organs can lose functionality without open wound. Impacting an organ's functionality doesn't cause it to bleed, wounds do cause bleeding, hence the strange conclusion: wounds don't merely impact an organ's functionality, they also affect a separate, bleeding-related subsystem.
During my first post I assumed that organs could have separated "main" functionality and vascular functionality; right now I realize the "Vascular system" as a whole could be an organ and thus would have a single, centralized functionality, impacted by wounds to organ anywhere and responsible for bleeding. Perhaps I'll change my mind again later.
In any case, the possibility of bleeding open unexpectedly deep questionings, at least to me.

(Irrelevant thought:
Quasar wrote:
Tue Dec 11, 2018 12:14 am
creatures have a Functionality Bar for each body part, which affects the creatures stats (or causes them to fall unconsious and ultimately die in the case of the brain).
A dark part of me was wondering if creatures could suffer strokes and similar ailments; now I know. :P )


Natural_20 wrote:
Wed Dec 12, 2018 7:11 pm
White parrot wrote:
Sun Dec 09, 2018 9:45 pm
EDIT: how does Dwarf Fortress do it? Quasar once mentioned he wanted to take inspiration from DF for the combat aspect, and given the reputation for complexity of the game, any system more developed than DF's is probably too much developed. :mrgreen:
Dwarf Fortress defines different types of tissue, starting with a base material and then adding tags for nervous and vascular density. Then it uses a body plan to layer them on in the right order. Hits actually affect a skin layer, then fat, then muscle, etc, until they can't penetrate deeper. DF's system is probably too detailed in some respects and not enough in others; it doesn't really care about things like biochemistry, but it's also designed with smaller numbers of units than Species has to deal with.
I was specifically wondering how they deal with bleeding: do dwarves have a definite amount of blood, this kind of things.
I see your point, but I hope DF can still be a relevant inspiration regarding combat: the game may not be about biochemistry, but it sure does care about bloodsheds!
At this point, we shouldn't be surprised by anything nature does. She's like a meth addict whose drug-fueled rampages unfold in slow motion and span millions of years.
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Re: "Circulatory system", you say?

Post by Natural_20 » Thu Dec 13, 2018 8:02 pm

White parrot wrote:
Thu Dec 13, 2018 5:06 pm
Natural_20 wrote:
Wed Dec 12, 2018 7:11 pm
White parrot wrote:
Sun Dec 09, 2018 9:45 pm
EDIT: how does Dwarf Fortress do it? Quasar once mentioned he wanted to take inspiration from DF for the combat aspect, and given the reputation for complexity of the game, any system more developed than DF's is probably too much developed. :mrgreen:
Dwarf Fortress defines different types of tissue, starting with a base material and then adding tags for nervous and vascular density. Then it uses a body plan to layer them on in the right order. Hits actually affect a skin layer, then fat, then muscle, etc, until they can't penetrate deeper. DF's system is probably too detailed in some respects and not enough in others; it doesn't really care about things like biochemistry, but it's also designed with smaller numbers of units than Species has to deal with.
I was specifically wondering how they deal with bleeding: do dwarves have a definite amount of blood, this kind of things.
I see your point, but I hope DF can still be a relevant inspiration regarding combat: the game may not be about biochemistry, but it sure does care about bloodsheds!
Yes, dwarves have a total blood value derived from mass. A laceration to a given tissue causes it to bleed based on severity and vascular score, and death occurs when the blood value hits 0. Funny enough, giant mosquitoes don't drain more blood than regular ones due to blood drain being defined as an attack property and Toady applying generic templates to make the giant versions of things.

Not defining blood results in creatures that don't bleed.
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