Theory: Tau blood is blue and here's why..
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Thanks to Shas'El Tael for this.
Tau Blood - A Theory on Cyan colouration of Tau blood, a much more accurate reality than 'Red' blood as we know things.
The Basics of It
To have cyan blood, the tau would need to have a circulatory system comprising of a plasma with free floating oxygen carrying Hemocyanin, which unlike human blood is not infused in corpuscles cells (hemocytes) , due to it's size (larger) and nature (relation to oxygen and carrying it). To be as efficient as hemoglobin, most Hemocyanin users have high density of blood. Which for tau, would account for their strong grey-blue complexion. Therefore, the less oxygen a tau has, they more grey faced they become..then white and die.
Tau, by this (I believe far better fitting) theory, have a blue blood circulatory system based on Hemocyanin usage.
All these findings suggest this Hemocynanic based cardiovascular circulatory system affects and accounts for a notable traits in Tau:
Like humans, we are coloured by our blood (put a torch behind your finger and turn it on), our skin pigmentation the only altering factor. Tau also exhibit this.
General lack of strength
Smaller muscles require less oxygen, a system using hemocyanin would starve larger muscles, this physiological adaption suits their circulatory system.
I will confirm this, but so far, there may be a connection between poor vision and this form of oxygen carrying circulatory system. Also may effect eye colouration.
Cool, Leather-like skin
The evolved need to retain water more than a human, on the hot world they live on would encourage them to produce such a trait. The cool, clammy feel, reported on those touching the tau would correlate well with the lower heat carrying ability of this physiology.
Prefer hot, moisture rich environs
By their own physiology, suggested here, a tau would prefer an environment that is not actively drawing water through his skin and is in general warmer, which like us, is more comfortable than the cold, which effects our bodies.
Prefer salty foods
With the type of blood pressure they require to make efficient use of their circulatory system, tau need salt and water in higher amounts, for in humans these two affect blood pressure. Too much is bad, which effects our (humans) flow of bodily fluids. For a tau, this is actually beneficial, their system supporting pressures to a higher degree. This is not to say Tau gulp down water more than humans, more they inherently have higher amounts and evolution wise have adapted ways to retain water more strictly than humans.
Hemocyanin is a bluish, copper-containing protein with an oxygen-carrying function similar to that of hemoglobin (at least it is blue when it is oxygenated, but colorless after the oxygen is released) which is present in the blood of certain animals such as crustaceans. Hemocyanin is much like hemoglobin except that the iron atom in the protein molecule is replaced by one of copper
In my readings quite a few animals have varying coloured blood (unsurprising) and some creatures simply bathe their organs in the finer-sized hemoglobin, just like a Hemocyanin species. But it is a poor usage of hemoglobin. Hemocyanin is far better in this role. Still, a blue-blooded creature's system can, among other tasks, devour bacteria, foreign substances and bits of dead tissue. Just how a human blood system, with red corpuscle cells infused with hemoglobin, would perform.
Human Blood = Red as it carried Haemoglobin; Tau blood = Cyan as it is more viscous
Hemocyanins (also spelled haemocyanins) are respiratory proteins containing two copper atoms that reversibly bind a single oxygen molecule (O2). Oxygenation causes a color change between the colorless Cu(I) deoxygenated form and the blue Cu(II) oxygenated form. Hemocyanins carry oxygen in the blood of most molluscs, and some arthropods such as the horseshoe crab. They are second only to hemoglobin in biological popularity of use in oxygen transport.
Although the respiratory function of hemocyanin is similar to that of hemoglobin, there are a number of differences in its molecular structure and mechanism. Whereas hemoglobin carries its iron atoms in porphyrin rings (heme groups), the copper atoms of hemocyanin are bound as prosthetic groups comprised of histidine peptides. Hemocyanin binds with oxygen non-cooperatively and is only one-fourth as efficient as hemoglobin at transporting oxygen. Hemoglobin binds oxygen cooperatively due to steric conformational changes in the protein complex, which increases hemoglobin's affinity for oxygen when partially oxygenated. Hemocyanin does not have an increased affinity for oxygen when only partially oxygenated.
Hemocyanin is made of individual subunit proteins, each of which contains two copper atoms and can bind one oxygen molecule (O2). Each subunit weighs about 75 kilodaltons (kDa). Subunits are arranged in chains or bundles in weights exceeding 1500 kDa. Because of the large size of hemocyanin, it is usually found free-floating in the blood, unlike hemoglobin, which must be contained in cells because its small size would lead it to clog and damage blood filtering organs such as the kidneys. This free-floating nature allows for higher densities of hemocyanin in the blood (as compared to hemoglobin), and helps offset its low efficiency.
With a thicker blood plasma, carrying the more numerous protien Hemocynanin, a tau would require a higher blood pressure than a human.
When blood enters the arteriole end (pushed from the heart) of a capillary, it is still under pressure (the Turgor Pressure, made by the heart pumping is measured as Torr, in this case 35 'torr') produced by the contraction of the ventricles of their heart. As a result of this pressure, a substantial amount of water, oxygen and some plasma proteins filter through the walls of the capillaries into the tissue space.
Thus fluid, called interstitial fluid, is simply blood plasma minus most of the proteins, eg. the larger Hemocyanin protiens, which would no be lose their blue hue after passing on the oxygen.
Interstitial fluid bathes the cells in the tissue space and substances in it can enter the cells by diffusion or active transport. Substances, like carbon dioxide, can diffuse out of cells and into the interstitial fluid.
Near the venous end (returning to heart) of a capillary, the blood pressure is greatly reduced (15 torr). Here another force comes into play. Although the composition of interstitial fluid is similar to that of blood plasma, it contains a smaller concentration of proteins than plasma and thus a somewhat greater concentration of water. This difference sets up an Osmotic Pressure. Although the osmotic pressure is small (~ 25 torr), it is greater than the blood pressure at the venous end of the capillary. Consequently, the fluid reenters the capillary here.
Now for humans, too much salt in your system causes high blood pressure, affecting this carefully balanced transfer of interstitial fluid. For tau, we know they like salt and enjoy salty foods (Kill Team, Fire Warrior), it is beneficial for them to have a higher sodium content than a human to maintain a higher blood pressure for more effective interstitial fluid transfer. Also, a higher water content adds to pressure. A tau would be wise not to dehydrate to quickly, possibly more so than a human.
This actually supports the rather generalist evolutionary traits of humans. Whilst highlighting the marked preference tau have for hot, humid worlds.
Overall, with these assumptions based on human systems, we can consider that the tau have stronger circulatory muscles and blood vessel walls to withstand the higher blood pressure. They have higher salt and water content which means their filtering organs differ in chemical output, types of chemicals and even their bodies intake is markedly different to a human. Interestingly, these needs also lead to aging issues and problems in humans, a possible shorter lifespan ?
With regard to a tau's nasal gill slit and providing oxygen, it is much like our nose in breathing ability, just carries more sensory receptors. They would probably have similar lungs to us even.
In the various reading I did, from marine biology to arachnid circulatory systems, I think the Tau have evolved a very good median system. Basically, as this is all fiction mind you, I am led to believe they have a circulatory system not unlike a humans.
Crayfish for example have a system much like a humans, veins, artery's, heart etc but more robust for the viscosity of bodily fluids they push about their bodies. However, at some point the plasma is simply suffused through the organs and 'flesh' of the crayfish. From this and several other creatures, including some lizards usage of their hemoglobin based circulatory system, I'm fairly positive on the idea the tau have evolved a complex circulatory system which has the better traits of a body that uses hemocyanin infused blood.
They would also have differing muscular components and some chemical differences, for carrying the heavier, larger and higher number of hemocyanin protein, a tau's copper content is very high and all the issues of that have to be countered I imagine. Human's need a small amount of copper, but too much is -really- unhealthy.
With regard to healing and wound 'scabbing', a plasma system is actually a little better. For sea life, the healing properties and sealing ability is paramount. A wound, without the ability to dry, for a human is a bad thing(tm).
For crustaceans, their plasma leaks and clots quickly, due to being a thicker consistency than the sea water. The oceans' salt factor also adds to the healing rapidity. A nice merging of environment and the creatures biology. Much like we need open air to dry out the blood in our wound. Interesting thing is, an open wound for a human (referring to a light wound here) seals nicely after a dip in open ocean.
Most plasma heavy circulatory systems I read about are pretty similar to a human system, but the general lack of hemocytes (for us Red Blood Cells) means a plasma heavy system (for our tau say) carries not only the Hemocyanin proteins (and the oxygen) but also nutrients, platelets and immune system cells. It would react to infection and bacteria just like our blood.
Water wise, tau having a dry, leathery skin may work well for us. It prevents loss of fluids, which for a system that suffuses its outer 'flesh' is a major issue. Not to mention, this type of system doesn't carry heat easily to the outer surfaces. So a tau would feel cold to the touch on outer extremeties.
RCanine's post mentioning the tau's personal environmental conditions, matches this deficiency well. A warm, damp environment would be comfortable for a tau. ;-)