I'm not a doctor or an expert in biology, but here is my thought experiment in some new elements that could help us simulate macroscopic living organisms. There is no change to current elements and no saves are affected. Please don't be afraid to suggest or point out things. I tried to put up a model considering what PT is capable of, the guidelines and what people want. So here's my 2 cents.


-SOLIDS

FLSH-
Flesh.
Flesh is the basic "building block" of organisms. It is the color of the average skin pigmentation of humans. It is a very sensitive thing, it can Die (turn into Dead Flesh) if any of these conditions are met:
1. Pressure of over 10 or under -10
2. Temperature <= -5 or >50 (or if it touches ICE)
3. Oxygen deprivation (LIFE reaches 0)
4. Is in contact with anything toxic (Oil, Diesel, Isotope, Uranium, Plutonium, ect)
5. Gets irradiated with a Neutron

Flesh melts at over 100 degrees and burns at over 350 degrees. It also has life. It's life is the level of oxygenation of it's tissues. It constantly requires a steady supply of oxygenated blood. If it's life reaches a negative value, it dies off.

Also, exposed flesh (with nothing surrounding it) will slowly clone SKIN. This means that the outer layer of a fleshy structure will grow a skin.


SKIN-
Organic, impermeable cover for FLESH. Prevents some types of poisons from being absorbed; but not all.


LUNG-
One of the most important part of this "system" is Lung. It absorbs Oxygen and inputs Life into the Blood. Inversely it also takes Co2 out of de-oxygenated blood to be expelled.It absorbs Oxygen from the blood and rejects Co2, in practice  it's like a reverse Plant. Subject to the same restrictions as Flesh, turns into Dead Flesh if conditions become hostile. Has no Life of it's own. It can also be killed by Smoke and Gas. It can only exchange gasses to blood not further than 1 pixel.

O2 -> Blood Life+100
Blood Life=0 + O2 -> CO2 (Gas exchange)


VEIN-
This is an organic (can die) solid that can transport blood to where it is needed whitout touching the oxygen in it, in a kind of insulator. It is as vulnerable as Flesh.


NERVE-
An organic conducting wire. Pretty simple. It is only conductive when it's temperature is between 1 and 45. Sparks travel twice as fast as in normal metal/wires; it uses PSCN for sending and NSCN for receiving other wise it is not conductive to anything else including water. Emits a spark when it is hit by a photon or electron. Disintegrates or dies into DEAD flesh.



-POWDERS


DEAD-
Dead flesh. Turns into DUST when heated above 200 degrees; (or into PLNT when mixed with water?). It spreads from cell to cell slowly but surely (necrosis). Can be destroyed by SOAP, Electrons and Arrays.



-LIQUIDS
BLOOD-
The centerpiece. It's LIFE level does not drop until it transmits oxygen to flesh. It is subject to color variation from light blue (depleted) to dark red (rich) depending on it's oxygen levels. Freezes into ICE and evaporates into steam (WTRV).


THINGS TO NOTE
Now you'll notice that most organs from the body are missing. That's because we can simulate the rest using what we already have.