The fact that blood plasma exists in Medicine, should not be confused with the fact that Plasmas exist, that are defined in Physics, and which all matter can be converted to. In short, a Plasma is what becomes of a gas, when its temperature is too hot, for it to be a gas.
The long form of the answer is a bit more complex. In Elementary School, Students are taught that there exist three familiar phases of a given substance: Solid, Liquid and Gas. But according to slightly more advanced knowledge in Physics, there is no real guarantee, that there will always be these three phases. A gas first results when the thermal agitation between molecules becomes stronger – i.e. their temperature hotter – than the force that holds individual molecules together. At that point, the molecules separate and a gas results, the physical behavior of which is approximately what one would obtain, if a swarm of particles was to exist through collisions but through few other interactions.
Similarly, Liquids will form, when the molecules are forced from occupying fixed positions, but when they still don’t expand.
Well, as the degree of thermal agitation (of a Gas) is increased further, first, molecules become separated into atoms, and then, the electrons get separated from their nuclei, as a result of ordinary collisions with other atoms. This results in the negative particles – electrons – following different trajectories than the positive particles – the nuclei. And the result of that is that the collective behavior of the fluid changes, from that of a gas.
When a charged particle crosses the lines of force, of a magnetic field, a force is generated which is perpendicular to both the velocity vector and the magnetic field vector. As a result, the particles can travel without restriction along the lines of magnetic force, but their motion at right angles to it is deflected, and becomes helical. Not only that, but the direction in which the paths of the particles becomes curved, is opposite for the negative and positive particles.
For this reason, Plasmas can be confined by magnetic fields, except along the lines of the magnetic field. Increasing the strength of an applied field will also cause a Plasma to become compressed, as these helices become narrower.
A good natural example of this type of Plasma, is what becomes of the substance of the Sun. Its temperatures are easily hot enough to cause the transition from Gas to Plasma, especially since the temperature inside the Sun is much higher, than the temperatures which are observed at its surface. At 5000K, gasses are still possible. But at hundreds of thousands Kelvin, or at a Million degrees Kelvin, the bulk of the Sun’s substance becomes a Plasma.
Now, if the reader is a skeptic, who has trouble believing that ‘other phases’ can exist, than Solid, Liquid and Gas, there is an example that takes place at lower temperatures, and that involves Oxygen, namely, O2. We’re aware of gaseous O2 as well as liquid O2 that gets used in rocketry. But as the O2 is cooled further, to 54.36K at 1 atmosphere, it solidifies. Thus, it has already demonstrated the 3 phases which we’re taught about in Elementary School. But, if we cool already-solid O2 below an even lower temperature, 43.8K at 1 atmosphere, its phase changes again, into yet another phase, which is also a solid one. It’s currently understood that solid O2 has 6 phases. (:1)
At the same time, many fluids are known to exhibit Supercritical Behavior, which is most commonly, a behavior of a fluid which is normally differentiated between Liquid and Gaseous, losing this differentiation, due to its critical pressure being exceeded, but at temperatures at which fluids are commonly boiled. This has nothing to do with Plasmas, but without any distinction between Liquid and Gaseous, a substance which is ordinarily though to have three phases – such as water – ends up demonstrating only two: Fluid and Non-Fluid.
So there is no ultimate reason for which matter needs to be in one out of three phases.
(Updated 10/14/2018, 10h25 … )