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Midichlorian's are chemical compounds found within every living cell that forms the basis between the connection life and the Force. Without midichlorians, life would be impossible. The amount of midichlorians in a life form represents its ability to understand, comprehend, and manipulate the Force.

I have decided to absorb more of these cells into my own body to make my force sensitive abilities stronger. To accomplish this, I have kidnapped numerous force sensitive users who match my blood type. When their midoclorian count has been measured, I will perform a blood transfusion from them to myself in order to absorb these cells. After I have attained enough of these cells, I will become the most powerful force user in the world and create my Sith empire. Then you will use this method to strengthen my apprentices and students.

However, after a number of tries, I have noticed that my powers have not increased. No matter how many Jedi I murder and drain of blood, my abilities remain average. Why would this be the case?

Because the blood level midichlorian equilibrium point is determined by your body's biochemistry. You can't alter this by adding more midichlorians. All that does is cause a temporary imbalance (way too short for your body to make use of it) that your body quickly fixes. Excess midichlorian gets filtered out by the kidneys and ends up down the drain. Literally.

Midichlorian is not what gives you the force, it is a by product of being strong in the force. All the excess psychic energy goes into making it. All that blood is wasted because the midichlorians come from the force, not the force from midichlorians.

Obviously blood can only contain a certain density of midichlorians or it would clog up and ultimately become solid. Your body will naturally break down any excess of midichlorians and convert them into fat. This is what happened to Jabba the Hutt.

Because they are genetically foreign.

They immediately start to reject the body, and the body rejects them. Welcome to a force fight at the cellular level.

Immuno-suppressants allow the transfusion to succeed without ripping your body apart, but the downside is that they significantly hamper even your own ability to interact with the force, injecting extra barely raises you back to functioning.

Cloning ;)

Okay, genetically identical. Except that the Midichlorian's are already at saturation in your body, adding more of the same simply causes them to die off.

You might, through an ingenuous system of continuous transfusions keep yourself in a state of over-saturation. This would amp up the basic capacity for force manipulation (you will still need practice as with any fine motor control), but there are (semi-)permanent side-effects. The Midichlorian's learn that massive spikes occur, and reduce their overall numbers so as to survive long-term. You might recover to normal levels, it might be a permanent reduction.

Divergence

As the force wielder learns, the Midichlorians alter themselves physically. Even if they were cloned and successfully duplicated, reintroduction back into the body causes the equivalent of a theological argument between religious sects. At best nothing changes, at worst valuable skills (learning) are lost as the two sides duke it out, or the host dies.

In short you are extremely lucky to be alive.

Homeostasis. Your body maintains an equilibrium, like blood or temperature

A normal human has around 5 litres of blood in their body. If you transfer a litter of to your body (and not die), would now you have 6 litres? Well... maybe, but not for much longer.

Or temperature, you are at 36℃, if you increase/decreases your body heat from your surrounding, will it stay in that forever? Of your not. Your body maintains a level of temperature, by warming up or cooling down at certain thresholds.

Your blood amount is also determined by your body, not your blood transfusions, if you have a lack of blood, your body will produce more to compensate it, and if you have too much, your body will dispatch the blood's water from kidneys, and red cells' will be recycled into bilirubin (yellow) in the spleen and that dispatched from duodenum as stercobilin (brown). Interesting transformation of colours, right?

Midichlorians are like blood, and a lot more things in our body, like hair, adipose tissue, temperature, etc. A transfusion may only increase the Midichlorians during a very short time (if the body is able in the first place to take advantage of them and don't die/get sick due an increased level), then the additional amount will be quickly destroyed/dispatch/recycled by your own body...

...Or maybe not, they aren't destroyed, but anyways they have a certain lifespan, so if you transfuse them to your body, they will work in you until they "get old" and die, like red cells (120 days). Then you should transfer more from the original source... or steal their Midichlorians's stem cells. That is up to you.