Where does one draw the line between medical ninjutsu and other jutsu that affect the body?
Medical Ninjutsu
Medical ninjutsu is a jutsu style which uses chakra to repair any harm the body has suffered by guiding, accelerating and enhancing the body's natural healing processes to restore their physical condition. As a type of Yang Release, it uses chakra mostly composed of physical energy to manipulate the body's cells and biological functions.
Fundamentally, it requires one to convert their chakra into a state that can be accepted by the recipient's chakra system without rejection. Otherwise channeling their chakra into their target would merely harm their body. Doing this allows them to restore the target's physical and mental energy so that they return to a physical state that can better mend their injuries and ailments. It is this requirement to transfer chakra without harm that separates medical ninjutsu from other jutsu that manipulate the body.
Then one sends chakra into a wounded area to make it restore itself by rapidly increasing its healing ability to repair the damaged cells of both external and internal injuries. The greater the injury then the more difficult and time-consuming it is to heal and not every wound can be healed. Scratches, bruises, burns and stab wounds are the easiest wounds to heal, followed by broken bones, torn muscles and severed tendons. The most difficult injuries are ones that are widespread across the body or damaged organs due to the time, effort and chakra required to restore them to a state without danger of death. And a significantly injured or destroyed organ is impossible to heal.
Beyond merely healing wounds, medical ninjutsu can also be used to treat drugs, poisons, sickness and disease by flushing out foreign substances, detoxify bodies and treat negative physical symptoms.
This ninja art is considered one of the most difficult as it requires precise chakra control, above average intelligence and advanced medical knowledge to effectively use it to heal without accidentally causing more harm. Those who specialize in medical ninjutsu - as well as other fields of medicine - are known as medic-nin.
And while it is known for being used to heal others, it works best the user's own body rather than that of another person as the chakra does not need to be changed to support a foreign chakra system and has direct access to the wounded area. In addition to healing one's own body, medic-nin typically learn jutsu that allows them to enhance their own bodies to better survive battle such as activating adrenal glands for surges of adrenaline, stimulating the nervous system to enhance their natural reflexes, triggering sweat glands and muscle to lower body temperature and other such manipulations.
Advancements in medicine and specialized training for medic-nin during the last century have led to the development of cooperative medical ninjutsu. By having training medic-nin work together on a patient, they are able to perform feats of healing that would be impossible for any of them individually such as regrowing lost flesh, healing internal organs or reattaching severed limbs. However these procedures can take hours to perform and are far more taxing than an ordinary healing session on everyone involved.
Fuuinjutsu is essential for this practice and is more closesly associated with medical ninjutsu than any other ninja art. As the primary role of fuuinjutsu is to seal out-of-control power it allows medical chakra to flow in a controlled state even when being provided by multiple users at once. Furthermore, fuuinjutsu can help the body remain in stable state when injured to avoid any further decline in health as well as sealing off afflicted body parts.
And just as an antidote can also a poison, medical ninjutsu can also a weapon in the right hands. The most obvious example of this are chakra scalpels which creates an extension of chakra from the user's hands. Often mistaken for wind release, this jutsu is actually composed of non-elemental yang charged chakra. Despite the name, a chakra scalpel is not actually sharp. Rather, the "blade" selectively severs the chemical bonds holding together any cells which it passes through. Chakra scalpels offer several advantages over conventional surgical instruments - Firstly, because the chakra scalpel does not actually cut living tissue, the lacerations it creates are much cleaner and finer than those created by even the sharpest wind blade, making them much easier to heal with other medical ninjutsu such as the mystical palm technique. And secondly, because the use decides what cells to sever and which to pass through harmlessly, a chakra scalpel can be used to operate on internal organs and tissue without causing unnecessary damage and while completely eliminating the risk of infection.
In addition, the very same properties that make the chakra such a versatile and powerful medical tool also make it a very effective weapon, although the chakra control and focus needed to wield them under such stressful conditions is quite high. In heated combat, even the greatest medical-nin won't be able to exert the precision needed to make fatal cuts, but it can still effectively cut muscle tissues and tendons, rendering the target immobile. No amount of body hardening or muscle training can protect someone from the effects of a chakra scalpel - if they are made of cells, they are vulnerable. Furthermore, reinforcing the hands with chakra is relatively easy for a ninja already skilled enough to fight with chakra scalpels, especially as they are already channeling chakra into their hands to use the jutsu in the first place, so a truly skilled medical ninja can not only use their hands for inflicting deadly wounds but also for blocking and deflecting swords, kunai and other ninja weapons.
And being able to manipulate bodily functions means that they can force their target's body to experience reactions that could hurt or kill them. One could cause their blood to clot within their veins, weaken the integrity of their bones, rewire their nervous systems or even cause cellular death throughout their body. While most medic-nin are best suited for support roles rather than battle, those with enough skill can be remarkably deadly enemies to face.
Most Famous Medics
Tsunade - The Slug Princess of Konoha's Sannin will undoubtedly be remembered as the greatest healer in history. Possessing a natural talent for chakra control, a keen intellect and a deep fascination with medicine, Tsunade was perfectly suited for the role of a medic-nin. She is a genius of medical ninjutsu who achieved greater contributions and achievements during her lifetime than anyone else had in centuries. This was especially true during the Shinobi World Wars where her medical skills were relied upon to save many lives.
In the Second Shinobi World War, Tsunade regularly fought on the front lines to protect her comrades. She healed their injuries both during and after battles to keep them alive and treated many different types of wounds, sicknesses and poisons used in the war. And as feared as she was for her superhuman strength, her ability to use medical ninjutsu in close combat could be just as dangerous. Then in Third Shinobi World War, she worked almost exclusively within Konoha rather than participating in battle. Yet her reputation as the greatest medic-nin only grows as she nevertheless developed new drugs, antidotes and medicine, trained more medic-nin, wrote advanced medical articles and create new medical ninjutsu up until the conclusion of the war.
She retired after the war as both a ninja and medic. She left Konoha with her chuunin apprentice, Shizune, and chose to travel the world having fulfilled her duties to her hidden village and nation. Even so Tsunade would go on to create her greatest medical ninjutsu yet, Creation Rebirth.
Haruno Sakura - She was the one of Tsunade's apprentice and proved herself to be the perfect successor as she learned all of her master's medical and combat skills. Her greatest achievement was completing the Strength of a Hundred Seal herself and using its chakra for the Creation Rebirth. While this was not so great as Tsunade inventing these jutsu, Sakura was able to master them before she was even twenty years old which made her a far greater healer than Tsunade herself at that age even if she didn't truly surpass her master until years later. Completing the seal in the Fourth Shinobi World War even allowed Sakura to become a more powerful fighter as she could use even more chakra to enhance her strikes without fearing that she would run out of chakra instead of holding back to carefully conserve it.
Yakushi Kabuto - Kabuto's own skill as a medic-nin is often overshadowed by his history as a spy and war criminal but he is considered by some to be the equal to Tsunade or her successor, Sakura. He spent most of his career using these skills almost entirely for the sake of self-preservation or preserving the victims of experimentation rather than simply healing the wounded, but that made him a more deadly fighter than most medics as he wielded those skills in battle.
One of his best medical ninjutsu was his self-invented Yin Healing Wound Destruction. It is named as such because it does not require yang chakra but regular chakra, meaning that it has far more mental energy than other medical ninjutsu. When Kabuto focuses his chakra into the area of the body that he anticipates a wound, it uses that mental energy to trick the cells into preparing for cellular repair before it has even been injured something that would negatively impact the body if tried with yang chakra. By building up a lot of chakra for this "pre-healing" it means that once the attack lands, it instantly starts to heal the wound even as the damage is being inflicted. This means that there is no delay in recovering from injury and grants an element of surprise as Kabuto counterattacks. Its only drawbacks are that it consumes far more chakra than an ordinary medical ninjutsu and if Kabuto fails to anticipate where the attack is going to hit then he suffers its full effect and wastes a lot of chakra for nothing.
Kabuto's medical skill was even pivotal to his later success as he delved into Orochimaru's research to assimilate the cells of other ninja, adopting their abilities for his own to make himself into a far more dangerous ninja than anyone expected.
