Eight Gates

The Eight Gates are eight specific tenketsu along the chakra pathway system.

Overview
The Eight Gates exist in order to limit the flow of chakra within an individual's body. Ordinarily, individuals can use no more than 20% of their body's full potential, the brain's way of protecting the individual from harming their body through overexertion. With training, individuals can learn to remove these limiters, referred to as "opening" the gates. With each opened gate, individuals are given access to more and more of their body's chakra, thus increasing their physical strength and speed. Ninja able to open the Eight Gates have existed since at least the time of Madara Uchiha. Since Madara's era, there have only been a few known users:
 * Might Duy learned to open all Eight Gates after twenty years of extensive training.
 * Might Guy, Duy's son, was taught how to open all Eight Gates by his father.
 * Rock Lee, Guy's student, was taught how to open some of the Eight Gates by Guy. During Part I, he's able to open the first five gates. By Part II, he's able to open the sixth gate. By adulthood, Lee had mastered all eight gates.
 * In The Last: Naruto the Movie, Rock Lee and the Suicide Corps could open the Sixth Gate to use a collaboration technique called the Crane Wing Formation.
 * In the anime, Metal Lee, Rock Lee's son, is shown subconsciously opening the first gate.
 * Kakashi Hatake is seemingly able to open the first gate, as seen when he activates it during a Cliff Climbing Practice. The anime later shows him opening it explicitly.

Learning to open any of the Eight Gates is apparently very difficult, as Kakashi is surprised when he's told that Lee - only thirteen years old at the time - can open five gates; Kakashi notes that such an accomplishment is impossible through hard work alone. Part of Kakashi's surprise is also because he does not believe it was wise for Guy to teach Lee to open the gates in the first place: although opening the gates does briefly give the user power greater than what they're normally capable of, this increased power causes the user serious harm. At a minimum, the user will be fatigued, which can leave them vulnerable to attack if this occurs during combat. In more extreme cases, muscles can tear and bones can break from the strain, which can be treated with medical ninjutsu. When opening the eighth and final gate, users will die as a consequence, their bodies being unable to survive the immense amounts of chakra flowing through them.

Using the Eight Gates is described as a double-edged sword, empowering users by sacrificing their own health and safety; such risks have caused the Eight Gates to be likened to kinjutsu. Because of this, Duy developed a self-rule to dictate when it was acceptable to open any of the Eight Gates, a rule he passed to Guy, and that Guy subsequently passed to Lee: to only open the Eight Gates when protecting something that is precious enough to give one's life. Often this "precious something" is a family member, a loved one, or a teammate. Rock Lee has also been known to use the Eight Gates to defend his nindō, the dream by which he leads his life.

Gates
When opening one of the Eight Gates, users are typically shown opening all preceding gates as well, though it is not clear if this is actually required. To have all of the Eight Gates open at once is known as Eight Gates Released Formation.
 * 1) The, located in the brain's left hemisphere, removes the user's mental inhibitions. Opening this gate allows the user to perform the Front Lotus, a technique that tires them afterwards.
 * 2) The, located in the brain's right hemisphere, increases the user's physical strength. This has the added effect of re-energising the body, enabling it to rapidly recover from exhaustion.
 * 3) The, located on the spinal cord, turns the user's skin red when opened. Other effects include the user's pupils disappearing, their hair standing on end, and bulging veins all over their body. Opening this gate allows the user to perform the Reverse Lotus. In the anime, the chakra released is enough to create a buffer zone to allow safe passage through severe adverse weather.
 * 4) The  is located on the spinal cord. Rock Lee opens this gate as he's about to use the Reverse Lotus, seemingly to increase his speed; as he performs the Reverse Lotus, his muscles start to tear. Opening the gate on this occasion also causes Lee's nose to start bleeding.
 * 5) The  is located in the abdomen. Rock Lee opens this gate as he's about to deliver the final blow of the Reverse Lotus, seemingly to increase his strength; after completing the Reverse Lotus, Lee's limbs receive compound fractures.
 * 6) The, located in the stomach, increases all the user's abilities. The power that emanates from the user is enough to displace surrounding bodies of water. Opening this gate allows the user to perform the Morning Peacock, wherein the user's punches are so fast that friction with the air produces flames. Opening this gate has sometimes been shown to create an aura around the user; in the anime, the aura is green.
 * 7) The, located below the stomach, causes the user's body to secrete a blue sweat. The sweat immediately evaporates, creating an aura around the user that can be mistaken for chakra. Opening this gate allows the user to perform the Daytime Tiger, a single punch that creates a pocket of compressed air. After opening this gate, Guy's muscle fibers rupture, causing him intense pain if anyone touches him.
 * 8) The  is located at the heart. Users open this gate by puncturing their chest with their thumb in order to direct the chakra there. While open, the user's heart pumps at maximum speed, granting them power a hundred times greater than they are usually capable of, far beyond that of the Five Kage. A  is emitted from the user's pores and hair, resembling a flaming red aura. Opening this gate allows the user to perform the Evening Elephant and Night Guy. The eighth gate causes the body to continually heat up until it's been cooked from the inside-out, reducing the user to ash.

Trivia

 * The Eight Gates are seemingly based off the real-life display of Hysterical Strength, in which a person pushes well-past their normal physical limits in moments of extreme desperation of life-or-death situations.
 * After Might Guy opens all Eight Gates, he is saved from death by Naruto Uzumaki's Yin–Yang Release. However, damage done to his leg from using Night Guy leaves him wheelchair-bound for the rest of his life. Despite the handicap, Kakashi Hiden, Konoha Shinden, and the Boruto anime all show Guy continuing his life as a ninja as best as he can.
 * In Naruto Shippūden the Movie: Bonds, Shinnō claims that his Body Revival Technique, combined with Dark Chakra, allows him to open all eight gates without any drawbacks.
 * In Naruto: Ultimate Ninja 2 and Naruto: Ultimate Ninja 3, Neji Hyūga uses Last Resort: Eight Gates Crumbling Attack to specifically block access to the Eight Gates.