The chakra pathway system.
Chakra Pathway System (経絡系, Keirakukei, English TV: Chakra Network, literally meaning: Meridian System) is the term for the channels within the body that transfer and channel chakra. Simply put they serve the same function for chakra as blood vessels do for blood. Along the chakra pathways, there 361 nodes called tenketsu that control the flow of chakra like circuit breakers, and within those tenketsu are Eight Gates which control body function and strain levels.
The chakra pathway system is enmeshed into the body and like blood vessels it touches and interacts with every single living cell and passes through every organ. If the chakra flow to a section of the body is blocked then that portion of the body will be unable to release any chakra from any of the tenketsu. Neji Hyūga was shocked that Naruto Uzumaki was still able to stand after being hit with Eight Trigrams Sixty-Four Palms as his body should have been in a virtually quadriplegic state.
Understanding this system is the key to the Hyūga Clan's Gentle Fist fighting style which forces chakra into an opponent's chakra pathway system. Proper application uses the system as a conductor sending harmful chakra directly to vulnerable vital organs. As there is no way to train a person's organs to resist physical or energy attack this method of fighting is extremely effective. An analogy could be made by comparing it to being attacked with a tazer.
Though Neji and more advanced users are known for attacking tenketsu, basic Gentle Fist is also extremely powerful as the chakra forced into the chakra pathways need not be specific as it affects everything in its vicinity. By using Gentle Fist, Neji was able to damage Hinata's heart to the extent that she was still coughing up blood up to a month later.
Naruto's Wind Release: Rasenshuriken has also shown the ability to destroy the enemy's chakra pathway system, and could do similar damage to his own right arm.