Chakra circulatory system. They show it often enough in the anime. Neji hits certain spots on a person's body that are things like pressure points. Hitting them directly and with enough force can temporarily seal off the chakra's flow to those areas. If you close all of them, chakra won't flow at all. But the trick to using chakra is becoming in tune with one's body and directing the flow of chakra through themselves, and regulating it. This is shown when they learn to walk on water and run up trees. They pool the chakra to their feet in order for them to be able to scale the trees. In walking on water they had to choose the right amount of chakra to collect at their feet in order to not fall in. It flows through the body, there aren't just like lakes of chakra in one's body. That's my understanding of it.