There are eight different Extraordinary Vessels and they connect the Six Extraordinary Yang Organs (Uterus, Brain, Marrow, Bones, Blood Vessels and Gallbladder) with the other Organs. For example, the Uterus connects to the Kidneys through the Directing and Penetrating Vessels.
These Extraordinary Vessels are like reservoirs of energy, as opposed to the main Channels which are like rivers. They connect to Channels and balance Yin and Yang flow between them. In effect, Channels can "tap into" the Extraordinary Vessels reserves or store Yin and Yang in them, depending on the needs.
The Penetrating (Chong Mai 冲脉), Governing (Du Mai 督脉) and Directing (Ren Mai 任脉) Vessels are the most important of the eight Vessels because they are the origin of all. Each of the three start directly from the space between both Kidneys because their main task is to supply and circulate Kidney Essence. They can be seen as the three branches of the same Vessel originating from the Kidneys and then taking different pathways. Compared with the other Extraordinary Vessels, these three can be used in clinical practice to influence a patient’s status at a deep fundamental level.
The Penetrating Vessel is a complex one because it has many different functions. Regarded as the ‘Sea of the 12 channels’, it branches out in many small capillary-like vessels and connect with the Channels. It has 5 main branches: the internal branch (through the lower abdomen, the Uterus and the perineum), the abdominal branch (along the Kidney Channel and dispersing in the chest and breasts), the head branch (ascending alongside the throat, chin, lips and eyes), the spinal branch (ascending inside the lumbar spine) and the descending branch (along the inner thigh, the lower leg, the internal malleolus and the foot).
It is also considered as the ‘Sea of Blood’ as it controls all the Blood Connecting Channels. It helps circulate and balance Qi and Blood within these Channels. It governs the Uterus Blood and the transformation of Kidney-Essence into menstrual Blood (also called Tian Gui).