The sea of blood, or the thoroughfare vessel, is one of the four seas where the blood of the twelve meridians concentrates and distributes. Connected with the twelve meridians, the thoroughfare vessel could infuse the yang meridians above and the yin meridians below, and thus is called the sea of the twelve meridians. Meridians are the pathway of qi and blood in the zang-fu organs, so it is called the sea of five zang-organs and six fu-organs. All the qi and blood of meridians and organs go downward to infuse the thoroughfare vessel, and that is why the thoroughfare is the sea of blood. When the yin blood is sufficient, the uterus could produce menses and conceive babies, fulfilling its physiological functions.