Heart yin refers to the yin essence of the heart. Opposite to heart yang, it is inhibitive, quiet, restraining, nourishing, and moistening. Heart yin is one of the basic substances to maintain the physiological functions of the heart. It is primarily involved in nourishing the heart and restricting heart yang to avoid the hyperactivity of heart fire so that heart yang can be held back from over- control and regular pulse as well as calm mind can be kept. Both heart yin and heart blood are of yin attribute, but they differ in function. While heart blood nourishes the heart, heart yin calms the mind and restricts heart yang.