The vessels, the routes for blood circulation, keep blood to circulate normally inside the vessels all through the body. The heart governing the vessels means that the heart bears close relationship with the vessels in structure, function and pathology. In structure, the heart is directly connected with the vessels. In function, the heart propels blood to flow inside the vessels by beating. 

In fact normal blood circulation is maintained by the heart and vessels working together. Thus normal function of the heart ensures smoothness of the vessels which is prerequisite to the normal flow of blood and the normal state of the pulse. However, abnormal function of the heart or un-smoothness of the vessels will disturb blood circulation and affect the sate of the pulse. For example, insufficiency of heart-blood and emptiness of blood vessels will lead to thin and weak pulse; insufficiency of heart-yang will bring on deep and indistinct pulse.

The idea that the heart opens into the tongue indicates that there is a special relationship between the heart and the tongue. For example, the sate of the tongue can reflect the physiological functions and pathological changes of the heart because a branch of the heart meridian runs into the tongue. So the condition of heart-blood displays itself on the tongue. On the other hand, heart-spirit directly influences the sensation and the movement of the tongue. 

Thus whether the heart functions well in governing the vessels or in controlling the mind can be understood from the color and shape of the tongue. That is why it is said that "the tongue is the sprout of the heart" in TCM. If heart-qi, heart-blood, heart-yin and heart-yang are normal, the tongue is ruddy and lustrous in appearance, agile in movement, soft in texture, keen in taste and fluent in speaking. So insufficiency of heart-blood leads to lightly whitish tongue without taste; up-flaming of heart-fire brings on deep colored tongue or ulcerated tongue; abnormal heart-spirit results in curled tongue, stiff tongue and dysphasia.

Since the condition of heart-qi, heart-blood, heart- yin and heart-yang mainly display on the face, observation of facial color and shape can enable one to know whether heart-qi, heart-blood, heart-yin and heart-yang are normal or not. If heart-blood is sufficient and flows smoothly, the face is ruddy and lustrous; if heart-qi is insufficient and heart-blood is deficient, the face is pale and lusterless; if heart-blood is stagnated, the face is cyanotic.