Xin 
, located in the thorax above the diaphragm and under Fei 
, is physically wrapped within the Pericardium. It is shaped like a downward-pointing sphere or a dangling lotus bud, and seems like a red fireball (with a red shining aura), full of energy and luster.
Xin is the residence of the Spirit 
, and the controller of Blood and the vessels. It corresponds to Fire in the Five Elements Theory and Yang within Yang in Yin–Yang Theory. It shows a dominant role in the vital activity of the human body. The Basic Questions of the Inner Canon of the Yellow Emperor says, “The Xin is like the monarch, and it governs the Mind …”. Meridians of Xin and the Small Intestine are connected, and Xin and the Small Intestine are externally–internally related. Xin connects to the Small Intestine, face, and tongue.
The physiological and mental functions of Xin ()
(1) Xin regulates blood and vessels
Xin regulates Blood and the vessels which carry Blood. The energy of Xin –Qi pumps Blood ceaselessly through the vessels to nourish and maintain the body’s normal physiological functions. The Basic Questions says, “The Blood pertains to the Xin ” . A constitutional weakness of Xin may present as a long, shallow crack down the tip of the tongue and a weak pulse at both Xin and Shen 
.
Xin’s energy is reflected in the state of the Blood vessels, which rely on Xin –Qi. A feeble, irregular pulse indicates weakness of Xin –Qi. The Basic Questions says, “The Xin regulates the Blood vessels …”. In TCM, diseases of Xin can be diagnosed by the Xin beat, pulse manifestation, complexion, tongue picture, and subjective feelings within the chest.
When Xin –Qi, -Blood, -Yin, or -Yang is disturbed, Xin function will be affected. It will show pathological manifestations such as a pale complexion, or a thin, weak pulse due to insufficient Xin –Qi or stagnated Xin Blood. Xin also acts in Blood production, termed “reddening” in TCM. After food is digested and absorbed, the refined nutrients, mixed with blood cells, are transported up to Xin and Fei 
. Fresh oxygenated air, inhaled into Fei, combines with the Original Qi, is warmed and steamed by Xin –Yang, and transformed into red Blood.
(2) Xin regulates mental activities
Xin regulates the activities of the Spirit or, it can be said, Xin houses the Spirit or Mind 
. The Spirit acts as the supreme dominator of all life activities in the body. In another sense, the Spirit is the collective term for perception, cognition, thinking, consciousness, and the mental state. Xin regulates the Mind because it regulates mental activities by balance of the Blood supply. The Brain is an information center under the control of Xin .The Miraculous Pivot says, “The Xin … is the residence of the Mind”
In TCM, the entire universe, including human beings, has tangible and intangible (i.e. Qi and Spirit system) aspects. The tangible and intangible components are closely related and dependent on each other. The Spirit or Mind is an extremely ethereal spirit element having high energy, motivation, and information, which regulate the solid body. The Basic Questions says, “The Mind (Shen, 
) is a transformation of Essence (Jing, 
) and Qi; both congenital and acquired Essence contribute to form the Mind. The Corporeal Soul (Po, 
) assists Essence and Qi, but does not move in and out. The Ethereal Soul (Hun, 
) complements the Mind and Qi, but moves in and out. Thought (Yi, 
) corresponds to memory and depends on Xin Willpower (Zhi, 
) is a “purposeful focused mind”1. All mental, conscious, and thinking activities reflect visceral physiological functions. Thus, Xin houses the Mind (Shen, ), Fei houses the Corporeal Soul (Po, ), Gan 
houses the Ethereal Soul (Hun, ), Pi 
houses Thought (Si, ), and Shen 
houses Willpower (Zhi, ).
Mental activities closely relate to the Brain’s response to external stimuli and certain intrinsic Brain functions. In TCM, Xin closely contacts, influences, and regulates the Brain’s selection and decision processes. Xin is the center of the five Zang organs, promotes mental activities, and has certain Brain functions which are controlled by the Spirit 
. The Miraculous Pivot says, “The Xin is in charge of mental activities” and “combines and unifies the Essences of parental Yin–Yang to form the Spirit”. Xin regulates the Spirit, the living physical body houses the Spirit, and the Spirit is the reflection of the body’s functional activities. After birth, the Spirit depends on acquired Essence including vital Essence, Qi, and Blood. The Spirit arrives, develops, and leaves with the solid physical body, just as form itself grows, develops, and eventually dies. Xin , therefore, dominates vital human activities. Under Xin’s central command the viscera are unified and coordinate all vital activities; all mental, conscious, and thinking activities reflect visceral physiological functions. Xin is the most important organ. If Xin loses control of the Spirit, it will present as insomnia, dream-disturbed sleep, restlessness, delirious mania, slowed response time, amnesia, coma, and insensibility. Phenomenological research reveals that the mechanisms of Xin’s roles in mental function and information processing in human beings are mysteries that are only just beginning to be explored by Western science. For example, there are recent studies regarding the occurrence of personality changes in heart transplant recipients wherein their nature takes on recognizable aspects of the heart donor.
(3) Xin opens into the tongue
Xin regulates the color, form, and appearance of the tongue, especially the tip. In addition, it regulates the sense of taste. The tongue reflects the physiological function and pathological changes of Xin because a branch of the Xin Meridian runs up into the tongue, or “the tongue is the sprout of Xin .” So Xin -Blood’s condition is displayed on the tongue while Xin Spirit directly influences tongue sensation and movement. Xin’s (function can be diagnosed from the tongue’s color and shape, and thus tongue diagnosis is critical for understanding the condition of Xin. If Xin is functioning normally, the tongue appears ruddy, lustrous, and agile. Insufficient Xin -Blood presents as a light, whitish tongue with a reduced sense of taste; inflammation of Xin -Fire presents as a deeply colored tongue with a swollen red tip or as an ulcerated tongue; abnormal Xin -Spirit presents as a curled, stiff, or deviated tongue, and dysphasia. Xin also influences speech and laughter so that, if Xin is suffering from particular pathogens, a person may talk or laugh incessantly.
(4) Xin manifests on the face
Xin’s external reflection is the face, whose complexion mirrors Xin’s (condition, mainly the condition of Xin –Qi, -Blood, -Yin, and -Yang. Facial color and shape reflects the function of Xin If Xin -Blood is sufficient and flows smoothly, the face is ruddy and lustrous; if not, the face is pale and lusterless.
(5) Xin is related to the emotion of joy
“Visceral manifestation” means that emotional reactions to external stimuli come from the five Zang-viscera. These five emotional reactions, visible manifestations of the intangible system, are Joy, Anger, Obsessive Thinking, Sadness, and Fear, and they correspond to the five Zang-viscera (Xin, Gan, Pi, Fei, and Shen, respectively). Of the five emotions, Joy, a reaction to an enjoyable external stimulus, relates to and is pleasant for Xin . However, over-exhilaration may damage Xin and impair concentration. As Xin also affects speech and laughter, a disharmony of Xin can cause a person to talk or laugh incessantly.
(6) Sweat secretion relates to Xin since it regulates Blood, which in turn relates to body fluids (including sweat)
Sweat secretion depends on Defensive Qi’s (Wei Qi) function of opening and closing the striae. Sweat, Blood, and body fluids come from the Essence of ingested food and water. Hyperhydrosis (heavy sweating) will consume the Blood and Qi of Xin . Any profuse and continuous sweating with Xin deficiency should be treated immediately.
For example, chronic heart failure, also known as heart failure, is a complex syndrome caused by damage due to a variety of organic or functional heart diseases, ventricular filling or ejection, often arising from infection, excessive physical labor, emotional stimulation, and so on.