For the biological description, see Heart
The Heart (心, pinyin: xīn) is one of the zàng organs stipulated by Traditional Chinese Medicine (TCM). It is a functionally defined entity and not equivalent to the anatomical organ of the same name.
In the context of the zang-fu concept 
As a zàng, the Heart is considered to be a yin organ. Its associated yang organ is the Small Intestine. Both Heart and Small Intestine are attributed to the Fire element.
Regarding its stipulated functions, the Heart
- ‘’stores‘’ (藏, pinyin: cáng) the shén (神, ‘’Aggregate Soul‘’, usually translated as mind)
- governs xuě (blood) and vessels/meridians
- opens into the tongue
- reflects in facial complexion
- governs joy (喜, pinyin: xǐ)
The Heart's function is said to be strongest between 11am and 1pm. Disturbed function of the Heart typically presents as palpitations, arrhythmia, insomnia, dream disturbed sleep, poor memory, restlessness, or even delirium.
Further reading 
- Val Hopwood (1997). "Introduction to Traditional Chinese Medicine theory". In Val Hopwood, Maureen Lovesey, and Sara Mokone. Acupuncture and Related Techniques in Physical Therapy. Elsevier Health Sciences. p. 8. ISBN 0-443-05593-9.
- Yanhua Zhang (2007). Transforming Emotions with Chinese Medicine. SUNY Press. p. 39. ISBN 0-7914-6999-9.