In Traditional Chinese Medicine (TCM), the body is divided into six Yin and six Yang organ systems. The Yin organ systems, or Zang (臓), are Liver, Heart, Spleen, Lung, Kidney and Pericardium. The Yang organ systems, or Fu (腑), are Gall Bladder, Small Intestine, Stomach, Large Intestine, Urinary bladder and Triple Warmer (or Triple Heater). 

Each yin organ is paired with a yang organ and have interior and exterior relationship. For instance, the liver is paired with gall bladder, heart with small intestine, etc.. These relationships are important in the diagnosis and treatment of various conditions.

These organ systems, sometimes referred to simply as organs, are biological functions and not physical systems. For example, Kidney includes the physical kidney organs, but also includes the reproductive functions.  In order to fully appreciate the Chinese medicine system, the Western view of "form" has to be put aside.

Each organ system has a meridian associated with it.  In other words, there is a Liver meridian, Heart meridian, etc..  Each system is responsible for a group of functions in the body.  In addition, each Yin organ systems opens to an orifice, and governs certain tissues.  For instance, Kidney opens to the ears and governs bones.

The organs systems also follow the Five Element Theory in TCM, so they are interrelated and interact in certain manner.  For instance, Liver feeds into Heart, so Liver is said to be the Mother of Heart, and Heart is the Child of Liver.  In practice, when the Child is deficient, the Mother has to be tonified; and when the Mother is excessive, the Child has to be reduced.  This is one of the many relationships that exist and are of practical value in treating illnesses.