The meridians are the energetic pathways through the body that were codified in Chinese Medicine. There are twelve major channels that we deal with, each one being given the name of an Organ (in the Eastern, not the Western medical sense). The twelve Organs are: Lung, Large Intestine, Stomach, Spleen, Heart, Small Intestine, Bladder, Kidney, Heart Protector, Triple Heater, Gall Bladder and Liver.
Although the meridian does encompass the physical organ as we understand it from a Western perspective, the meridian also includes the functioning of the organ and its related structures. For example, the Heart meridian includes the actual heart as well as all the blood vessels and arteries throughout the body. The meridians are used extensively in acupuncture (and related disciplines) and many of the points that an acupuncturist needles are the acu-points that exist along the twelve major meridians.
In shiatsu, we also work on the meridians, although instead of using a needle as an acupuncturist would, we work along the meridian using various massage techniques, pressure and stretches. Shiatsu practitioners also use the acu-points that an acupuncturist uses, although, in shiatsu the acu-point would be stimulated with finger pressure instead of needles.