In our everyday language, the heart is the organ most closely linked to emotion. Think about all the terms we use every day to describe our state of mind: “heartsick,” “heartbroken,” “heartache.” The heart is not the director of subtlety; the emotions it encompasses seem to always be on the far end of the spectrum, either extreme sadness or extreme joy.
The physical job of the heart in traditional Chinese medicine is very close to what we know it to be in Western medicine. It controls the blood vessels, and also controls the sweat. Excess sweating is a sign that the heart needs to be tonified (built up and supported). It also controls the tongue, and has an effect on speech. Stuttering, loss of words, and other speech difficulties can result from a deficiency of the heart.
The major responsibility of the heart, however, is housing the mind and controlling the shen. “Shen” can be seen as the overall healthiness of the mind. When you look at a healthy person of good spirits, you know how you can see that in their eyes? There is a certain bright clarity and sense of health that shines from within. We acupuncturists would say that this person has good shen.
Have you ever looked into a person's eyes and noticed that they appeared, well, not “all there”? Maybe their eyes were shifting from side to side, or maybe they just seemed cloudy and dull, as if they were not really in the present moment. This is poor shen. Sometimes mild depression or distraction can cause this shen disturbance; if very severe, it can be a sign of mental instability.
The heart is both the cause of and the victim of extreme joy. I know that extreme joy looks like a positive thing, but think more in terms of manic depression. Or ADHD. This hyperactivity is a result of the heart not properly controlling the mind.
One of my professors once described the heart's job as maintaining appropriate timing in life. He used an example of a person wearing a bathing suit. If this person wore the suit out to the pool in the summer, his heart was doing its job. But if he put the bathing suit on for a business meeting, it meant that the heart was not allowing him to make the right choice for the circumstances. In short, the heart is all about maintaining appropriate behavior for the appropriate situation.