If you need to brush up on the lessons of supply and demand, U.S. emergency rooms provide an all too vivid example. Demand is up. Supply is down. The system is a mess.

Emergency room visits jumped more than 32 percent from 90.3 million in 1996 to 119 million in 2006, the most recent year statistics are available, according to the National Center for Health Statistics, a division of the CDC. About 13 percent of the patients get admitted to the hospital, meaning they are pretty sick. That proportion is pretty stable but the overall numbers are increasing. By 2006, more than half of all admitted patients had shown up first in the ER. That's a 36 percent increase since 1996, and it says a lot about the increasing problems people face in getting primary care.

And before you assume that this is the inevitable result of having 47 million uninsured Americans, think again. Increasingly, the ER patients are insured.

"There are more people arriving at the ERs. And there are fewer ERs," Dr. Stephen Pitts, author of the report and a CDC fellow who teaches emergency medicine at Emory University's School of Medicine was quoted by the AP as saying. The number of emergency departments fell from 4019 to 3833.

As the law of supply and demand would predict, waiting times have risen from an average of 38 minutes to 56 minutes. Pitts said that the average is inflated because some urban hospitals have such long waits; typically, a patient waits about a half hour.

If you've been reading posts from our colleague Dr. Guy Clifton, a brain surgeon who began working in health policy partly because of the Emergency Department crisis in his home city of Houston, this shouldn't surprise you. But the CDC's report reveals some unpleasantly stark statistics.

"The uninsured have long been more frequent users of (emergency rooms). That's not new. What's new is the rise ... in frequency in visits, and that's occurring in the insured," Pitts told the San Francisco Chronicle.

The rise in insured ER users reflects the shortage of primary care physicians, the wait for appointments, and their limited after-hours accessibility even for more affluent and well-insured patients. But they are showing up, as Guy Clifton has pointed out, in a world where hospitals are poorly managing their bed capacity and patient flow. That only adds to the stresses -- the patients piled up in the waiting room and hallways, the ambulances on diversion.

The ER is in many ways a proxy for measuring the overall health of our health system. We'd say it's time to call 911.