As it gets more difficult to make an appointment with a primary care doctor, one business model is picking up the slack: walk-in medical centers. Walk-in clinics — from drugstore chains that offer limited services from a nurse to stand-alone clinics operated by emergency room doctors — are increasingly treating people who either don’t have a regular doctor or can’t get in to see one when they need to. And if Congress succeeds in passing a nationwide health insurance mandate, the urgent care industry expects even more growth.