Dr. Lee Gross is worried. He has practiced family medicine in North Port, Florida, near Sarasota, for 14 years. But he and two partners are the last small, independent practice in the town of 62,000. Everyone else has moved away, joined larger groups, or become salaried employees of hospitals or health companies.

“We’re struggling to survive,” Gross, 47, said. “Our kind of practice is dying in this country, and medicine itself is changing so rapidly that doctors everywhere seem to be burning out.”

Indeed, in their professional journals, at conferences, on social media and health care blogs, and in comments to federal regulators, the nation’s doctors are expressing growing anger and frustration.

The focal point of their angst is a 2015 federal law that changes the way Medicare pays doctors.