Accountable care organizations — a vague phrase with deep implications — could revolutionize the nation’s health care, policymakers say, lowering medical costs and increasing quality.

If they work as planned.

While the concept could lead to huge changes in the system, many ACOs will be virtually invisible to the patient.

“I’m not seeing any differences,” said Jaime Pinto, 69, a retired boat mechanic with chronic spinal problems who has just become part of the new world envisioned by the federal health care overhaul. “I’m still seeing the same doctors, getting my medications the same way. I don’t want to be guided, and I’m not being guided.”

ACOs — and a related concept, “medical homes” — were pushed hard by creators of the Affordable Care Act to improve quality and lower costs for Medicare patients, but the concept is expected to spread to others in the health care system.

Superficially, the concepts seem much like the old health maintenance organizations, but there are major differences. HMOs generally get a specific number of dollars per year for each patient, and the fewer dollars they spend on health care, the larger their profit. HMOs often have gatekeepers who must approve visits to specialists.

With ACOs, quality is a major factor in measuring results. Hospital readmissions and other poor health outcomes could penalize the organizations. But if patients remain healthy and cost Medicare less, the ACO member providers — which can be hospitals, doctors, home health agencies and others — share in the savings.

The concept is meant to fix the present system in which many providers offer no coordination whatsoever. One example often cited by lawmakers is that Medicare patients leaving the hospital are usually given prescriptions to fill and an order to seek follow-up care with their doctor in a short time. Often, those orders go unheeded, and the patient ends up back in the hospital. Medicare is now tightening the screws on hospitals, threatening not to pay for needless readmissions.

With an ACO, a care coordinator can keep track of discharged patients, making sure prescriptions are filled and doctor appointments made — and providing direct help if necessary, such as getting the drugs from pharmacy to home. Coordinators, working with shared electronic records, can also be sure, for example, that different specialists don’t duplicate tests or treatments.

Such a concept is especially important for patients in Miami, where the average Medicare recipient has close to the highest costs in the country — costing the federal government about twice what the average senior costs in Minneapolis.

Repeated studies by Dartmouth Atlas researchers have shown that Miami’s high costs often come from seniors bounced from specialist to specialist, frequently with duplicate tests. One Dartmouth study found that in the last two years of life, Medicare patients in Miami with any of 12 chronic conditions saw almost twice as many specialists as similar patients in Fort Myers.

With the Affordable Care Act offering incentives for ACOs, many large players are entering the competition, including major hospital chains, health insurers and even Walgreens. Some hospitals have gone on hiring sprees to get doctors to help provide the integrated care that ACOs require.

But not all ACOs are alike, said Steven Ullmann, a health policy expert at the University of Miami business school. He said some highly successful ACOs have been around for a long time, such as the West Coast’s Kaiser Permanente, which is a comprehensive umbrella that includes insurance, hospitals and doctors in one system that emphasizes preventive care. The best long-standing ACOs are nonprofit and are well-accepted in their communities, creating a culture in which pregnant moms start with obstetrics and the kids grow up through pediatricians, primary care doctors and geriatricians. But these programs aren’t always easy to adapt to other locations.

In Miami, Florida Blue is trying an ACO as part of an informal organization with Baptist Health South Florida and a group of Miami oncologists. Others, such as the Miami group that Pinto is part of, are created by doctors and don’t include hospitals.

Pinto is a patient of the South Florida ACO, which was set up this year by 75 Miami-area doctors who wished to control their future rather than be forced into some big entity’s structure.

“We simply want to focus on the practice of medicine,” said Jorge Acevedo-Crespo, a Miami pulmonologist who heads the ACO.

About half of Acevedo-Crespo’s practice is internal medicine, meaning primary care, and one of his patients is Pinto, who already had a “bad experience” with an HMO that “promised many, many things,” but made it hard for him to see specialists and have tests done.

Pinto was asked to join the ACO’s board as a volunteer patient member and has been happy with the freedom it allows him. One example: Pinto continues to see a Coral Gables pain specialist who is not part of the ACO.

Unlike long-established systems such as Kaiser Permanente, Acevedo-Crespo said his ACO doesn’t restrict patients to doctors within the organization because its goal is to get other physicians — such as the Gables pain specialist — to join the South Florida ACO.

“Even as it is, we work closely together,” Acevedo-Crespo said of his relationship with the pain doctor. “I pick up the phone and talk directly to him, discuss what we’re going to do next. We are practicing state-of-the art, evidence-based medicine,” rather than being part of an ACO run by “big insurance or hospital chains or investors or have a Wall Street company asking where are the dividends going to come from.”