If medical training were available in infinite supply, it wouldn’t matter how many doctors worked part time or quit, because there would always be new graduates to fill their spots. But medical schools can only afford to accept a fraction of the students who apply.

An even tighter bottleneck exists at the level of residency training. Residents don’t pay tuition; they are paid to work at teaching hospitals. Their salaries are supported by Medicare, which pays teaching hospitals about $9 billion a year for resident salaries and teaching costs as well as patient care.

In 1997, Congress imposed a cap on how many medical residencies the government could subsidize as part of the Balanced Budget Act. Last year, the Senate failed to pass an amendment to the health care bill that would have created thousands of new residency positions. Even if American medical schools could double their graduating classes, there wouldn’t be additional residency positions for the new doctors. Federal and state financing to expand medical education will be hard to find in today’s economic and political climate.

We often hear the argument that nurse practitioners, nurse anesthetists and physician assistants can stand in for doctors and provide cheaper care. But when critical decisions must be made, patients want a fully qualified doctor to lead the health care team.

Policy makers could encourage more doctors to stay in the profession by reforming the malpractice system to protect them from frivolous lawsuits, safeguarding their pay from further Medicare cuts and lightening the burden of bureaucratic regulations and paperwork. And in a perfect world, hospitals and clinics could keep more female doctors working full time by setting up child care centers — with long operating hours — on site.

In the meantime, we can only depend on doctors’ own commitment to the profession.

Students who aspire to go to medical school should think about the consequences if they decide to work part time or leave clinical medicine. It’s fair to ask them — women especially — to consider the conflicting demands that medicine and parenthood make before they accept (and deny to others) sought-after positions in medical school and residency. They must understand that medical education is a privilege, not an entitlement, and it confers a real moral obligation to serve.