WITH A SENATE subcommittee deciding when human life begins, and a presidential commission recommending a uniform legal definition of when it ends, the trustees of the Medicare program reminded us last week that, tough as these issues may be, there are harder ones still to be addressed in keeping the organism healthy between these two signal events.

Medicare, the health insurance program covering most aged and disabled persons, is headed for hard times in the next decade and beyond. Already the program's costs have grown enormously -- from about $7 billion in 1970 to over $40 billion this year. They will probably double again in the next five or six years. While much of this growth is tied to increases in general prices and wages, the trustee's report points to another factor of almost equal importance to the program -- increases in the number and type of services provided to each patient. In other words, one major reason that medical costs have grown so rapidly -- and are likely to keep growing even if the general price level stabilizes -- is that people are getting more and fancier care.

The country now devotes almost 10 percent of its gross product to health care. That covers everything from corn pads to kidney dialysis, but hospital care is far and away the biggest and fastest growing item. People go to hospitals more, and spend more money while they're there, not because they like the accommodations but because, thanks to modern technology, there are many more types of diagnostic and treatment services that hospitals can provide, and these things are extremely expensive. While other things like diet, not smoking and modern sanitation are far more important contributors to general health than medical care, 10 percent of the GNP is probably not too much to be spending for the many benefits of modern medicine. As that percentage creeps up, however, as it is likely to do if present trends continue, the questions of how much is enough to spend on medical care and what to spend it on will come to be posed more sharply.

One school of thought holds that these difficult questions can be avoided by trying to make the medical market work better -- limiting insurance coverage, for example, so that people pay more attention to prices in deciding whether to seek medical care and from what source. Recent pilot programs do show that requiring more cost-sharing by patients somewhat limits the use of medical care, at least among the non-aged. Whether that affects their health hasn't yet been determined. In any case, there are obvious limits to this approach. When you or your child is the one with the failing kidney, heart defect or rapidly spreading cancer, it's not likely that you'll be shopping around for the cheapest doctor or hospital rather than the very best one you can get.

Past administrations have tried in various ways -- physician review groups, health planning agencies and direct cost controls -- to get a hold on medical costs. The Reagan administration thus far proposes only to shift part of the cost burden to other levels of government or to individuals. Shifting the costs of caring for the very sick and the aged, with their generally limited resources, won't make them go away. A better solution is needed for what is rapidly becoming one of the country's major policy dilemmas.