WASHINGTON (AP) _ Death claimed more Americans in 1985 than in any other year in the nation's history as the elderly, subject to the highest death rates, comprised an ever larger share of the population.

Thus, while better medical care extends life, it also results in a larger number of elderly, the age group most subject to the long-term effects of aging and chronic illness.

The National Center for Health Statistics recorded 2,084,000 deaths in the United States in 1985, about 37,000 more than a year earlier.

Medical improvements lowering the death rate for most people were balanced by an influenza epidemic and the larger share of elderly in the population, with that group's relatively high death rate, government statisticians reported.

The result in 1985 was an unchanged national death rate of 8.7 deaths per 1,000 people, with the increase in total deaths paralleling the growth of the population.

''Contributing to the increased number of deaths were the continuing increase in the proportion of older persons in the population and the influenza outbreak during the first quarter of 1985,'' the center reported.

The nation's population over age 65 increased by 2.5 million between 1980 and 1984, the Census Bureau reports, with an estimated 28,040,000 elderly as of July 1, 1984.

That total is up from only 16 million over age 65 in 1960. And the growth has been especially marked among the so-called ''old old'' - people 85 and over. That segment of society increased from only 900,000 people in 1960 to 2.7 million as of 1984.

Preliminary health center statistics for the 12 months ended November 1985 show that deaths of people aged 85 and over increased from the same period a year earlier.

For people aged 75 to 84, the death total also rose, and the 65-to-74 age group had a small increase, according to a sample survey.

Among younger Americans, most age groups experienced declines in total deaths between the two 12-month periods.

The exceptions were infants and children and the 35-to-44 age group. All of those increases were relatively small, however.

Despite a slight rise in infant deaths, though, the nation's infant death rate was 10.6 per 100,000 live births, down slightly from 10.7 a year earlier.

The rate fell despite a rise in deaths due to the fact that the total number of infants increased faster than the deaths - a reflection of the so- called echo of the Baby Boom, occurring as that large generation born after World War II passes through the prime childbearing ages.

While the national death totals were given for calendar 1985, the detailed figures are for the 12 months ended last November, since statisticians have not had the opportunity to analyze later figures.

During that 12-month period, the nation's single most common cause of death was heart disease, as it has been for many years, although the rate has been declining.

Major cardiovascular diseases - diseases of the heart and blood vessels - had a death rate of 410.5 of every 100,000 Americans. That is down from a rate of 410.9 a year earlier.

Cardiovascular diseases had a death rate of 436.4 in 1980, 496.0 in 1970 and 515.1 in 1960, according to government records. Lower rates, however, of between 350 and 400 per 100,000 people were recorded in the early years this century.

The lowest reported was 345.2 in 1900, after which it rose steadily to top 400 in 1926 and 500 in 1944. The worst year appears to have been 1963 with a cardiovascular death rate of 527.3.

Various forms of cancer had a death rate of 193.2 for 1985, up from 191.1 in the earlier sample, the center reported.

In contrast to recent declines in the death rate from heart disease, the cancer rate has been rising in recent years as people live longer.

The cancer death rate was 183.9 in 1980, 162.8 in 1970 and 149.2 per 100,000 people in 1960. Cancer was blamed for only 64 deaths per 100,000 people in 1900, the rate climbing steadily ever since, and topping 100 in 1932.

Lung and chest cancers accounted for the largest segment of cancer deaths last year at a rate of 53.1 per 100,000 people, up from 51.8 in the 12 months ending in November, 1984.

Other cancers with high death rates included cancer of the digestive organs, 49.1, up from 48.7; cancer of the genital organs, 20.8, up from 20.4; breast cancer, 16.7, down from 16.9; cancer of the lymph system, 10.4, up from 10.2, and cancers of other unspecified sites, 24.5, up from 24.3.

Other major causes of death in 1985 listed in the new report, and the rates per 100,000 people, are:

Accidents, 38.3, down from 39.6; chronic lung disease, 31.3, up from 29.5; pneumonia and influenza, 27.8, up from 24.5; diabetes, 15.8, up from 15.1; suicide, 11.6, down from 11.7; liver diseases, 11.4, down from 11.5; kidney diseases, 9.3, up from 8.7; murder, 8.2, down from 8.3, and blood poisoning, 7.1, up from 6.2.