“Superficially, it might be said that the function of the kidneys is to make urine; but in a more considered view one can say that the kidneys make the stuff of philosophy itself.” —Homer William Smith

 “All the soarings of my mind begin in my blood." —Rainer Maria Rilke

Kidney Function

The kidneys play a critical role in filtering the blood and removing waste. Their mass decreases by about 25 to 30 percent with age, resulting in reduced filtering surface. Studies have shown kidney function declines steadily with age, at about one percent per year from ages 40 to 90. Some studies suggest a steeper decline at very advanced ages. Reduced filtration leads to reduced clearance of some drugs and reduced urinary acidification. The kidney’s abilities to maximally dilute and to maximally concentrate urine are reduced to a greater extent than is the GFR. In addition, the kidney’s hormonal system that regulates salt and water and blood pressure can be more easily disrupted by dehydration in older people.

Although these changes are typical, they are not inevitable and do not necessarily have a major impact on quality of life. Some studies tracking people for as long as 18 years have found older people who exhibit no decline in kidney function, and even a few instances of functional increases.


Generally, blood volume is maintained throughout life and the tissues that produce blood retain a remarkable capacity for regeneration. The normal values for red blood cell numbers, size, hemoglobin concentration and hematocrit are essentially unchanged as we age. The average lifespan of red blood cells remains constant although red cells in older people may be more fragile. The amount of active bone marrow diminishes with age and marrow fat increases. Older people generally have a reduced capacity to accelerate the production of red blood cells, but the response to major blood loss, although impaired, usually remains adequate.

Anemia, although common, is not a normal physiologic consequence of aging. It always has a cause other than age, most commonly malnutrition, blood loss or the presence of a malignancy. White cell and platelet numbers are unchanged with age, though some of the function of blood cells that protect the body against infections and toxic substances may be affected by aging. 

What Does All This Mean for You?

These last four blogs present an overview of the normal changes that you are likely to experience as you grow older, as well as some of the increased health risks you may face. What are the general implications of these changes in terms of your own aging and vitality?

One lesson that I’ve drawn from my decades as a geriatrician is that aging is far from homogeneous. In fact, as we age we become increasingly differentiated and biologically unique. There is much more biologic variability among octogenarians than among neonates. Because of this increasing differentiation over time, algorithmic approaches, clinical pathways, rigid guidelines and other “one-size-fits-all” strategies of diagnostic investigation and resource allocation are likely to be less than optimal if they are based solely on age. Health providers, clinical investigators and health policy makers must begin to recognize this and take it into account. As a patient, you should work to build relationships with your health care providers and ensure they are considering your whole health picture rather than only your age.

A second lesson is that biologic systems that are minimally affected by age are often profoundly influenced by lifestyle factors such as smoking, physical activity, nutrition and economic advantage. Although the precise mechanisms by which these factors induce physiologic changes are unknown, it is often worth trying, where possible, to reduce those factors that seem to accelerate aging.

A third lesson is that we must—as individuals and as a society—recognize the inherent challenges of living with progressively diminishing resources while our environmental demands become increasingly complicated. Age-related functional declines are often compounded by losses of social status, income, self-esteem and family support (such as through the death of a spouse). Disease processes, increasingly common with age, may further reduce physical or mental capabilities. These changes in capacity may appear magnified or have greater impacts as a result of societal changes. For example, computer literacy is becoming an increasingly important social skill but can be challenging for some older people due to barriers such as vision problems or financial constraints. The complexity of changing social expectations may be especially problematic for those who have developed a self-reliant lifestyle and self-image. In addition, some older people are victims of changes in the physical environment, such as the deterioration of a once-fashionable neighborhood and resulting increases in crime.

Aging is not the accumulation of disease, although aging and disease are related in subtle and complex ways. The fundamental principal to keep in mind is that biologic and chronological age are not the same. Each of us ages at different rates and within each person aging affects different systems in different ways, primarily as a result of environmental factors such as lifestyle. Because aging is so variable and individual, we must each develop and implement an individualized personal aging plan. A “one size fits all” strategy simply will not fit.

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