Module 6: Exploring Age-Related Body Systems Changes

6.6: Renal System

6.6.5: Renal Changes with Age

Renal mass declines about 20% between the ages of 40 and 80. Loss in mass is predominant in the renal cortex and includes a decrease in functioning glomeruli and telomere DNA (Melk, 2000). (Also see the Telomere Theory of Aging—Note: This link will open in a new browser window.)

Renal blood flow (RBF) also decreases, about 10% per decade after the age of 20. By age 80, RBF may be 600ml/min as compared to 1200ml/min in young adults (30-40). Like changes in mass, this is most prominent in the cortex. The decrease in RBF does not appear to be just related to a change in renal mass because there appears to be an actual decrease in flow per gram of tissue (Rowe, 1988; Beck, 1998).

Animal data suggest the following:

• Cortex: atrophy of afferent and efferent arterioles of the glomerular tuft
• Medulla: sclerosis resulting in the formation of direct channels between the afferent and efferent arterioles.

These changes maintain blood flow to the medulla over that of the cortex. How does this affect renal function?

The highly concentrated medulla, maintained by a sluggish blood flow, allows the kidney to form concentrated or dilute urine. An increased rate of blood flow to the medulla can ‘wash out’ some of the hypertonicity, decreasing the concentrating capacity of the counter current mechanism.

Other structural changes, some of which are from animal data, are noted in Table 1:

Why do these structural changes occur? We don’t know for sure yet, although some researchers have investigated possible reasons, including hypertenion and diet. Nor do we know whether it is possible to avoid these changes. However, we do know that there is heterogeneity in actual renal function in older adults.

Some suggest the changes in renal structure are caused by increases in blood pressure (Tracy, et al., 1992); others suggest the high protein diet often consumed in Western societies (Brenner, et al. 1989; Anderson and Brenner, 1988). A high protein diet could lead to chronic hyperperfusion of the kidney and thus hyperfiltration, which would in turn possibly potentiate renal deterioration. However, much of these data come from animal research and are not proven. Hyperperfusion may be an adaptive mechanism that maintains renal function with age (Lindeman, 1998). Further, a recent cross sectional study comparing 29 individuals in Boston (age range 19-79, x=52) with 16 island dwelling indigenous Kuna Amerinds of Panama (age range 18-86, x=51) who consume a low protein diet and did not evidence the same increase in blood pressure with age found that both groups had similar declines in GFR and renal perfusion with age; the Kuna Amerinds actually evidenced a steeper decline, although the actual differences in the rate of decline were not statistically different (Hollenberg et al. 1999).

What are the functional implications of the structural changes?

The most important change is a Decrease in Glomerular Filtration Rate (GFR is defined as—filtration of plasma/unit of time; about 125ml/min. or about 180L/day in a young, healthy kidney. However, there is wide inter-individual variation, and over 99% of the glomerular filtrate is reabsorbed).

The decline noted varies depending on whether cross sectional or longitudinal data are reviewed. In the Baltimore Longitudinal Study on Aging (BLSA), Lindeman, et al (1985) showed that, while there was a decrease with age in creatinine clearance, some individuals appeared to have a rapid rate of decline while 1/3 of the sample evidenced minimal decline. Further longitudinal data are needed to determine the clinical significance of these findings.

Changes in renal function do not significantly compromise the well-being of older adults in un-stressed circumstances (Castellani et al., 1998). However, vulnerability to environmental and disease challenges increases, and is enhanced by changes in homeostatic controls.