Thus, under physiological conditions, the kidneys are highly active metabolic organs, and so are susceptible to the ageing process. In actuality, the kidneys are amongst the organs with the most prominent changes during the normal ageing process (Long et al., 2005).
Large scale (macroscopic) changes in the ageing kidney
On the macroscopic scale, renal ageing shows itself as roughness of the kidney surface as well as formation and increased size of simple renal cysts.
Simple kidney cysts can form in one or both kidneys but do not result in enlargement of the kidneys. Their formation is seemingly associated with ageing because they are common among people greater than 40 years old (upto 3 per kidney is normal). Simple cysts are usually harmless and have long been considered of little clinical significance.
However some studies have demonstrated that simple renal cysts are linked to hypertension, decreased renal size and functional changes.
Kidney volume is an important indicator of renal impairment. Two early studies used ultrasound or computed tomography (CT) scan in hundreds of adult volunteers and patients with no kidney disease and demonstrated that kidney volume progressively declined with age (Emamian et al., 1993).
Also it was estimated that the parenchymal thickness of a kidney decreased 10 % per older decade of age regardless of gender.
In addition, Wang, 2014 evaluated 1344 potential kidney donors by contrast-enhanced CT imaging and demonstrated that kidney volume declines at 22 cm3 per decade after the age of 50.
Interestingly, Wang also found that renal cortical volume progressively declines with age whereas medullary volume increased until 50 years of age, resulting in a net decline in total kidney volume after 50 years of age in normal individuals.
Microscopic changes in the ageing kidney
These large scale changes are associated with underlying changes at the microscopic level.
Nephrosclerosis (kidney scarring) is the pathological hallmark of ageing kidneys, characterised by nephron loss, hypertrophy of remaining nephrons, and arteriosclerosis, which is thought to be the initiating factor for ageing-related renal changes.
Other features of nephrosclerosis include global glomerulosclerosis, tubular atrophy, and interstitial fibrosis, which are reminiscent of the changes observed in progressive chronic kidney disease.
After 30 years of age, approximately 6,000-6,500 nephrons will be lost every year due to nephrosclerosis, or more specifically glomerulosclerosis (Denic, 2016).
In support of this contention, a study of 1638 healthy kidney donors showed that the number of nephrons was reduced by 48 % in 70-75 year-olds as compared with 18–29 year-olds, whereas the number of globally sclerotic glomeruli increased only 15 % (Denic, 2017).
Summary
We have described changes in the ageing kidney – on a large and microscopic scale. We hope it has been helpful.
