101 Facts About Ageing #41: Blood Filtration Rate Of The Kidneys And Ageing

As Daniel Patrick Moynihan, an American sociologist, politician, and diplomat once said: “Everyone is entitled to his own opinion, but not his own facts”. And we wholeheartedly agree. A shared set of facts is the first step to building a better world with longevity for all. In that spirit, we are creating a series that covers 101 indisputable facts about ageing, health and longevity.

We need our kidneys to filter the blood, allowing waste products to be lost in the urine while reabsorbing substances that are necessary for our bodies. The kidneys also play a central role in controlling the volume and salt content of the blood, by altering the extent to which they reabsorb water and sodium.

The rate at which the kidneys filter blood is called the glomerular filtration rate (GFR), in reference to the glomerulus, the network of small blood vessels at which blood filtration occurs. GFR declines by roughly 50% between the ages of 30 and 90 in healthy individuals. In other words, the kidneys of a 90 year-old are able to clear substances from the blood at half the rate of the kidneys of a 30 year-old. This happens for several reasons.

The rate of blood flow to the glomerulus declines with age and the glomeruli undergo scarring, reducing both the rate at which blood enters the glomerulus and the efficiency with which that blood is filtered. The reabsorption of molecules from the blood filtrate occurs within structures called nephrons – microscopic, winding tubes which begin at a capsule surrounding the glomerulus and end by emptying into a duct that carries the urine to the renal pelvis. With advancing age, the number and size of these nephrons decline. From young adulthood (18–29 years) to old age (70–75 years), healthy adults lose almost half of their nephrons.

Age-associated loss of GFR is not the same thing as chronic kidney disease, in which GFR drops enough that dialysis/kidney transplant may eventually be needed. Diabetes, age-associated increases in blood pressure, cholesterol, and inflammation increase the risk of chronic kidney disease. Chronic kidney disease is the world’s 12th leading cause of death, but normal age-related loss of GFR is thought to have relatively little impact on mortality.