Age-related structural changes in the kidneys
Like other organs, the kidneys undergo significant physical changes over time. One of the most notable is a decrease in overall size, primarily affecting the outer region known as the cortex. This loss of cortical volume is directly linked to the gradual decrease in the number of functioning nephrons, the tiny filtering units within the kidneys.
- Nephron reduction: The number of filtering units, or nephrons, decreases with age. By the time a person reaches their 70s, as many as 30% of their glomeruli (the filtering part of the nephron) may have become sclerotic, or scarred.
- Arterial narrowing: The small arteries supplying blood to the kidneys can narrow and harden, a process known as arteriosclerosis. This decreases blood flow to the kidneys, which directly impacts their ability to filter waste.
- Glomerular changes: The walls of the capillaries in the glomeruli thicken and the overall surface area for filtration decreases. This makes the filters less efficient and contributes to the decline in overall kidney function.
- Fibrosis: Fibrous tissue can deposit between the nephrons, a condition called tubulointerstitial fibrosis. This further contributes to the loss of functional kidney tissue.
Functional decline and reduced reserve
These structural changes lead to a measurable decline in kidney function. The most common measure of this function is the glomerular filtration rate (eGFR), which indicates how well the kidneys are filtering blood. Studies show that for many people, eGFR begins a slow decline after the age of 40, though the rate of decline varies significantly among individuals.
One of the most important consequences of this functional decline is the loss of renal reserve. A young, healthy kidney has a large reserve capacity, meaning it can function normally even under stress, such as during an illness or dehydration. With age, this reserve is diminished, and even a minor health issue can temporarily cause a more significant loss of kidney function, a condition known as acute kidney injury (AKI).
- Impaired water balance: Older kidneys are less efficient at concentrating or diluting urine and conserving water, which can increase the risk of dehydration.
- Electrolyte imbalances: The kidneys' ability to manage electrolytes like sodium and potassium is also impaired with age, making older adults more vulnerable to imbalances during illness.
- Medication vulnerability: Many medications are cleared by the kidneys. A reduced eGFR means medications can build up in the body and become toxic. Dosage adjustments are often necessary for older patients.
Comparing an aging kidney with a healthy young kidney
| Feature | Healthy Young Kidney | Aging Kidney (60+) |
|---|---|---|
| Overall Size | Normal weight and size, often peaking in the fourth decade. | Progressive decrease in mass, especially cortical volume. |
| Nephrons | A high number of functional nephrons (approx. 1 million per kidney). | Loss of functional nephrons and increased glomerular scarring. |
| Blood Vessels | Healthy, elastic blood vessels with good blood flow. | Arteriosclerosis (hardening) of small arteries, reducing blood flow. |
| Filtration Rate (eGFR) | High GFR, typically above 90 mL/min/1.73m$^2$. | Gradual decline in GFR, with average eGFR below 90 mL/min/1.73m$^2$ for those over 60. |
| Renal Reserve | High reserve, allowing kidneys to easily handle stress like illness or dehydration. | Reduced reserve, increasing vulnerability to acute injury during stress. |
| Risk of Problems | Low risk for kidney-related complications under normal circumstances. | Increased risk of chronic kidney disease (CKD), AKI, and medication side effects. |
How to protect your kidneys as you age
Even with the inevitable changes of aging, you can take proactive steps to support your kidney health and slow the rate of decline.
- Manage underlying health conditions: Uncontrolled diabetes and high blood pressure are leading causes of kidney disease. Keeping blood glucose and blood pressure within target ranges is critical for protecting renal function.
- Adopt a healthy lifestyle: Regular exercise, maintaining a healthy weight, and not smoking are all important for kidney health. Quitting smoking is especially vital, as tobacco damages blood vessels and reduces blood flow to the kidneys.
- Hydrate consistently: Proper hydration helps flush waste and sodium from the body. While a daily fluid goal depends on individual health, it's important to drink consistently, as the sensation of thirst may decrease with age.
- Be cautious with medications: Long-term, heavy use of nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen can damage the kidneys. Always consult a healthcare provider or pharmacist about the safe use of medications, especially if you have reduced kidney function.
- Get screened regularly: Early stages of chronic kidney disease often have no symptoms. For those over 60 or with other risk factors like diabetes, annual screening with a blood test (eGFR) and a urine albumin test is recommended.
Conclusion
Aging naturally brings about structural and functional changes to the kidneys, including a reduced number of filtering units, decreased blood flow, and a lower glomerular filtration rate. While these changes are not necessarily a sign of disease, they reduce the kidneys' reserve capacity, making them more susceptible to damage from stress factors like illness, dehydration, and certain medications. However, by proactively managing underlying health conditions, adopting a healthy lifestyle, and getting regular screenings, older adults can help preserve kidney function and maintain their quality of life. The key is to be aware of the risks and take protective measures to support these vital organs as you age.
