A 2011 study of differences in electrolyte and fluid balance in Advances in Chronic Kidney Disease between young adults and the elderly found that the most notable difference is the decreased ability of the elderly to regulate kidney function. The study suggests that individuals experience difficulties in excreting a concentrated or a diluted urine, inclusive of regulated amounts of waste ammonium, sodium or potassium.
The article claims that it's normal for kidney function to begin decreasing around age 40, including larger amounts of cortical glomerulosclerosis and declines in both renal plasma flow and glomerular filtration rate. Cortical glomerulosis results in the hardening of the glomeruli, which are renal structures responsible for filtering waste products out of the blood for the production of urine. Decreased blood flow into the glomeruli results in a slower filtration of the blood and decreased urine production. This decrease in regulatory function of the glomeruli can result in hyponatremia, which is a dangerously low level sodium in the blood, or hypernatremia, which is a dangerously high level of sodium in the blood. The study found a high correlation of mortality in the elderly with occurrences of hyponatremia or hypernatremia. Under normal conditions, these imbalances of sodium in the blood are not enough to cause death on their own, but they can result in death when compounded with illness or certain medications.