The kidneys are organs located on either side of the body that filter out and remove waste. According to the National Kidney Foundation, the kidneys process about 200 quarts of fluids every single day. About two quarts of the processed fluids are removed as waste. Exercise and physical exertion put a particular strain on your kidneys, and so their function must adapt in order to deal with the increased demands of your body's metabolism, which is the set of all chemical reactions that occur in your cells.
Function
The kidneys remove waste through millions of functioning units called nephrons. Each nephron consists of tiny blood vessels known as a glomerulus attached to a tubule. The task of the glomerulus is to filter out chemicals and water from the blood which are then taken to the bladder for excretion as urine. The rest is reabsorbed back into the blood. Kidney function is measured by its ability to remove a product called creatinine that is broken down during the course of your body's normal metabolic processes in the muscles.
Significance
According to a 1994 study published in the journal "Sports Medicine" by researchers from Belgium, the amount of blood flowing to the kidneys during exercise is reduced overall, but the rate of filtration can actually double. The glumerulus is therefore processing about the same amount. However, excretion rates tend to increase as a whole. Furthermore, despite large increases in lactate -- a metabolic waste product that is responsible for the "burning" you feel in your muscles -- the actual excretion of lactate is limited. This might be caused by a saturation that limits the cells' ability to transport lactate. Lastly, the amount of protein in the blood increases up to an hour after physical activity. This is directly related to the intensity of exercise rather than its duration. The increased clearance of proteins suggests an increased glumerulus permeability and an inhibition to fluid reabsorption.
Dehydration
Dehydration is defined as an excessive loss of body fluids such as water and electrolytes. Despite the body's attempt to prevent dehydration, it can occur from the excessive excretion of fluids during exercise. According to Dr. Gabe Mirkin, a practicing physician and graduate of Harvard University, dehydration is known to put repeated stress on the kidneys. Increasing the intensity of exercise only makes it worse.
Considerations
A 2008 study published in the journal "Clinical Journal of Sports Medicine" by Italian researchers appears to demonstrate, at least preliminarily, that exercise may actually help to prevent kidney damage that often occurs with aging. Regular exercisers have lower blood creatinine levels, which means that their bodies are more capable of clearing out creatinine from the bloodstream. This suggests that kidney function improves in spite of the repeated stress during exercise and perhaps even because of it. The improved creatinine clearance was even more pronounced in professional cyclists, who tend to endure the greatest stress of dehydration to the kidneys during exercise.
