Urinary Excretion Of Phosphates After Exercise

When you exercise, skeletal muscle is broken down in microscopic amounts. These changes may be experienced as soreness or fatigue afterward. Normally, this amount of damage is easily repaired and your muscles adapt by growing in size and strength. However, undergoing a repetitive, strenuous exercise session without proper training and preparation may lead to a syndrome known as exertional rhabdomyolysis. Changes in urinary excretion of muscle cell contents, including phosphate, may result if the damage is severe enough.

Exertional Rhabdomyolysis

In some cases of strenuous exercise, your muscle cells may be severely strained to the point of rupturing their cell membrane. This condition is known as rhabdomyolysis and is characterized by the release of muscle cell contents into the blood. Muscle cells are loaded with specific types of proteins and ions, including potassium and phosphate, which allow them to function as contractile units. Rhabdomyolysis consequently results in elevated blood potassium and phosphate concentration.

Symptoms

In addition to ions, when skeletal muscle is severely damaged, muscle cell proteins will leak into the blood. Notably, the muscle protein myoglobin, which normally acts to bind oxygen in muscle cells, is released into the circulation. Eventually, this protein will be filtered out of the blood by the kidney, resulting in cola-colored urine. Along with extreme fatigue and muscle tenderness, dark urine is a classic symptom of exertional rhabdomyolysis.

Changes in Urine Content

Other changes in urine content are only appreciated with laboratory tests. During a typical exercise routine, the small changes in blood phosphate levels would be of little consequence. During exertional rhabdomyolysis, however, the flood of phosphate ions out of damaged muscle cells would overwhelm the kidney, causing some to spill over into the urine. Therefore, hyperphosphatemia, or elevated levels of phosphate in the blood, will increase urinary excretion of phosphates.

Complications

Your kidneys are constantly working to maintain the concentration of ions and minerals in your blood. As a microscopic filter, the kidneys are susceptible to damage when large proteins, such as myoglobin, are released into the circulation. In some cases, the damage may be so severe that the patient presents with symptoms of acute kidney failure. Exertional rhabdomyolysis is a relatively rare condition, typically presenting in cases of extremely strenuous exercise, such as during marathons and military basic training.